physical geology02

REVIEW CLASS FOR

PRINCIPLES OF GEOLOGY

Geology - from the Greek words geo (earth) and logos (discourse); literally the

science of the Earth, dealing with the study of its composition, structure, history, past

life forms and processes responsible for the Earth’s present configuration.

Geology is already recognized to be of important use in economics, engineering,

environmental studies, agriculture, land use, urban planning, and even for

seemingly unrelated disciplines such as sociology and politics.

The two main branches of Geology are:

1) Physical or Dynamic Geology – involved in the study of the material

composition, appearance, structure and processes of the Earth.

2) Historical Geology – deals with the history of the Earth. This includes the Earth’s

origin, relative and absolute timing of events that has shaped the Earth, as well as the

life forms that has appeared in Earth’s history.

PRINCIPLES OF GEOLOGY REVIEW

Geological Branches and Specializations

• Geophysics • Geochemistry • Petroleum geology • Economic geology • Hydrogeology and hydrology • Engineering geology • Environmental geology • Seismology • Geochronology • Geomorphology

• Planetary geology or cosmogeology

• Glaciology

• Marine geology

• Mineralogy

• Paleontology

• Petrology

• Sedimentology and stratigraphy

• Structural geology

• Volcanology

1) Ca. 4.7 B years ago - accumulation of the planet by

the gathering of planetesimals (unsorted

conglomeration of Si compounds, and Fe and Mg

oxides and smaller amounts of natural chemical

elements.

Earth origins

2) Gravity compression leads to temperature

rise; heat accumulated in the Earth’s interior,

probably averaging 1000°C.

3) Spontaneous disintegration of radioactive

elements (e.g., U, Th, K) further caused heating

of the Earth’s interior.


Planetary differentiation

At about 1 billion years after the Earth was formed,

the T at depths of 400-800 km was enough to melt

Fe.

Large drops of Fe have fallen toward the center,

displacing the lighter minerals.

About 1/3 of the material sank to the center, a large

part being converted to a molten state.

The molten material floated upward to cool and form a primitive crust

Such differentiation resulted in the Earth’s internal layering

Differentiation probably initiated the escape of gases from the interior which

eventually led to the formation of the atmosphere and oceans, and ultimately,

life.

the transfer of internal heat to the surface was accomplished by convection,

even when the mantle solidified.


The major structural units of Earth


Layers Based on Composition

The Crust consists of:

1) The continental crust - relatively light ―granitic‖ rock that includes the oldest

rock of the crust; generally richer in Na and K; thickness ranges from 30 to 80

km, sometimes attaining 100 km in some portions.

2) The oceanic crust - composed of dark, dense volcanic rocks (basalt) with

densities much greater than that of granite; more Fe-rich than the continental

crust and thinner, ranging from 3 to 10 km in thickness; young and relatively

undeformed by folding.

The Mantle - surrounds or covers the core; constitutes the great bulk of Earth

(82% of its volume and 68% of its mass); composed of iron and magnesium

silicate rock.

The Core - central mass about 7000 km in diameter; density increases with depth

but averages about 10.78 g/cm3; constitutes only 16% of Earth’s volume but

accounts for 32% of Earth’s mass; mostly composed of iron.


Internal Layers Based on Physical Properties

Lithosphere - the strong rigid outer layer consisting of the crust and a portion of

the upper mantle.

Asthenosphere (―weak sphere‖)- a major zone within the upper mantle where

temperature and pressure are at just the right balance so that part of the material

melts. The rocks become soft plastic in behavior and flowing like warm tar. The

boundary between the lithosphere and the asthenosphere is distinct but does not

correspond to a compositional change but due to a major change in the physical

properties of the rock. It is as much as 200 km thick.


Mesosphere – layer below the asthenosphere; the region between the

asthenosphere and the core-mantle boundary; stronger and more rigid because

the high pressure at this depth offsets the effect of high temperature.

The Core - marks a change in both physical properties and composition;

composed mostly of iron and is therefore distinctly different from the silicate

(rocky) material above. On the basis of physical properties, the core has two

distinct parts—a solid inner core and a liquid outer core.

EARTH’S OUTER LAYERS

The outermost layers of Earth are the atmosphere, hydrosphere, and

biosphere. The continents and ocean basins are Earth’s major surface

features.


The Continents and Ocean Basins

The ocean basins occupy about two-thirds of Earth’s surface; characterized by a

spectacular topography.

The continents rise above the ocean basins as large platforms.

The difference in elevation of continents and ocean basins represents a

fundamental difference in rock density.


Major features of the continents:

1. Most continents are roughly triangular in shape.

2. They are concentrated in the Northern Hemisphere.

3. Although each may seem unique, all continents have three basic components:

(a) a shield - large areas of highly deformed igneous and metamorphic rock

(basement complex)

(b) a stable platform or craton - extensive flat, stable regions of the

continents in which complex crystalline rocks are exposed or buried

beneath a relatively thin sedimentary cover

(c) folded mountain belts – uplifted mountain ranges that are sites of tectonic

convergence

4. Continents consist of rock that is less dense than the rock in the ocean basins.

5. The continental rocks are old, some as old as 3.8 billion years.

6. The climatic zone occupied by a continent usually determines the style and

variety of landforms developed on it.


Shield

Stable platform

Continental crust

Flood basalt

Young mountain belt

Old mountain belt

Oceanic crust

Trench

Rift Zone


Major features of the ocean floor

1. Mostly basalt, a dense volcanic rock, and its major topographic features are

somehow related to volcanic activity The oceanic crust, therefore, is entirely

different from the continental crust.

2. The rocks are young in a geologic time frame; most are less than 150 million

years old

3. The rocks have not been deformed by compression.

4. The major provinces of the ocean floor are

a. Oceanic ridge - most striking and important feature on the ocean floor;

extends continuously from the Arctic basin down the center of the Atlantic

Ocean, into the Indian Ocean, and across the South Pacific; A huge,

cracklike valley, called the rift valley, runs along the axis of the ridge

throughout most of its length

b. The abyssal floor - vast areas of broad, relatively smooth, deep-ocean

basins on both sides of the ridge; lies at depths of about 4000 m; consists

of:

c. Seamounts - isolated peaks of submarine volcanoes.

d. Trenches - the lowest areas on Earth’s surface; adjacent to island arcs or

coastal mountain ranges of the continents.

e. Continental margins - zone of transition between a continental mass and

an ocean basin consisting of continental shelf and continental slope


PLATE TECTONICS

The lithosphere is broken into a series of separate plates that move relative to each

other.

The Earth’s lithosphere floats on the denser, plastic asthenosphere beneath, and it

rises and sinks in attempts to maintain equilibrium.

Tectonics – study of the origin and arrangement of the broad structural features of

the earth’s surface (e.g., continents, mountain belts, island arcs, earthquake belts,

faults, folds, etc.) .

Plate – a large, mobile slab of rock that is part of the earth’s surface. It may be

made up entirely of sea floor (e.g., Nazca plate) or both continental and seafloor

(e.g., North American plate).

Plate tectonics – the principle that the earth’s surface is divided into large, thick

plates that move slowly and change size relative to one another.

Plate boundary – narrow areas of intense geologic activity where plates move

away from one another, past one another or toward one another.


The major plates of the world


Eurasian Plate Pacific Plate

African Plate Indian-Australian Plate

North American Plate Philippine Sea Plate

South American Plate Nazca Plate

Antarctic Plate Cocos Plate

Arabian Plate Scotia Plate

Distribution of earthquakes and volcanoes


1. Continental

extension

2. Continental rifting

3. Ocean spreading

Diverging plate boundaries -where plates move away from each other, either within the ocean or

continent.


Converging plate boundaries - where two plates move

toward each other

OCEAN-OCEAN CONVERGENCE


OCEAN-CONTINENT CONVERGENCE


CONTINENT-CONTINENT CONVERGENCE

1. Ocean-continent

convergence

2. Ocean closing

3. Continent-continent

collision


Transform boundaries – where one plate slides horizontally past another

plate along a fault or a group of parallel faults.

- the displacement along the fault abruptly ends or transforms into another kind of

displacement

MOR-MOR

MOR-Trench Trench-trench


Plates move by mantle convection


ROCKS AND MINERALS

Rock – the material or substance, consisting of a mineral or aggregate of minerals,

the Earth is made of.

Mineral – a solid chemical compound that is characterized by a definite composition

or a restricted range of chemical compositions and by a specific, regular

architecture of the atoms that make it up.


The nature of minerals

Minerals are the major solid constituents of Earth. A precise definition is difficult

to formulate, but for a substance to be considered a mineral, the following

conditions must be met:

1. It occurs naturally as an inorganic solid.

2. It has a specific internal structure; that is, its constituent atoms are precisely

arranged into a crystalline solid.

3. It has a chemical composition that varies within definite limits and can be

expressed by a chemical formula.

4. It has definite physical properties (hardness, cleavage, crystal form, etc.) that

result from its crystalline structure and composition.

The differences among minerals arise from the kinds of atoms they contain and

the ways the atoms are arranged in a crystalline structure.


The Structure of Minerals

Law of constancy of interfacial angles - each mineral has a characteristic

crystal form. Although the size and shape of a mineral crystal form may vary,

similar pairs of crystal faces always meet at the same angle.

Polymorphism - ability of a specific chemical substance to crystallize with more

than one type of structure. Example: Diamond and graphite, pyrite and

marcasite, etc.

Physical Properties of Minerals

1) Crystal Form - natural crystal faces that assumes a specific geometric form.

2) Cleavage - tendency of a crystalline substance to split or break along smooth

planes parallel to zones of weak bonding in the crystal structure.

3) Hardness - measure of a mineral’s resistance to abrasion. (Review Moh’s

Scale)

4) Specific Gravity - the ratio of the weight of a given volume of a substance to

the weight of an equal volume of water.

5) Color - most minerals are found in various hues, depending on such factors

as subtle variations in composition and the presence of inclusions and impurities.

6) Streak. It is the color of a mineral in powder form; usually more diagnostic

than the color of a large specimen.


Cubes of pyrite


Prismatic crystals of quartz

Perfect cleavage in mica Gypsum scratched by fingernail

SILICATE MINERALS

contain a basic building block called the silicon-oxygen tetrahedron, a complex

ion [(Si04)4 -] in which large oxide ions (O2

-) arranged to form a four-sided pyramid

with smaller silicon ion (Si4) fitted into the cavity between them. The major groups

minerals differ mainly in the arrangement of such silica tetrahedral crystal

structures.

The silica tetrahedron


Silicon-oxygen tetrahedra combine to form minerals in different ways. In the

simplest combination, the oxygen ions of the tetrahedra bonds with other

elements, such as iron or magnesium (e.g., Olivine). Most silicate minerals are

formed by the sharing of an oxygen between two adjacent tetrahedra.

The sharing of oxygen with the silicon ions results in several fundamental

configurations of tetrahedra defining the major silicate groups:

1.Single chains - pyroxenes

2.Double chains - amphiboles

3.Two-dimensional sheets - micas, chlorites, and clay minerals

4.Three-dimensional frameworks - feldspars and quartz

REVIEW THE ROCK FORMING MINERALS (e.g., silicates, carbonates,

sulfates, etc.)


IGNEOUS ACTIVITY

An igneous rock is one that formed from the solidification of magma.

Magma - is the hot-liquid molten material, generated within the Earth, that forms

igneous rocks when solidified

Magmas may be:

1) Intrusive (plutons) - magmas stored within the crust

2) Extrusive – magma erupted on the surface either as lava or as fragments sent

into the air (pyroclastic material).

Plutons may be emplaced as concordant or discordant bodies in relation to the

layering of the intruded rock or host rock.

- Sills are concordant plutons; they are flat, tabular bodies intruded parallel to the

layering of the host rock.

- Dikes are discordant plutons that cut across the layering of the host rock. When

no layering in the host rock is evident, the pluton is called a dike.

- A volcanic neck is an intrusive structure apparently formed within the throat of a

volcano.


-Laccoliths are mushroom-shaped bodies that rises near the surface and domes

the overlying layers while it spreads laterally.

- Batholiths are enormous, complex rock bodies that cover at least 100 km2.

- Stocks are plutons similar to batholiths but smaller in size (<100 km2).


Igneous rock textures

Phaneritic texture – coarse grained; the

mineral components are visible to the naked

eye; charactersitic of deep intrusive rocks that

slowly cooled.

Aphanitic texture – fine grained; the

mineral components are not visible to the

naked eye (i.e., microscopic); formed by

relatively fast cooling of some volcanic

rocks (e.g., basalt).


Glassy – texture of igneous rock with a high

glass content; formed by very rapid cooling, such

that minerals had no time to form crystals.

Porphyritic – igneous texture in which

crystals visible to the naked eye are embedded

in a matrix of aphanitic texture; it represents

a solidifying magma that has suddenly

erupted to the surface.


Classification of igneous rocks

Peridotite family (Ultramafic rocks).

Peridotite - a dark, coarse-grained intrusive rock composed mainly of olivine, with

lesser amounts of pyroxene with little or no plagioclase, believed to form the bulk of

the upper mantle. Mg and Fe aare dominant constituents

Basalt-gabbro family (Mafic rocks).

Basalt - a fine-textured, dark brown to black extrusive rock, composed primarily of

Ca-plagioclase feldspar, pyroxene and olivine.

Gabbro - coarse-textured, deep intrusive equivalent of basalt.

Dolerite or diabase - intermediate between basalt and gabbro, as it is intruded near

the surface.

Mg and Fe minerals remain important components but in lesser amounts than those

in ultramafic rocks. They compose the entire oceanic crust, basalt forming the upper

layers and dolerite and gabbro forming the thicker internal layer upon which the

basalt rests. Hotspot volcanoes and some arc volcanoes also erupt basalts.


Andesite-diorite family (Intermediate rocks)

Andesite - a gray, fine-grained volcanic rock consisting of plagioclase, ± pyroxene,

amphibole and/or biotite (mica). The plagioclase has about equal amounts of Ca and

Na ions.

Diorite - the coarse-grained intrusive equivalent.

The andesite family is typical of subduction-related magmatism.

Granite-rhyolite family (silicic or felsic rocks)

Granite - a light-colored, coarse grained intrusive rock consisting primarily of quartz,

K-feldspar and/or Na-plagioclase. Ferromagnesian minerals such as hornblende and

biotite may or may not be present in subordinate amounts. Granite and its slightly

more mafic variety, granodiorite, are the most common igneous rocks of the

continental crust.

Rhyolite - the extrusive equivalent of granite and is also generally confined to the

continental crust.


How magma forms Magma originate principally by partial melting of pre-existing rock.

Sources of heat:

1) Geothermal gradient

2) The hotter mantle – geothermal gradients are higher in hot spots, where mantle

plumes, which are narrow upwellings of hot material within the mantle occur.

Factors affecting melting temperatures:

1) Pressure – In general, the melting point of a mineral increases with increasing P.

Upwelling mantle material originating from deeper high pressure portions would

melt at shallower portions where there is lower P.

2) Water – water vapor under high pressure can lower the melting T of rocks.

How magmas of different composition evolve 1) Differentiation

Magma stored within the earth’s crust, if allowed to remain liquid, will undergo

differentiation, the process by which different ingredients separate from an

originally homogenous mixture. Differentiation is attained when minerals crystallize

and separate from the mother magma, altering the magma composition in the

process.


According to experiments by N.L. Bowen in the early 20th century, it is possible to

derive mafic and felsic magma from a common parental source through

differentiation.

Bowen’s reaction theory

Bowen’s reaction series


2) Source rock – the composition of the resultant magma is, in general, more felsic than the

parent magma. Thus, peridotite melting produces basaltic magma, while melting a basaltic

source will give rise to intermediate to felsic rocks, depending on partial melting degree.

3) Partial melting – The first minerals to melt are those in the later portion in the Bowen

reaction sequence. Thus, the lower the partial melting degree is, the more felsic the rock

becomes.

4) Assimilation – A very hot magma may melt the country rock and assimilate the newly

molten material into the magma.

5) Magma mixing – If two magmas meet and merge in the crust, the combined magma will

be compositionally intermediate.


Geologic settings of igneous activity

Andesitic island arc

Trench

Basaltic shield

volcano

Basaltic MOR

Trench

Andesitic/rhyolitic

volcanoes

Granitic

pluton

Basaltic lava

plateau Rift

valley

Rhyolitic ash flow


WEATHERING AND SOILS

Weathering – changes that take place in minerals and rocks at or near the surface

of the earth in response to the atmosphere, water and plant and animal life.

Some definitions:

1)Bedrock – the solid rock underlying all parts of the land surface.

2)Regolith – soil and loose fragments that may cover the bedrock

3)Soil – surface accumulation of sand, clay and decayed plant material (called

humus)

Types of weathering

1)Mechanical weathering – also called disintegration process by which a rock is

broken down into smaller and smaller fragments as the result of energy developed

by physical forces.

a) Expansion and contraction – changes in T, if they are rapid and great

enough, may bring about the mechanical weathering of a rock.

b) Frost action – When water trickles down into the cracks, crevices and pores of

a rock mass and freezes, its volume increases about 9%. This expansion sets up

pressures directly outward from the inside of a rock and frost wedging results. A

second type of frost action is frost heaving, which occurs when moisture

absorbed by loose soil or fragments freezes at shallow levels, heaving the ground

above.



c) Exfoliation – process in which curved plates of rock are stripped from a larger

rock mass by physical forces.

- it develops two types of landscape features:

1) exfoliation domes – joints parallel to the surface of a rock mass may develop.

This may be accomplished through exhumation of the deeper portions of the

rock mass by erosion (e.g., sheeting).

2) spheroidally weathered boulders – boulders that have been rounded by the

spalling off of a series of concentric shells of rock. The shells develop from

pressures set up within the rock by when minerals become altered (or

chemically weathered) and expand. Rocks that have considerable amount of

feldspar are susceptible to spheroidal weathering because of the expansion of

these minerals during chemical weathering.


Other types of weathering

Plants also play a role in mechanical weathering. The roots of trees and shrubs

growing in rock crevices sometimes exert sufficient pressure to dislodge previously

loosened fragments of rock.

The mechanical mixing of soil by ants, worms and rodents, makes the soil more

susceptible to chemical weathering.

2. Chemical weathering – also called decomposition; more complex process

involving chemical alteration or changes, transforming the original material into

something different. These changes either involve the transformation of a mineral to

clay or another mineral or the solution of soluble minerals.

Factors influencing chemical weathering

1) Particle size – The greater the surface area of a particle, the more vulnerable it is

to chemical attack.


2) Composition of original material – Minerals respond at different rates to

weathering

3) Climate – rocks respond to to different climate conditions

4) Moisture – when moisture is accompanied by warmth, rate of chemical

weathering is faster.

5) Plants and animals – they produce oxygen carbon dioxide and certain acids

that enter into chemical reactions with earth materials.

Chemical weathering of minerals

1) Quartz – very slowly affected, relatively stable mineral.

2) K-Feldspars – Feldspars are among the first minerals to break down under

chemical attack. - Aluminum silicate, derived from the chemical breakdown of

feldspar, combines with water to form hydrous aluminum silicate minerals, or

clays.

Products of orthoclase weathering:

a) Clay – the H ion from the water forces the potassium out of the orthoclase,

disrupting its crystal structure. The H ion combines aluminum silicate to

form the new clay mineral. The mineral undergoes hydration, the process

by which waer combines with other molecules.

- clays are very fine, sometimes colloidal (0.2-1 mm)

Types of clay:

1) kaolinite – derived from the Chinese kao-ling, or ―high hill‖, the name of

the hill from which the first kaolinite shipped to Europe came.

2) montmorillonite – first described from a town in central France,

Montmorillon.

3) illite – named by state geologists of Illinois in honor of their state.

b) Potassium ions – some of the potassium is carried away by water, some

are absorbed by plants, and some are absorbed by clay minerals or taken

into their crystal structure.

c) Silica - appears as silica in solution or finely divided quartz of colloidal size.


3) Plagioclase – the weathering products are similar to those of K-feldspars, but

instead of K, either Na or Ca carbonate is formed. These are soluble in water,

and may eventually reach the sea.

4) Ferromagnesians – they produce similar products when weathered: clay,

soluble salts, and silica. The presence of Fe and Mg, however, makes the

formation of other minerals possible. Fe may combine with O to form hematite

(Fe2O3), a deep red color mineral (from the Greek word haimatités or ―bloodlike‖).

The Fe may unite with O and a hydroxyl ion to form goethite (FeO(OH)) (named

after the German scientist Goethe). Another substance produced is limonite or

plain rust. It is not a true mineral because its composition is not fixed. Mg is

removed in a solution as a carbonate or taken up in illite and montmorillonite

clays.

Rates of weathering

- Some rocks weather rapidly and others only slowly. Rate of weathering is

governed by the rock type, mineral composition, moisture, temperature,

topography, and plant and animal activity

- Minerals commonly found in igneous rocks can be arranged according to the

order in which they are chemically decomposed at the surface.


Rate of weathering of minerals in relation to Bowen’s reaction series


SOIL Definitions:

Soil - the residue of weathering. It is the layer of weathered unconsolidated material

above bedrock.

Bedrock - the unweathered rock beneth the soil; also termed as parent rock.

Regolith – loose fragments that may cover the bedrock; soil is the upper part of the

regolith

Loam – soil of approximately equal amounts of sand, silt and clay; they are well

drained and often fertile.

Topsoil – the upper fertile protion of soil.

Subsoil – Stony part of the soil, lacking organic matter.

Soil consists of:

1) Clay minerals – help to hold water and plant nutrients in soil.

They are negatively-charged microscopic plates that attracts

water and nutrients (e.g., Ca++ and K+) in the soil.

2) Quartz – form sand grains that help keep soil loose and aerated,

allowing good water drainage

3) Water molecule – is neutrallly charged, but with posititve and

negative end; its positive end is attracted to clay to make it

available for uptake by plant roots.


Soil Horizons – are the soil layers that can be distinguished from one another by

appearance and chemical composition. Their boundaries are transitional.


O horizon – dark-colored layer rich in organic materialand forms just below surface

vegetation. It contains humus or decomposed plant material, and contributes to the

formation of organic acids that accelerate the leaching of the underlying A horizon.

A horizon – this is the zone of leaching, where there is downward movement of

water. Water that percolates leach or carry dissolved chemicals and clay downward

to lower levels. Leaching may make this horizon pale and sandy, but the uppermost

part is often darkened by humus.

B horizon – where leached material from A horizon accumulates. It is often clayey

and stained red or brown by hematite and limonite. Calcite may build up in B

horizons.

C horizon – incompletely weathered parent material or bedrock; it is the transition

between unweathered bedrock below and developing soil above.

Residual soil – soil that develops in situ; it developed on the rock beneath it.

Transported soil – soil that developed from parent material that was brought to a

site from other region such as sediment deposited by running water, wind or ice. Mud

deposited by a river during flooding may form fertile soil


Effect of parent rock

Soil from granite – develops sand and clay; sand is composed of quartz, while clay

comes from weathering of the feldspars.

Soil from basalt – is never sandy but always clayey

Effect of time

• Given enough time, soil will mature and parent material becomes immaterial. The

presence of quartz grains is the only characteristic with long term significance.

• With time, soils become thicker.

• A new deposit of volcanic ash maybe covered with grass in just a few years but a

new lava flow weathers much more slowly.

Effect of slope

• Soils tend to be thick on flat land where erosion is slow and water collects; they are

thin on steep slopes where gravity pulls water and soil particles downslope.

Effect of organic activity

• Organic activity aides mechanical and chemical weathering


Effect of climate

Soil tends to be thick in wet climates, and characterized by downward movement of

water to form pedalfer, which has a high content of aluminum and iron.

Soils are thin in arid climates where pedocals are developed. They are characerized

by little leaching, scant humus and the upward movement of soil water beneath the

land surface by evaporation. The evaporation of water beneath the surface can

cause the precipitation of calcium salts.


Hardpan

• This is a general term for a hard layer of earth material that is often clayey.

• Hardpan in wet climates are usually formed of clay minerals, silica and iron

compounds that have accumulated in the B horizon.

• In arid climates, caliche forms from the cementing of soil by calcium carbonate and

other salts that precipiitate as water evaporates.

• Hardpans are very hard and impermeable; they can break plows, prevent water

drainage and act as barrier to plant roots.

Laterites and bauxites

Laterites – highly leached soils that develop in tropical climates. Intense and deep

weathering develops red soil composed entirely of iron and aluminum, which are

the least soluble products of rock weathering. Iron laterites may form from basalt,

and other iron-bearing rocks. Nickel laterite is produced from weathering of

ultramafic rocks. Laterites are unproductive soil. If exposed to the sun, it is apt to

bake into a very hard layer that may be quarried to make bricks.

Bauxite – forms from the weathering of aluminum-rich rocks. If found in thick pure

layers, they are mined for aluminum. The aluminum is concentrated residually by

removal of other components.


Paleosol – soil buried by younger deposits of lava, volcanic ash, dust or sediment.

They may be extensive layers useful for interpreting past climates and topography.

Laterite soil

Bauxite formation


SEDIMENTARY PROCESSES Sediment – particles that have been mechanically transported by water, wind or

ice or chemically precipitated from solution, or secreted by organisms, and

deposited in loose layers on the Earth’s surface.

- they contain the entire fossil record of the Earth.

- in them are recorded the composition, climate, topography of former

landmasses, as well as physical, chemical and biological conditions of the

oceans that no longer exist.

- they provide the means of retaining the chronological record of the past

- also of significant economic importance: groundwater, gold, copper, zinc, iron,

lead, diamonds, limestone, sand, gravel, clay, oil, gas, coal.

Size classification:

1) Gravel – includes particles coarser than 2 mm in diameter (boulder > 256 mm,

cobble 256-64 mm and pebble 64-2 mm)

2) Sand – grains from 1/16-2 mm

3) Silt – grains from 1/256—1/16 mm; too small to see without a magnifying glass.

4) Clay – finest sediment, at least 1/256 mm in size.


3)Types of sediments:

1)Detrital – fragments derived from the weathering of rocks, transported by water,

wind or ice and deposited in loose layers on the Earth;s surface.

2)Chemical – particles precipitated directly from water.

3) Biochemical – precipitated directly or indirectly by the activities of organisms

Origin of sedimentary rocks:

Weathering – the physical disintegration and chemical alteration and

decomposition of rocks exposed to the atmospheric influences at the Earth’s

surface.

Transport – The disintegrated rock particles are transported by water, wind or ice.

During transport, the sediments are subjected to:

a) rounding – the grinding away of sharp edges and corners of rock fragments

during transport.

b) Sorting – process in which sediment grains are selected and separated

according to grain size or grain shape and specific gravity.


Deposition – when transported material settles or comes to rest as the medium of

transport loses energy and can no longer transport its load.

- also refers to the accumulation of chemical or organic sediment, such as shells, in

the sea floor or plant material on the floor of a swamp.

- Deposition of salt crystals can take place as seawater evaporates.

environment of deposition – the location in which deposition occurs (deep sea

floor, desert valley, river channel, coral reef, lake bottom, beach, sand dune).

Preservation – sediments are preserved when they are protected from further

erosion, usually by being buried by later sediments.

Lithification – The conversion of sediment into rock trough such processes as

compaction, cementation and recrystallization.

compaction – reduction in volume of sediments resulting from the weight of

newly deposited sediments above.

The spaces between grains of sediments are called pores, which may be empty or

filled with finer sediment called matrix. If the matrix consists of clay and silt,

compaction will harden the matrix.

If the matrix is filled with groundwater or saltwater saturated with silica, calcium

carbonate or iron oxides, these compounds will precipitate and bind the grains

together in a process called cementation.

Recrystallization – the formation of new crystalline mineral grains in a rock.


COMPACTION AND CEMENTATION


Classification of sedimentary rocks

Sedimentary rock – rock that has formed from: (i) lithification of sediment; (ii)

precipitation from solution; consolidation from the remains of plants and

animals.

Types of sedimentary rock:

1) Clastic (or detrital) – formed from cemented sediment grains that are

fragments of preexisting rocks (e.g., conglomerate, sandstone, shale)

2) Chemical – deposited by precipitation of minerals from solution (e.g., rock salt,

limestone

3) Organic or biochemical – rocks formed by the accumulation of the remains of

organisms.


Clastic (or detrital rocks) Breccia and conglomerate

Sedimentary breccia – coarse-grained

sedimentary rock formed by the cementation of

coarse, angular fragments of rubble; formed not

far from the source (e.g, landslides, talus, etc.)

Conglomerate – coarse-grained sedimentary

rock formed by the cementation of rounded

gravel; though formed not far from the source,

some transport was necessary to round the

angular edges.


Sandstone – a medium-grained sedimentary rock formed by the cementation of

sand grains.

Quartz sandstone – sandstone in which more than 90% of the grains are

quartz Arkose – sandstone in which more than 25% of the grains are feldspar; the rock

has not underegone severe chemical weathering; transportation distance is

relatively short.

Graywacke (lithic sandstone) – more than 15% of the rock’s volume consists of

fine-grained matrix; often tough and dense, generally dark grey or green in color.

The sand grains consist primarily of lithic (or rock) fragments, feldspar, ± quartz.

Maturity of detrital sediments


Turbidite deposits

Most graywacke probably formed from sediments transported by turbidity

currents, dense masses of sediment-laden water that flow downslope along the

sea floor. The sediments that result are called turbidites.


Fine-grained rocks

Shale – fine grained sedimentary rock notable for its splitting capability (called

fissility). Splitting takes place along the surfaces of very thin layers called

laminations within the shale. They deposit on lake bottoms, at the ends of rivers in

deltas, beside rivers in floods, and on the quiet parts of the ocean floor. Depending

on the size of particles fine grained rocks are called siltsone, shale and mudstone.

Carbonate rocks

Limestone – a sedimentary rock composed mostly of calcite (CaCO3), usually

precipitated in shallow seawater through the actions of organisms. Carbonate

sediments are directly precipitated in the warm, shallow waters of tropical to

subtropical seas

Types of limestone:

1) Biochemical limestone

a) chalk – fine-grained limestone consisting of billions of microscopic organisms

that settled in shallow water.

b) coquina – composed of large, poorly cemented shell fragments.

c) micrite – fine-grained limestone formed from the lime mud; deposited in water

devoid of current or wave action (e.g., tidal flat or quiet lake)

Coral reefs –built of the myriad skeletal secretions of tiny colonial coral. Live coral

must grow close to sea level where light can penetrate.

Chemical limestone Oolitic limestone – directly precipitated limestone consisting of caviar-sized

particles called ooids. An ooid consists of a minute sand-grain nucleus, around

which are wrapped layers of calcium carbonate.


Other sedimentary rocks

1) Evaporites – rocks that formed from crystals that precipitate during

evaporation of water (e.g., rock gypsum and rock salt)

2) Chert – A hard, compact, fine-grained sedimentary rock formed almost entirely

of organisms that secrete silica for their shells. These organisms, radiolaria

and diatoms, are very tiny single celled.

3) Coal – a sedimentary rock formed from the consolidation of plant material; it is

rich in carbon and usually black; it burns readily. Coal usually develops from

peat, a brown, lightweight unconsolidated deposit of moss or of the plant

material that accumulates in wet bogs.

Sedimentary structures Bed (or stratum) – The smallest division of stratified (or bedded) sedimentary

rock, consisting of a single distinct sheetlike layer of sedimentary material,

separated from the beds above and below by relatively well-defined planar

surfaces called bedding planes which mark a break in sedimentation.

Stratification – the condition shown by sedimentary rocks of being disposed in

horizontal layers of beds.

Lamina – the thinnest or smallest recognizable unit layer of original deposition in

a sediment or sedimentary rock<1 cm in thickness


Types of bedding:

1) Current or cross-bedding – the development of internal laminations within a

stratum inclined at an oblique angle to the main bedding planes, resulting from

changes in the direction of water or wind currents during deposition. It is most

commonly developed in sandstones.


2) Graded bedding – Sedimentary bedding in which particles show a size

distribution. The coarsest material forms the base and the sequence becomes

progressively finer upwards. It is often present in turbidity deposits.


3) Mud cracks – also known as dessication cracks,

they are irregular fractures in a crudely polygonal

pattern formed by the shrinkage of clay, silt or mud

in the course of drying under the influence of

atmospheric surface conditions.

Ripple marks - most common minor beach morphological form, consisting of fairly

regular and generally small ridges formed in sediment on a river bed, in the inter-

tidal zone, or on the seabed seaward of low-water mark. Ripples are caused by

water or wind flow, and are aligned more or less perpendicularly to the flow

direction.


SEDIMENTARY ENVIRONMENTS


Glacial deposit – Boulder-clay or till is the dominant deposit, consisting of unsorted

and unstratified heterogenous mixture of clay, sand, gravel and boulders varying

widely in size and shape.

Alluvial fan – a low, outspread, relatively flat to gently sloping mass of loose rock

material, shaped like an open fan or a segment of a cone, deposited by a stream at

the place where it issues from a narrow mountain valley upon a plain or broad valley,

or where a tributary stream is near or at its junction with the main stream, or

wherever a constriction in a valley abruptly ceases or the gradient of the stream

suddenly decreases.

Flood plain – flat surfaces adjacent to streams over which streams spread in times

of flood. It is built of alluvium (recent clastic sediments) carried by the river during

floods. The sediments are called flood plain deposit.

River channel deposit – may consist of sediments of all sizes and shape. Abundant

load can result in the formation of channel bars, elongate deposits of sand and

gravel located in the course of the stream.

Lake deposit – sedimentary deposit laid down conformably on the floor of a lake,

usually consisting of coarse material near the shore and sometimes passing rapidly

into clay and limestone in deeper water; most of it is fluvial or glacial in origin.

Sand dunes – consists of loose sand piled or heaped up by the wind, commonly

found along low-lying seashores above high-tide level, more rarely on the border of

a large lake or river valley, as well as in various desert regions.


Delta – The low, nearly flat alluvial tract of land deposited at or near the mouth of a

river, commonly forming a triangular or fan-shaped plain of considerable area.

Beach – a shore of a body of water, formed and washed by waves or tides, usually

covered by sandy or pebbly material.

Lagoon - A shallow stretch of seawater, near or communicating with the sea and

partly or completely separated from it by a low, narrow, elongate strip of land such as

a reef, barrier island, sandbank, etc. Sand and silt dominate the sediments

deposited, with or without organic matter.

Barrier island – a long, low, narrow wave-built sandy island representing a

broadened barrier beach that is sufficiently above high tide and parallel to the shore,

and that commonly has dunes, vegetated zones, and swampy terranes extending

lagoonward from the beach.

Shelf – continental shelf; a relatively wide, shallowly submerged platform

Slope –continental slope; the steep part of the continental margin abutting the

continental shelf.

Abyssal fan – a terrigenous (land-derived sediments) cone- or fan-shaped deposit

located seaward of large submarine canyons. They often form turbidites.

Pelagic sediments – deep-sea sediments without terrigenous material; they are

either inorganic clay or organic ooze (pelagic sediment consisting of at least 30%

skeletal remains of pelagic organisms, and clay).


METAMORPHISM Metamorphism – the set of processes by which preexisting rocks (also called

parent rocks) are changed by pressures, temperatures and chemical conditions

that prevail deep within the earth.

- alters the mineralogy, texture and structure of the rocks, to give rise to

metamorphic rocks.

- Occurs at pressures and temperatures well above those prevailing at the Earth’s

surface but below melting temperatures.

Factors controlling the characteristics of metamorphic rocks 1)Composition of the parent rock – usually, no new chemical compounds are

added to the rock, except, perhaps, water (except for contact metasomatism).

2)Temperature – any mineral is stable only within a given temperature. High T

causes ions of the minerals to vibrate more rapidly, causing chemical bonds to

break, forcing them to realign in combinations suitable with the high-energy

environment; New minerals form while old ones are destroyed. Heat also causes

plastic deformation.

3) Pressure and stress - buried rocks are subjected to confining pressure (or

geostatic pressure), or the pressure applied equally on all surfaces of a body as a

result of burial.


Differential stress: the stronger or weaker stress acting on a body at different directions; often

caused by tectonic forces. Ther are 2 types:

a) Compressive stress – due to compression

b) Shearing stress – due to sliding of one body past another.


Compressive stress Shearing stress

Foliation – planar texture that develops as a result of

differential stress. Minerals that are subjected or form

under differential stress tend to follow the direction

of shearing or align themselves perpendicular to the

compressive stress.

4) Fluids – hydrous fluids charged with dissolved gases greatly accelerate chemical

reactions. The water may come from the rocks themselves or from an outside

source.

5) Time - Minerals that form by metamorphism need time to grow. Silicates have

very slow chemical reaction rates, often requiring millions of years to attain

significant growth.


Classification of metamorphic rocks

Non-foliated rocks (Name based on mineral content)

Usual parent

rock

Rock

Name

Predominant

minerals

Identifying characteristics

Limestone

Dolomite

Marble

Dolomitic

marble

Calcite

Dolomite

Coarse interlocking grains. Calcite (or

dolomite) has rhombohedral cleavage.

Calcite effervesces in weak acid.

Hardness between glass and fingernail.

Quartzose

sandstone

Quartzite Quartz Interlocking small granules of quartz;

sugary appearance, vitreous luster;

scratches glass.

Shale

Basalt

Hornfels

Hornfels

Fine-grained

micas; ferromags,

plagioclase

Fine-grained dark rock that generally

will scratch glass; may have a few

coarser minerals present


Non-foliated rocks

Foliated rocks (Name based on kind of foliation)

Texture Rock

Name

Identifying characteristics

Slaty Slate Fine-grained rock with earthly luster. Splits easily into thin, flat

sheets

Intermediate

(slaty to

schistose)

Phyllite Fine-grained rock with silky luster. Generally splits along wavy

surfaces.

Schistose Schist Visible platy or elongated minerals with planar alignment

Gneissic Gneiss Light and dark minerals are found separate, parallel layers or

lenses. Dark layers commonly include biotite and hornblende;

light-colored layers consist of quartz and feldspars. Layers may

be folded or contorted.


a) Slaty – rock splits easily along nearly flat and parallel

planes indicating microscopic platy minerals pushed

into alignment.

Types of foliation texture:

b) Schistose – visible platy or needle-shaped minerals

have grown essentially parallel to a plane due to

differential stress.

c) Gneissic – minerals separate into distinct light and dark

layers or lenses.


Metamorphic facies

Facies – Assemblage of mineral, rock

(or fossil) features reflecting

environment in which rock was formed;

such features are used to differentiate

one rock facies from other neighboring

units. Rocks having the same mineral

assemblage that formed within a well-

defined set of pressure-temperature

conditions are regarded as belonging to

the same metamorphic facies.

Such minerals that characterizes a

given intensity of metamorphism are

called index minerals.


Distribution of facies across a convergent plate boundary


Types of metamorphism:

1) Regional metamorphism – metamorphism of an extensive area of the crust;

generally associated with intensive compression and mountain building; induced

during subduction and collision that produce fold mountain ranges. Rocks are

almost always foliated due to differential stress (e.g., from green schist to

amphibolite)

2) Shear metamorphism – the transformation of rocks within the shear zone

associated with active fault movement; mainly involves grinding, pulverizing and

recrystallization of the rocks. Shear faulting produces a rock type called

mylonite.

3) Contact metamorphism – the transformation of rocks caused by heat escaping

from an igneous intrusion. It may be accompanied by metasomatism, which is

a replacement process whereby the elements of a rock are exchanged with

those of a magmatic fluid.

4) Burial metamorphism – results in response to the pressure exerted by the

weight of the overlying rock; occurs deep into thick sedimentary basinsThe

deeper rocks are subjected to higher temperature and pressures, giving rise to

progressive metamorphism.


5) Shock metamorphism – changes in rock and minerals caused by shock waves

from high-velocity impacts, mainly from meteorites.

Metamorphism and plate tectonics

Specific metamorphic

rocks are associated with

specific tectonic

environment. Examples:

regional metamorphism at

subduction zones, shear

metamorphism long faults,

contact metamorphism due

to rising magma, and

hydrothermal

metamorphism of the sea

floor.


Seafloor metamorphism

Fractures that develop within

the MOR act as

passageways for seawater

circulating within the crust.

The seawater heated by

magma rises and reacts with

the basaltic crust, converting

it to hydrous rocks such as

serpentinite. Metals extracted

from the crust are

redeposited and

concentrated high within the

crust and on the surface.


Stress, Strain and Strength of Rocks

Deformation of the Earth’s crust can be described in terms of change in volume,

change of shape, or a combination of both.

Compression produces

change in volume

without change in shape

Shear causes change

of shape without change

of volume

Deformation may involve a combination of both

CRUSTAL STRUCTURE AND DEFORMATION


Stress – A force applied to a material that tends to change that material’s

dimensions.

Unit stress – total force divided by the area over which the force is applied.

Strain – the amount of deformation caused by stress.

Rocks are said to be elastic if deformation disappears when stress is removed.

However, rocks may may not regain their former shape at an instant, as there is a

retardation of recovery. A materials’s resistance to elastic shear is called rigidity.

Plastic deformation is permanent. It involves a property of rock called viscosity.

A material that is deforming plastically does so by the propagation and movement of

dislocations (or small structural defects of the material).

If the rate of flow is proportional to the stress causing it, the material is said to be

viscous.

Viscosity is an important property in some geological processes; it governs the

ability of magma to flow and enables the mantle to adjust to crustal loads.

There are different types of stress:

Extension – stretching stress, can cause increase in volume of material

Compression – tends to decrease the volume

Shear – produces changes in shape.

A stress beyond a material’s strength can cause rupture.

Strength – is the limiting stress that a solid can withstand without failing by rupture

or continuous plastic flow (compressive strength, shear strength, and tensile

strength.)


Structural geology – deals with deformed masses of rock, their shapes and stress

that caused the deformation.

Relief – the difference in elevation between the highest and lowest points in a

specified area.

Structural relief – the difference in elevation of parts of a deformed bed; it is used

as a measure of deformation

In describing the attitudes of structural features, it is convenient to use two special

terms:

Angle of Dip – the acute angle that a rock layer or linear structure forms with the

horizontal.

Strike – the course of bearing of the inclined rock layer or structure, measured along

the line of intersection that the inclined layer or structure makes with a horizontal

plane.

Direction of dip – direction in which a rock layer is inclined downward from a

horizontal plane. Dip is measured perpendicular to the strike.


Horizontal Vertical Inclined Overturned


Folding – is the bending or warping of rock strata caused by compressive stress.

The structure that develops is called a fold.


Types of fold: Monocline – a double flexure

connecting strata at one level

with the same strata at another

level.

Anticline – an arch-shaped fold

Syncline – a trough-shaped fold.

Anticlines and synclines are

symmetrical if their limbs have

approximately equal dips.

If one limb is steeper than the

other, the fold is asymmetrical.

Overturned fold – An

aysmmetrical fold in which one

limb has been tilted beyond the

vertical.


Recumbent fold – a fold in

which the axial plane has been

overturned.

Isoclinal fold – fold in which

the beds on both limbs are

nearly parallel, whether

upright, overturned or

recumbent.

Fold nomenclature:

a) Limbs – the two sides or legs of

the fold

b) Axis – the direction of an

imaginary line connecting the

points of maximum flexure of the

fold

c) Axial plane – an imaginary

plane containing all the fold axes

within a deformed layer of rock

layers.

The axes of most folds are inclined.

The angle of dip of its axis is the

plunge.


The sedimentary rocks covering much of the continental interiors have been mildly

warped into:

Domes – circular or elliptical structural or topographic highs in which beds dip

away to all directions; when eroded, the oldest rocks are exposed at the center.

Basins – circular or elliptical structural or topographic lows or downwarps in which

beds dip towards the center; when eroded, the youngest rocks are exposed at the

center.

A warped plane

Outcrop pattern of an eroded

dome and basin structure


Joints - fractures or cracks developed on the rock, along which no significant

movement of blocks has occurred.

Columnar joints - joints resulting from the cooling of dikes, volcanic flows and

volcanic necks (similar to mud cracking

Sheeting – A pattern of essentially horizontal joints. This is especially common in

the shallow portions of granites, and may be related to pressure release upon

exhumation.

Faults – deformation by rupture in which the blocks of rock on each side of the

break move relative to each other.

Types of faults:

Dip-slip fault – usually, faults are inclined. When the displacement occurs is along

the direction of the dip, it is called dip slip fault.

The block above the fault plane is called the hanging wall, while the opposite

block is the footwall.


Dip slip faults are classified according to

the relative movements of these blocks.

Normal fault – a dip-slip fault in which the

hanging wall appeared to have moved

down with respect to the footwall. Also

known as gravity fault. It is mostly

associated with extensional stresses.

Thrust fault – or reverse fault, a dip-slip fault in

which the hanging wall appears to have moved up

relative to the footwall. It is largely the result of

horizontal compressive stresses.


Hanging wall – the block at the top of a dipping fault or structure.

Footwall – the corresponding block below a dipping fault or structure.

Extension or stretching of the crust may cause a series of related normal faults

that would be expressed as:

Graben – a structure in which a central block dropped down in relation to two

adjacent blocks; they form topographic basins marked by relatively straight

parallel walls.

Horst - a structure in which a central block was upraised in relation to two

adjacent blocks; ahorsts commonly blocklike plateaus or mountain ranges

bounded by faults.


Strike-slip fault – a fault along which the

movement is essentially horizontal, i.e., parallel

to the strike; also called transcurrent fault

Right lateral fault – a strike-slip fault in which

the block on the right appears to have moved

towards the observer; also known as dextral

fault.

Left-lateral fault – a strike-slip fault in which

the block on the left appears to have moved

towards the observer; also known as sinistral

fault.

Sag pond

Oblique-slip fault – a fault in

which movement has both vertical

and horizontal components

Hinge fault – a fault in which one

block appears to have rotated with

respect to another block, such

that the displacement dies out at a

definite point or axis.


Unconformity – a structural feature representing the relationship between a

buried erosional surface and younger overlying sediments. There are several

types:

a) Angular unconformity – formed when the older strata dip at an angle from that

of the younger strata.


b) Nonconformity – formed

when the underlying eroded

rocks are crystalline, either

igneous or metamorphic.


c) Disconformity – develops when the eroded rocks are parallel to the overlying

younger rocks. The top portions of the buried rocks are typically irregular because

of erosion.


EARTHQUAKES

Earthquake – trembling or shaking of the ground caused by the sudden release

of energy stored in the rocks beneath the earth’s surface

2 types:

1) Volcanic – due to volcanic activity (eruption or rising magma under a volcano)

2) Tectonic – due to movement of rocks past one another along faults; when a

rock breaks, waves of energy are sent out or produced, known as seismic

waves, causing earthquakes.


Elastic rebound theory: involves the sudden

release of progressively stored strain in rocks,

causing movement along a fault. 1) deep-seated tectonic forces act on a mass of rock

over many decades

2) The rock bends but does not break.

3) More and more energy is stored in the rocks as the

bending becomes severe.

4) The energy stored exceeds the the breaking strength

of the rock, and the rock breaks suddenly, producing

an earthquake.

5) The movement may be vertical, horizontal or both

(diagonal or oblique).

Seismic waves Focus or hypocenter – the point

within the earth where seismic

waves originate; the point of initial

movement of a fault.

Epicenter – the point on the

earth’s surface directly above the

hypocenter.


2 types of seismic waves:

1. Body waves – travel trough the earth’s interior, spreading outward from the

hypocenter in all directions (like sound in air).

2. Surface waves – travel on the earth’s surface away from the epicenter (like

ripples on water); slowest wave, can cause more property damage.

2 types of body waves:

P wave – compressional; parallel to

direction of wave propagation

S wave – secondary wave; transverse

to direction of wave propagation.


P wave S wave

Very fast at speeds of 4 to 7 km/sec. Slow, at 2-5 km/sec.

First wave to arrive at a station Arrives at a later time than P wave does

Can pass through solid and fluid (gas or

liquid)

Can pass through solid but not fluid

Recording earthquakes Seismometer – the instrument used to detect seismic waves. A heavy suspended

mass is held as motionless as possible, suspended by springs or hanging it as a

pendulum. When the ground moves, the frame of the instrument moves with it.


Seismograph – a seismometer with a recording device that produces a

permanent record of earth motion, usually in the form of wiggly line drawn on a

moving strip of paper. There are numerous seismograph stations all over the

world.


Seismogram – the paper record of earth vibration. The different waves travel at

different rates, so they arrive at seismograph stations in a definite order: first P

waves, then S waves, and finally, the surface waves. Analysis of seismograms can

reveal the location and strength of the earthquake.


REVIEW LOCATING EARTHQUAKES

Measuring earthquake strength 2 methods of measurement:

1) Intensity – finding out how much damage the quake has caused. Intensities are

expressed in Roman numereals from I to XII on the modified Mercalli scale; higher

numbers indicate greater damage

disadvantage: (i) damage lessens away from the epicenter, so different locations report

different intensities; (ii) damage to buildings and infrastructure depend on geology and

quality of building.

2) Magnitude – the amount of energy released by the quake is calculated and assigned a

number; usually done by measuring the height or amplitude of one of the wiggles in a

seismogram – the larger the quake, the bigger the wiggle. It is now conventional to report

such measurement on a Richter scale


THE EARTH’S INTERIOR

Layers of different compositions If the Earth’s composition was homogenous, the velocity of P and S waves would

increase smoothly at depth.

Distinct boundaries (or discontinuities, as they are called in geology) can be

readily detected by reflection and refraction of seismic waves.


Seismic body

waves as they

are reflected

and refracted in

the Earth’s

interior.

The Crust

In the early 20th century, Mohorovičić discovered that for shallow ( 40 km depth)

focused earthquakes two distinct P and S waves are recorded by seismographs

that lie within a distance of 800 km from the epicenter.

One set of waves have traveled directly from the focus to the seismograph

station, and another set arrived slightly earlier, leading to the conclusion that the

latter was refracted by another layer in which the waves traveled faster (high

velocity zone) before being refracted back to the surface.

Conclusion: there is a distinct compositional boundary that separates an outer

layer (the crust) from a deeper layer (the mantle) of higher density. This boundary

is called the Mohorovičić discontinuity, also known as M-discontinuity or the

Moho.


Seismic wave speeds can be measured for different rock types in the laboratory

and the field. With this data, the composition and thickness of the crust (or the depth

of the Moho) can be measured.

Seismic velocities in the continental crust, in general, are markedly different

(slower) than those in oceanic crust.

P-wave speed in the crust ranges from 6 to 7 km/s (similar to granite, diorite and

gabbro).

The Mantle

We cannot see the mantle but we can infer its composition from seismic studies.

Beneath the Moho, speeds are greater than 8 km/s (similar to rock with denser

minerals called peridotite).

We conclude that the mantle must be made up of peridotite.

The Core

At a depth of about 2900 km, P and S waves are so strongly influenced by another

discontinuity.

P waves are so strongly reflected and refracted that the boundary casts a P-wave

shadow, or an area on the Earth’s surface opposite the epicenter where no P-waves

are detected.


Geoscientists consider that this

discontinuity corresponds to the

boundary of the mantle and the core.

P-wave reflections indicate the

presence of a solid material in the

inner portion of the core. Thus, the

core consists of a liquid outer layer

and a solid inner portion.

The core density is about 10-11

g/cm3, corresponding to iron.

The same boundary casts an even

more pronounced S-wave shadow,

because it cannot traverse this

boundary. It is concluded that

beyond this boundary is liquid

material.


Discontinuities in the mantle

The low velocity zone

From the base of the crust to a depth of about 100 km, P-wave velocity rises to

~8.3 km/s. Then velocity drops slowly to just below 8 km/s down to depths of 350

km.

At about 400 km depth, the seismic waves increase in velocity, but not so

much as to indicate a change in composition.


The 670 km seismic discontinuity represents another increase in seismic

velocity, the cause of which is unknown. Several hypotheses are given, including

polymorphic and compositional transition and the presence of cold subducted slab at

these depths.


Gravity and Isostasy

Gravity

The earth is not a perfect sphere, but an ellipsoid. The radius at the equator is 21

times longer than that at the poles.

The pull of gravity is greater at the poles than at the equator because gravitational

attraction is inversely proportional to the square of the distance between their

centers of mass.

The gravitational pull of the earth can be

measured by an instrument called a

gravimeter.

A simple gravimeter

If the rock masses between the gravimeter and the center of the earth were

everywhere uniform in thickness and composition, there would be no variations in

gravitational pull.

However, large and significant variations do exist, and these are called gravity

anomalies.

The anomalies are due to bodies of rock having differing densities.


How a gravimeter works

Example of a gravity anomaly


Crustal thickness profile

Seismic analysis

Gravity analysis


Isostasy - balance or equilibrium between adjacent blocks of brittle crust

―floating‖ on the upper mantle.

Analogy of the

principle of

isostasy


Isostatic

compensation

due to erosion

Isostatic

rebound due

to glacier

melting


Crustal thickening due to magma underplating


A region in isostatic balance gives a uniform gravity

reading (or measurement)


A region held up out of

isostatic equilibrium gives a

positive gravity anomaly

A region held down out of

isostatic equilibrium gives a

negative gravity anomaly


Crustal blocks float on the mantle They tend to rise or sink gradually until they are

balanced by the weight of displaced mantle material. This concept of vertical

movement to reach equilibrium is called isostatic adjustment.

The rise of the crust after removal of its load is called crustal rebound.

Isostatic compensation or adjustment may occur when the crust thickens by

magmatism, compression or collision.


Magnetism

The Earth is one gigantic magnet. Thus, it

is surrounded by a magnetic field.

The field has north and south magnetic

poles, near the geographic poles.

Magnetic poles are displaced about

11º30’ from the geographic poles.

The strength of the magnetic field is

greatest at the poles – coming out from the

south and entering the north.

Magnetic reversals - The earth’s magnetic field has

periodically reversed its polarity in

the past.

- During a time of normal

polarity, magnetic lines of force

leave the earth near the south

pole and enter near the north pole

(like it is today).

- opposite happens during a time

of reversed polarity.


Magnetic anomalies The instrument used to measure the

strength if the earth;s magnetic field is called

a magnetometer.


The strength of the earth’s magnetic field varies from place to place. The

deviation from the average reading is called a magnetic anomaly. It may

be positive or negative.

The Earth’s Internal Heat Geothermal gradient – the temperature increase with depth into the earth.

The average T increase is 25ºC/km

near the surface.

The T of the asthenosphere is

estimated at 1150ºC/km.

The geothermal gradient must taper

off sharply a short distance into the

earth, to as low as 1ºC/km

The transition at 400-670 km depth

must have a T of around 1500 ºC at the

top and about 2000 ºC at the bottom.

Estimates of T at the outer core-

mantle boundary is around 4500 to

4800 ºC.

The inner core-outer core limit has a T

of 6600 ºC, and the center of the earth

is approximated to be 6900 ºC.


Heat flow – a small but measurable amount of heat from the earth’s interior that is

being lost gradually through the earth’s surface.

What is the origin of the heat?

1. Original heat (or thermoremanent heat) if the earth formed as a hot mass that is

cooling down.

2. Radioactive decay may actually be warming the earth.

Some regions on earth have high heat flow indicating either the presence of hot

material underneath (e.g., magma) or rocks with high content of radioactive

elements (e.g., uranium).

Regional patterns of high and low heat flow on the seafloor may also be

explained by convection of mantle rock.


PLATE TECTONICS AND SEAFLOOR SPREADING

CONTINENTAL DRIFT • Soon after the first reliable world maps were made, scientists noted that the

continents, particularly Africa and South America, would fit together like a jigsaw

puzzle if they could be moved. Antonio Snider-Pelligrini, a Frenchman, showed

in 1858 how the continents looked before they separated and cited fossil evidence

in North America and Europe, but based his reasoning on the catastrophe of

Noah’s flood.

• Alfred Wegener, a German meteorologist, was the first to make an exhaustive

investigation of the idea of continental drift and based his theory not only on the

shapes of the continents, but also on geologic evidence such as similarities in

the fossils found in Brazil and Africa. He drew a series of maps showing three

stages in the drift process, beginning with an original large landmass, which he

called Pangaea (meaning ―all lands‖).

Alexander du Toit (friend of Wegener) divided Pangea into two parts which initially

separated:

Laurasia – North America, Greenland and Eurasia

Gondwanaland – South America, Africa, India, Australia, Antarctica


200 million years ago 180 million years ago 135 million years ago 65 million years ago Present


Wegener believed that the continents, composed of light silicic rock, somehow

plowed through the denser rocks of the ocean floor, driven by forces related to the

rotation of Earth. Most geologists and geophysicists rejected Wegener’s

theory, for the lack of a plausible explanation of how such a process could happen.

The early arguments concerning the breakup of the supercontinent Pangaea and

the theory of continental drift were supported by some important and imposing

evidence, most of which resulted from regional geologic studies.

Paleontological Evidence

• The striking similarity of certain fossils found on the continents on both sides of

the Atlantic is difficult to explain unless the continents were once connected.

• Floating and swimming organisms could migrate in the ocean from the shore of

one continent to another, but the Atlantic Ocean would present an

insurmountable obstacle for the migration of land-dwelling animals, such as

reptiles and insects, and certain land plants.


Examples:

a) Fossils of Glossopteris, a fernlike plant, have been found in rocks of

the same age from South America, South Africa, Australia, and India and

within 480 km of the South Pole, in Antarctica.

b) Lystrosaurus, strictly a land dweller. Its fossils are found in abundance

in South Africa, South America, and Asia, and in 1969 a United States

expedition discovered them in Antarctica.

c) Remains of Mesosaurus, a freshwater reptile, were found in both Brazil

and Africa.

d) Fossils of Cynognathhus, a Triassic land reptile, were also found in

Argentina and southern Africa.


• The continents on both sides of the

Atlantic fit together, not only in

outline, but in rock type and

structure.

• The geologic similarities on

opposite sides of the Atlantic are

found only in rocks older than the

Cretaceous period, which began

about 137 million years ago. The

continents are believed to have

split and begun drifting apart in

Jurassic time, about 200 million

years ago.

Jigsaw-fit of continental margins

and their structure and rock types


• During the latter part of the Paleozoic Era

(about 300 million years ago), glaciers

covered large portions of the continents in

the Southern Hemisphere. The deposits

left by these ancient glaciers can be

readily recognized, and striations and

grooves on the underlying rock show the

direction in which the ice moved.

• Except for Antarctica, all of the continents

in the Southern Hemisphere now lie close

to the equator. In contrast, the continents

in the Northern Hemisphere show no trace

of glaciation during this time. In fact, fossil

plants indicate a tropical climate in that

area.

• If the continents were grouped together as

Wegener proposed, the glaciated areas

would have comprised a neat package

near the South Pole and Paleozoic

glaciation could be explained nicely.

Evidence from Glaciation


Similarity in sedimentary record


• Great coal deposits in

Antarctica show that

abundant plant life

once flourished on that

continent, now mostly

covered with ice.

• On the other

continents, salt

deposits, formations of

windblown sandstone,

and coral reefs provide

additional clues that

permit us to reconstruct

the climatic zones of

the past.

Evidence from Other

Paleoclimatic Records


DEVELOPMENT OF THE THEORY OF PLATE TECTONICS

The plate tectonics theory was developed during the early 1960s, when new

instrumentation permitted scientists to map the topography of the ocean floor and to

study its magnetic and seismic characteristics.

The Geology of the Ocean Floor

• In the 1950s and 1960s, newly developed echo-sounding devices enabled marine

geologists and geophysicists to map the topography of the ocean floor in considerable

detail.

• They revealed that the ocean basins are divided by a great ridge approximately

64,000 km long and about 1500 km wide.

• At the crest of the ridge is a central valley, from 1 to 3 km deep. This feature appears

to be a rift valley, which is splitting apart under tension.

• Other evidence shows that ocean basins are relatively young.

• Seismic studies have established that the oceanic crust (composed largely of basalt)

has a completely different composition from the continental crust and is much thinner.

• The oceanic crust is not deformed into folded mountain structures and apparently is

not subjected to strong compressional forces.

• In 1960, H. H. Hess, a noted geologist from Princeton University, proposed a theory

of sea-floor spreading which suggested a possible mechanism for continental drift.


He postulated that the ocean floors are spreading apart, and are moving

symmetrically away from the oceanic ridge. This continuous spreading produces

fractures in the rift valley, and magma from the mantle is injected into these fractures

to become new oceanic crust. The continents are driven away from the oceanic

ridge. The oceanic crust descends into the mantle and is reabsorbed at deep-sea

trenches. He pointed to mantle convection as the mechanism to propel seafloor

spreading.

EVIDENCES SUPPORTING THE THEORY

Paleomagnetism

• Certain rocks, such as basalt, are fairly rich in iron and become weakly

magnetized by Earth’s magnetic field as they cool.

• The mineral grains in the rock become ―fossil‖ magnets, oriented with respect to

Earth’s magnetic field at the time when the rock was formed, and thus preserve a

record of paleomagnetism.

• Similarly, the iron in grains of red sandstone becomes oriented in Earth’s

magnetic field as the sediment is deposited, so red sandstone also can indicate

the orientation of the paleomagnetic fields.

• These rocks therefore retain an imprint of Earth’s magnetic field at the time of

their formation.


Apparent Polar Wandering. Studies of

paleomagnetism of widely different

ages demonstrate that Earth’s north

magnetic pole apparently has steadily

changed its position with time.

Paleomagnetic studies at different parts

of the globe indicate several apparent

polar wandering curves.


• It is impossible that there

were numerous magnetic

poles migrating

systematically and

eventually merging.

• The most logical

explanation is that there

has always been only

one magnetic pole,

which has remained

fixed while the

continents moved with

respect to it.


Magnetic Reversals on the Sea Floor.

Recent studies of the magnetic properties of numerous samples of basalt from

many parts of the world demonstrate that Earth’s magnetic field has been reversed

many times over.

Epochs of normal polarity, when the magnetic field was oriented as it is today,

alternated with epochs of reverse polarity.

The major intervals of alternating polarity (about 1 million years apart) are termed

polarity epochs, and the intervals of shorter duration are termed polarity events.


Determining magnetic anomalies on the seafloor


From the sequence of magnetic anomalies and their radiometric ages, a reliable

chronology of magnetic reversals has been established.


• If Earth’s magnetic field reversed

intermittently, new basalt forming at the

crest of the oceanic ridge would be

magnetized according to the polarity

at the time when it cooled.

• As the ocean floor spreads, a

symmetrical series of magnetic

stripes, with alternating normal and

reversed polarities, would be

preserved in the crust along either

side of the oceanic ridge. Subsequent

investigations have conclusively proved

this theory proposed by Vine and

Matthews and by Morley.

• In 1963, Fred Vine and D. H. Matthews

saw a way to test the idea of sea-floor

spreading. Sea-floor spreading should be

recorded in the magnetism of the basalts

in the oceanic crust. (The same idea was

developed independently by L. W

Morley.)


The patterns of magnetic reversals away from the rest of the ridge are the same as those

found in a vertical sequence of rocks on the continents, from youngest to oldest.

• An important aspect of these reversal patterns is that

they enable us to determine rates of plate movement.

• From the pattern of magnetic reversals, the rate of

sea-floor spreading appears to range from 1 to 17

cm per year.


• Magnetic surveys have now determined patterns of magnetic reversal for much of the ocean

floor, and from these patterns, the age of various segments of the sea floor has been

established. These studies show that most of the deep-sea floor was formed during

Cenozoic time (during the last 65 million years).

• It now seems probable that very little or none of the present ocean basin was formed before

the Jurassic.


• As is predicted by the plate

tectonics theory, the youngest

sediment resting on the

basalt of the ocean floor is

found near the oceanic ridge,

where new crust is being

created. Away from the ridge,

the sediments that lie directly

above the basalt become

progressively older, with the

oldest sediment nearest the

continental borders.

• Also, the sediments become

thicker away from the MOR.

• The ages of the sediment

matches the age of the ocean

floor that it directly covers.

• Equatorial microorganisms are

found even north of the equator

in the Pacific Ocean.

With no seafloor spreading

With seafloor spreading

Evidence from Sediment on

the Ocean Floor


Evidence in topography

Depth of the ocean

floor increases, and

heat flow decreases

away from the MOR.


Evidence from seamounts

Ages of the volcanic

Hawaiian islands and

the Emperor seamount

chain increase steadily

as they approach the

Aleutian trench.


Each volcano probably formed over a stationary magma source, a hotspot (an

area of volcanic activity produced by a plume of magma rising from the mantle),

then moved away as the seafloor spread.


The major plates of the world

Individual plates are not permanent features. They are in constant motion and continually

change in size and shape. Plates that do not contain continental crust can be completely

consumed in a subduction zone. Plate margins are not fixed. A plate can change its shape by

splitting along new lines, by welding itself to another plate, or by accretion of new oceanic crust

along its passive margin.


PLATE BOUNDARIES

Three kinds of plate boundaries are recognized and define three fundamental kinds of

deformation and geologic activity: (1) divergent plate boundaries—zones of tension, where

plates split and spread apart, (2) convergent plate boundaries (also called subduction

zones) —zones where plates collide and one plate moves down into the mantle, and (3)

transform fault boundaries—zones of shearing where plates slide past each other without

diverging or converging

Processes at Divergent Plate Boundaries

Divergent plate boundaries, or spreading axes, form where a plate splits and is pulled apart.

Where a zone of spreading extends into a continent, rifting occurs, and the continent splits to

form a new and continually enlarging ocean.

Divergent plate boundaries are thus characterized by tensional stresses that produce block

faulting, fractures, and open fissures along the margins of the separating plates.

Basaltic magma derived from the partial melting of the mantle is injected into the fissures or

extruded as fissure eruptions.

The magma then cools and becomes part of the moving plates.

More than half of Earth’s surface has been created by volcanic activity along divergent plate

boundaries during the past 200 million years.


• Examples of continental rifting in various stages

are found in various parts of the world.

• The initial stage is represented by the system of

great rift valleys in East Africa. The long, linear

valleys, partly occupied by lakes, are huge,

downdropped fault blocks, which result from the

initial tensional stress.

• Magma rising from the mantle into the rift zone

produces volcanism, exemplified by the great

volcanoes of Mount Kenya and Mount

Kilimanjaro.

• The Red Sea illustrates a more advanced stage

of rifting. The Arabian Peninsula has been

completely separated from Africa, and a new

linear ocean basin is just beginning to develop.

• The Atlantic Ocean represents a still more

advanced stage of continental drift and sea-floor

spreading.


• If both plates at a convergent boundary

contain oceanic crust, one is thrust under

the margin of the other, in a process called

subduction. The subducting plate descends

into the asthenosphere, where it is heated

and ultimately absorbed in the mantle.

• If one plate contains a continent, the

lighter continental crust always resists

subduction and overrides the oceanic plate.

• If both converging plates contain

continental crust, neither can subside into

the mantle, although one can override the

other for a short distance. Both continental

masses are instead compressed, and the

continents are ultimately ―fused‖ or ―welded‖

together into a single continental block, with a

mountain range marking the line of suture.

Processes at Convergent Plate Boundaries

• Convergent plate boundaries, or

subduction zones, where the plates collide

and one moves down into the mantle, are

areas of complicated geologic processes,

including igneous activity, crustal

deformation, and mountain building.


The zone of convergence between two plates is a zone of deformation, mountain

building, and metamorphism. If the overriding plate contains continental crust,

compression deforms the margins into a folded mountain belt, and the deep roots of

the mountains are metamorphosed.

Metamorphism and

crustal deformation

Metamorphism-

High T, high P zone

Crustal

deformation

Andesitic

volcanism Granitic

intrusions


• As cold, wet oceanic crust is subducted into the hot asthenosphere, water is

driven off by metamorphic dehydration reactions at elevated temperatures.The

water was originally incorporated into the oceanic crust on its path from the ridge

to the subduction zone.

• As the light, water-rich fluids rise into the overlying wedge of mantle, they act

as fluxes and lower the melting temperature of the mantle sufficiently to

produce the distinctive magmas of subduction zones.

• The characteristic magmas of subduction zones are andesites, but more-silicic

magmas are found there as well.

• Some geologists think that the magmas of subduction zones are products of direct

melting of the oceanic crust as it becomes hot in the subduction zone.

• Some of this magma is extruded at the surface as lava and forms an island arc or

a chain of volcanoes in the mountain belt of the overriding plate.

• Usually most of the magma intrudes into the deformed mountain belt to produce

batholiths. Both extrusion and intrusion add new material to the continental plate,

and thus continents grow by accretion.

• This is an important mechanism in the differentiation of Earth, whereby less-dense

material, enriched in elements such as Si, Al, K, and Na, is concentrated in the

upper layers of the planet.


• Back-arc basin is characterized by crustal thinning and block faulting.

• The back-arc region is somewhat similar to a major spreading axis.

• The floors of the basins are young. Sediment is generally thin, and exposed rocks include

fresh basalt.

• Heat flow is high, but there is no well-defined ridge or rift valley, and magnetic anomalies

appear jumbled and unorganized.

• Back-arc spreading (extension and spreading of the sea floor behind the island arc) may

also result at convergent plate margins, presumably as a result of complex convective eddies

in the asthenosphere above the subducting plate or by pulling away of the adjacent plate


• Transform fault boundaries are zones

of shearing where plates slide past each

other without diverging or converging

and without creating or destroying

lithosphere.

• A transform fault, is simply a strike-

slip fault between plates (that is,

movement along it is horizontal and

parallel to the fault). The term transform

is used because the kind of motion

between plates is changed—

transformed—at the ends of the active

part of the fault.

• Transform faults connect convergent

and divergent plate boundaries in

various combinations.

Processes at Transform Fault

Boundaries


• Where segments of the oceanic ridge have been offset, a transform fault connects the two

divergent plate boundaries and creates a major topographic feature called a fracture zone.

• In fracture zones, the relative motion between the plates and seismic activity occur only in the

area between the offset segments of the ridge. This zone is the only place where the fault

forms a boundary between the plates. Beyond this zone, the plates on either side of the

fracture are moving in the same direction and at the same rate and can be considered to be

linked together.

• Note that the oceanic ridge is not being offset by motion along the transform fault. It was

offset previously and may represent an old line of weakness in the rifted continental crust that

preceded the development of oceanic crust.


GEOLOGIC TIME


Catastrophism

• CREATION was thought to have involved forces of tremendous violence,

surpassing anything experienced in nature, in so short a time.

• This was the generally accepted idea of how the Earth was formed

• A noted French naturalist, Georges Cuvier (1769-1832) , an able student of fossils,

concluded that each fossil species was unique to a given sequence of rocks.

• He cited this discovery in support of the theory that each fossil species resulted

from a special creation and was subsequently destroyed by a catastrophic event.

Uniformitarianism

• James Hutton (1762—1797) saw evidence that Earth had evolved by uniform,

gradual processes over an immense span of time.

• He argued that past geologic events can be explained by natural processes we

observe operating today, such as erosion by running water, volcanism, and gradual

uplift of Earth’s crust.

• The concept of Uniformitarianism - the laws of nature do not change with time .

UNCONFORMITIES Geologic time is continuous; it has no gaps. In any sequence of rocks, however there

are many major discontinuities (unconformities) that indicate significant interruptions

in the rock-forming processes.


At least four major events are involved in the development of an angular

unconformity:

(1) an initial period of sedimentation during

which the older strata are deposited in a

near-horizontal position.

(2) a subsequent period of deformation

during which the first sedimentary

sequence is folded

(3) development of an erosional surface on

the folded sequence of rock, and

(4) a period of renewed sedimentation and

the development of a younger sequence

of sedimentary rocks on the old

erosional surface.


The relationship, in which plutonic igneous or metamorphic rocks are overlain by

sedimentary shale, is called a NONCONFORMITY. This implies four major

events:

(1) the formation of an ancient sequence of rocks

(2) intrusion of granite and/or metamorphism

(3) uplift and erosion to remove the cover and expose the granites or metamorphic

rocks at the surface

(4) subsidence and deposition of younger sedimentary rocks on the eroded

surface.

When the rock strata above and below the erosion surface are parallel, a

DISCONFORMITY is formed


RELATIVE DATING

• Relative dating is simply determining the chronologic order of a sequence of

events.

• No quantitative or absolute length of time in days or years is deduced.

• An event can only be inferred to have occurred earlier or later than another.

• To establish the relative ages of these events is to determine their proper

chronologic order.

To apply relative dating, we utilize several principles of remarkable simplicity and

universality:

1) The Principle of Superposition

• It states that in a sequence of undeformed sedimentary rock, the oldest beds are

on the bottom and the higher layers are successively younger. The relative ages

of rocks in a sequence of sedimentary beds can thus be determined from the

order in which they were deposited


2) The Principle of Faunal Succession

• States that groups of fossil animals and plants occur in the geologic record in a

definite and determinable order and that a period of geologic time can be

recognized by its characteristic fossils.

• Fossils provide geologists with a means of establishing relative dates.

3) The Principle of Crosscutting Relations

• States that igneous intrusions and faults are younger than the rocks they cut.

Crosscutting relations can be complex, however, and careful observation may be

required to establish the correct sequence of events.

4) The Principle of Inclusion

• States that a fragment of a rock incorporated or included in another is older than

the host rock.


Succession in Landscape Development

• Many landforms evolve through a definite series of stages.

• The composite diagram in the Figure below shows several kinds of crosscutting

relationships as well as unconformities and superposition of major rock bodies.

• The major rock bodies, faults, and unconformities are labeled by letters arranged

in alphabetical order from oldest (A) to youngest (N). The same events are listed

in sequence from the youngest (N, top) to the oldest (A, bottom) in the

accompanying table.


1) The oldest rocks in the diagram are the metamorphic rocks, A.

2) the granite, B, intrudes these rocks and is younger; but the granite is not in

contact with the tilted strata, U, so their age relationship is not certain.

3) An erosional surface, C, developed on the metamorphic terrain

4) A sequence of sedimentary rocks, D, were deposited.

5) These rocks were then intruded by dikes and sills, E.

6) Faults, F, displaced the sequence D.

7) Widespread erosion then occurred, developing the unconformity, G, which cuts

across all of the units A-F.

8) The sequence of horizontal rocks H, was then deposited.

9) Two igneous intrusions, I and J, occurred. Intrusion I formed a laccolith, whereas

J formed a dike and sill.

10) From crosscutting relationships, intrusionJ is older than the fault, K, and the

volcanic rocks, M.

11) Lava flow, M, is younger than the alluvial fan, L. Both are cut by recurrent

movement on fault K. Note the amount of displacement along the fault of the

sedimentary rocks, H, and the small amount of displacement of the fan, L, and

lava flow, M. judging from the lack of erosion on the volcanoes, it would appear

that the cones are very young features.


THE STANDARD GEOLOGIC COLUMN

• Using the principles of superposition and faunal succession, geologists have

determined the chronological sequence of rocks throughout broad regions of every

continent and have constructed a standard geologic time scale that serves as a

calendar for the history of Earth.

• The original subdivision of the geologic column was based simply on the sequence

of rock formations in their superposed order as they are found in Europe.

• Rocks in other areas of the world that contain the same fossil assemblages as a

given part of the European succession are considered to be of the same age and

commonly are referred to by the same names.


Nomenclature of the Geologic Column

The Precambrian

represented by a group of highly complex metamorphic and igneous rocks, which

form a large volume of the continental crust. To produce these rocks, great

thicknesses of sedimentary and volcanic rocks were intensely folded and faulted

and were intruded with granitic rock. Precambrian rocks contain only a very few

fossils of the more primitive forms of life. Arrangement of individual rock layers in

their proper detailed stratigraphic sequence is therefore difficult if not impossible in

this group of rocks. The structure is too complex.

The Paleozoic Era

Rocks younger than the Precambrian are much less complex and contain great

numbers of fossils, permitting geologists to identify them worldwide.

Paleozoic means ―ancient life;‖ they contain numerous fossils of marine organisms,

primitive fish, and amphibians. The era is subdivided into periods distinguished

largely according to the rock formations of Great Britain.

Cambrian comes from Cambria, the Latin name for Wales, where these rocks were

first studied. In most areas of the world, Cambrian rocks rest on the highly deformed

Precambrian metamorphic complex.


Ordovician is derived from the name of an ancient Welsh tribe, the Ordovices.

Ordovician strata overlie the Cambrian but differ in the types of fossils they contain.

Silurian designates rocks are exposed on the border of Wales, a territory originally

inhabited by a British tribe, the Silures.

Devonian was first used to refer to rocks exposed in Devonshire, England.

Carboniferous is the name of a sequence of coal-bearing formations that lie above

the Devonian rocks. In the United States, Carboniferous rocks are subdivided into

two major units:Pennsylvanian (named after the state of Pennsylvania) and the

Mississippian (named after the upper Mississippi valley).

Permian was introduced to refer to rocks exposed over much of the province of

Perm, Russia, just west of the Ural Mountains.

The Mesozoic Era

Mesozoic means ―middle life.‖ The term is used for this period of geologic time

because the presence of fossil reptiles and a significant number of more modern

fossil invertebrates dominates these rocks. The Mesozoic Era includes three

periods:


Triassic refers not to a geographic location hut to the striking threefold division of the

rocks overlying the Paleozoic in Germany.

Jurassic was first introduced for strata outcropping in the Jura Mountains.

Cretaceous refers to the chalk formations in France and England, The name is

derived from the Latin creta, ―chalk.‖

The Cenozoic Era

Cenozoic means ―recent life.‖ Fossils in these rocks include many types closely

related to modern forms, including mammals, modern plants, and invertebrates. The

Cenozoic Era has two periods:

Tertiary is a term held over from the first attempts to subdivide the geologic record

into three divisions—Primary, Secondary, and Tertiary The companion divisions,

Primary and Secondary, have been replaced by Precarnbrian, Paleozoic, and

Mesozoic.

Quaternary is the name proposed for very recent deposits, which contain fossils of

species with living representatives.

The geologic column by itself indicates only the relative ages of the major periods in

Earth’s history. It tells us nothing about the specific duration of time represented by a

period.


RADIOMETRIC MEASUREMENTS OF ABSOLUTE TIME

Radiometrlc dating provides a method for measuring geologic time directly in terms

of a specific number of years (absolute dating). It has been used extensively during

the last 60 years to provide an absolute time scale for the events in Earth’s history.

Principle

• Radioactive isotopes are unstable: their nuclei spontaneously disintegrate,

transforming them into completely different atoms.

• Each radioactive substance disintegrates at its own rate and that for many

substances the rate is extremely slow.

• The rate of radioactive decay is defined In terms of half-life, the time it takes for

half of the nuclei in the sample to decay. In one half life, half of the original atoms

decay. In a second half life, half of the remainder (or a quarter of the original

atoms) decay. In a third half life, half of the remaining quarter decay and so on.


• The time elapsed since the formation of a crystal containing a radioactive element

can be calculated from the rate at which that particular element decays.

• The amount of the radioactive element remaining in the crystal (parent isotope) is

simply compared with the amount of the disintegration product (daughter

isotope).


• Another important radioactive clock uses the decay of carbon-14 (“C), or

radiocarbon, which has a half life of 5730 years. (REVIEW PRINCIPLE)

THE RADIOMETRIC TIME SCALE

The currently accepted geologic time scale is based on the standard geologic

column, established by faunal succession and superposition, plus the finite

radiometric dates of rocks that can he placed precisely in the column.

Each dating system provides a cross-check on the other because one is based on

relative time and the other on absolute time.


From this radiometric time scale we can make several general conclusions about

the history of Earth and geologic time.

1) Present evidence indicates that the age of Earth is about 4.5 to 4.6 billion

years.

2) The Precambrian constitutes more than 80% of geologic time.

3) Phanerozoic time (the Paleozoic and later) began about 570 million years ago.

Rocks deposited since Precambrian time can be correlated worldwide by

means of fossils, and the dates of many important events during their formation

can be determined from radiometric dating.

4) Some major events in Earth’s history are difficult to place in their relative

positions on the geologic column but can be dated by radiometric methods.


MASS MOVEMENT Mass movement – the movement of surface material caused by gravity.

Factors contributing to mass movement

1) Gravity – provides the energy for the downslope movement of surface debris and

bedrock

2) Water – surface tension of interstitial water gives a certain cohesion to the soil.

When heavy rain forces all the air out of the pore spaces, this surface tension is

completely destroyed and the whole mass becomes susceptible to downslope

movement.

3) Air – air trapped beneath rapidly moving masses of rock debris acts as cushion to

reduce the friction of the debris with the ground, making possible high velocity

movement of rock slides.

4) Angle of repose – the maximum slope at which rock or other loose material can

remain stable. If that angle is exceeded, the material begins to move downslope.

Coarser and more angular materials have higher angles of repose than finer and

rounded materials.



5) Vegetation – tends to stabilize the soil on a slope; its absence promotes

erosion and mass movements.

6) Climate – in higher latitudes, downslope movement may be promoted by

freeze-thaw cycles. Heavy rainfall in tropical climates tend to saturate the ground

to promote mass movements.

7) Rock type – certain rock types, such as shale, may become very slick when

wet so that overlying rock layers may slide along the shale.

8) Structures – bedding planes and alignment of crystals, as well as joints and

faults may present planes along which material may be weaker.

9) Seismicity – provides additional energy, especially where material is disposed

at a critical angle of stability.

i) Slump – also called slope failure; the

downward and outward movement of rock

or unconsolidated material traveling as a

unit or as a series of units along a curved

plane.


Types of mass movement

ii) Rockslide – the most catastrophic

of all mass movements; sudden, rapid

slides of bedrock along planes of

weakness.

iii) Rockfall – free fall of rocks

from steep cliffs.


iv) Debris slides – a small, rapid

movement of largely unconsolidated

material that slides or rolls downward and

produces a surface of low hummocks with

small intervening depressions.

b) Debris flow – consists of

mixtures of rock fragments, mud

and water that flows downslope

as a viscous fluid.


i) Mudflow – debris flow consisting of a large percentage of silt and clay

particles, usually resulting from sudden heavy rain or thaw. Their water

content may be as much as 30%.

ii) Lahar – or volcanic mudflow; consisting of abundant loose pyroclastic

material that has accumulated at the foot of a recently erupted volcano,

and which has been remobilized by heavy rain or thaw.

c) Earthflow – the plastic movement of unconsolidated material lying in solid

bedrock, usually helped along by excessive moisture.

d) Talus – a slope built up by the accumulation of rock fragments at the foot of a

cliff or ridge.


e) Subaqueous mass movement –

water-saturated sediments flowing

or sliding downslope of the seafloor.

f) Subsidence – downward movement of earth material lying at or near the

surface; movement is essentially vertical with little or no horizontal component.

Subsidence may be induced by solution of underlying rocks in limestone areas,

large underground mining (e.g., block-caving), or too much pumping of

groundwater.

2) Slow movements – more difficult

to recognize, they operate over

long periods of time. They are

probably responsible for the

transportation of more material

than rapid and violent

movements of rock and soil.

a) Creep – the slow downward

movement of surface material

that operates even on gentle

slopes with a protective cover of

grass and trees.

b) Solifluction – (from the Latin

solum or soil and fluere, to flow)

refers to the downslope

movement of debris under

saturated conditions; most

pronounced in higher latitudes,

where there is alternating freeze

and thaw of the ground.


GROUNDWATER Groundwater – water contained within the openings (pores, fractures, etc.) of the

rocks beneath the Earth’s surface.

POROSITY AND PERMEABILITY

Porosity – the percentage of the openings within a given volume of rock; it

determines how much water a rock mass can hold.

4 main types of pore spaces:

1) Spaces between mineral grains

In sand and gravel deposits, pores space can constitute from 12 to 45%. The

infilling of pore spaces by smaller grains reduces porosity.

2) Fractures

All rocks are cut by fractures, and in some dense rocks (e.g., granite), fractures

constitute the only significant pore spaces.

3) Solution cavities

some limestones have high porosity because this rock is soluble in water,

forming pits and holes. Movement of water along bedding planes and joints may

result in solution cavities which may become caves.

4) Vesicles

Vesicles are commonly concentrated near the top of a lava flow and form zones

of very high porosity.


Groundwater


Permeability – the capacity of a rock to transmit a fluid; it varies with the fluid’s viscosity,

hydrostatic pressure (the pressure within a given point of a fluid at rest), the size of the

openings, and the degree of interconnection between the openings.

THE WATER TABLE

Zone of aeration – the zone above the water table which is partly filled with air and partly

filled with water; the water forms a thin film, clinging to grains by surface tension.

Zone of saturation – below the zone of aeration, where all the pores are filled with water.


Water table – the upper surface of the zone of saturation.

• In general, the water table tends to mimic the surface topography.

• The water table is at the surface in lakes, swamps and most streams.

• In arid regions, most streams lie above the water table, so they loose much of

their water through seepage into the surface.

Perched water table - is produced when an impermeable layer (e.g., shale)

occurs within the zone of aeration. If a perched water table extends to the side of a

valley, springs and seeps occur.


The movement of groundwater

Hydraulic head – the difference in elevation between parts of the water table.

Base of groundwater – also called lower limit, occurring at considerable depths, all

pore spaces in the rocks are closed by high pressure, and there is no free water.

Natural and artificial discharge

Natural discharge – occurs wherever the water table intersects the surface of the

ground (stream channels, floors and banks of marshes, lakes). It provides the major

link between groundwater reservoirs and other parts of the hydrologic cycle.

Spring – a place where groundwater flows or seeps naturally to the surface.

Well – holes dug into the ground to reach the zone of saturation, in order to access

the fluid.


Cone of depression – the cone-shaped line of the water table as it is depressed due

to pumping of fluid from a well.


Artesian water – confined in an aquifer between impermeable beds. It is under

pressure, like water in a pipe; where a well or fracture intersects it, the aquifer

water rises in the opening, producing a flowing well or an artesian spring. The

occurrence of artesian water requires:

1) The rock sequence must contain interbedded permeable and impermeable

strata (e.g., sandstone and shale). The permeable beds are called aquifers.

2) The rocks must be tilted and exposed in an elevated area where water can

infiltrate into the aquifer.

3) Sufficient precipitation and surface drainage must occur in the outcrop area to

keep the aquifer filled.


Thermal springs and geysers

Groundwater migrating through areas of recent igneous activity or ―hot rocks‖

becomes heated and, when dicharged to the surface, produces thermal springs and

geysers.

Geyser – a thermal spring that intermittently erupts steam and boiling water.

(REVIEW HOW GEYSERS FORM)

Geothermal energy – energy useful to human beings that can be extracted from

steam and hot water found within the Earth’s crust. It can be used directly to heat the

homes in winter countries (e.g., Iceland), or steam can be used to run electric

generators to provide electricity.

Erosion by groundwater

Slow-moving groundwater can dissolve huge quantities of soluble rock (e.g.,

limestone, gypsum) and carry it away in solution. It transports the dissolved mineral

matter and either discharges it into other parts of the hydrologic system or deposits it

within the pore spaces within the rock.

Groundwater erosion starts with water percolating through joints, faults and bedding

planes and dissolving the soluble rock. In time, the fractures enlarge to form a

subterranean network of caves.


Sinkhole – produced when caves frow larger until the roof collpses and a crater-like

depression results.

Caves – a naturally formed subterranean open area, chamber, or series of

chambers, commonly produced in limestone by solution activity.

Karst topography – a distinctive type of terrain resulting largely from erosion by

groundwater, characterized by sinkholes, solution valleys, rounded hills and knobs

and other features produced by groundwater activity.

Tower karst – a particular type of karst topography developed in tropical areas

where dissolution is at a maximum because of the abundance of water from heavy

rainfall. It is characterized by steep, cone-like hills.

Deposition by groundweater

The mineral matter dissolved by groundwater can be deposited in a variety of ways.

The change from solution to precipitation is commonly caused by lowering of the

water table, because the main solution processes occur in the zone of saturation,

and precipitation occurs in the zone of aeration after the pore spaces and caves

have been drained.

Dripstone – collective term for groundwater deposits formed from by precipitation

from slow percolating and dripping groundwater rich in mineral matter.


Evolution of stalactites, stalagmites and columns


Stalactite – an icicle-chaped deposit of dripstone hanging from the roof of a cave.

Stalagmite – a conical deposit of dripstone built up from a cavefloor.

Drip curtain – a thin sheet of dripstone hanging from the ceiling or wall of a cave; it

may follow the trace of a fracture on the ceiling or wall.

Travertine terraces – terraced deposits of calcium carbonate deposited by flowing

pools of water on the cave floor or carbonate-rich water discharged along a slope.

These deposits, however, are trivial compared to the amount of material deposited in

the pore spaces of a rock.

Hot spring deposits – calcium carbonate deposition, usually of travertine, from hot

springs in geothermal areas.

PROBLEMS OF GROUNDWATER SYSTEM

Pollution

Any concentration of chemical or waste creates local pockets that potentially can

contaminate the groundwater reservoir.

Leaching – the process by which groundwater dissolves and transports soluble

components of rock or soil, usually downward or down-gradient.

Leachate – the solution produced by leaching. Material that is leached from waste

disposal sites, including both biological and chemical contaminants, can pollute the

groundwater system.

Saltwater encroachment – too much pumping in places near seawater may cause

incursion of saltwater into the water table

Changes in the position of the water table - Raising of the water table by too

much irrigation can produce springs where there were no springs before, inducing

erosion and mass wasting. Lowering of the water table, in turn could result in

droughts and subsidence.

Subsidence -the sinking or settling of a part of the Earth’s crust with respect to the

surrounding parts.


River Systems

River system – a network of connecting channels through which water, precipitated

on the surface, is collected and funneled back to the ocean.

MAJOR CHARACTERISTICS:

A river system or a drainage basin consists a main channel and all the

tributaries (a stream flowing into or joining a larger stream) that flow into it.

The surface of the ground slopes toward the network of tributaries.

It is bounded by a divide (ridge) beyond which water is drained by another

system.

Within a river system, the surface of the ground slopes toward the network of

tributaries, so the drainage system acts as a funneling mechanism for removing

surface runoff (water that flows over the land surface) and weathered rock

debris.

3 subsystems of a river system:

1) Collecting system – consists of a network of tributaries in the headwater (the

higher portions of the river), which collects and funnels water and sediment to the

main stream.

2) Transporting system – the main trunk stream, which functions as a channelway

through which water and sediment move from the collecting area toward the

ocean. It may also collect additional water and sediment.


Parts of a drainage system


3) Dispersing system - consists of a network of distributaries (stream branches

into which a river divides where it reaches its delta) at the mouth of a river, where

sediment and water are dispersed into an ocean, a lake or a dry basin. The major

process is deposition.

Order in stream systems

Individual streams and their

valleys are joined together into

networks. We can rank the

relationship of theses streams

using a hierarchy. Small

headwater streams can be

ranked as 1st order. Two 1st

order streams would combine

to form a 2nd order stream, and

so on.


Stream pattern

The overall pattern developed by a system of streams and tributaries depends

partly on the nature of the underlying rocks and partly on the history of the streams.

The most common stream patterns are:

1) Dendritic – or treelike, it develops

when the underlying bedrock is

uniform in its resistance to erosion.

2) Radial – streams radiate outward

in all directions from a central high,

likely to develop on the flanks of a

volcano.

3) Rectangular – occurs when the

underlying bedrock is criss-crossed

by fractures, the streams flow at

nearly right angles to each other.

4) Trellis – consisting of parallel

streams that develop on alternating

resistant and non-resistant rocks.

1

2

3

4


DYNAMICS OF STREAM FLOW

The most important variables in stream dynamics:

1) Discharge – the amount of water passing a given point during a specific interval of time,

usually measured in cubic meters per second. Groundwater seepage is important because it

can maintain the flow of water throughout the year. Continual seepage establishes

permanent streams. If the supply of groundwater is depleted seasonally, streams become

intermittent, dry during low rainfall (or dry) season, becoming alive again with increased

rainfall.

2) Velocity

The velocity of water is not uniform throughout the stream channel, and depends on the

shape and roughness of the channel and on the stream pattern. Velocity is usually greatest

at the center of the channel and above the deepest part, away from the frictional drag of

the channel walls and floor. As the channel curves, the zone of maximum velocity shifts to

the outside bend, and a zone of minimum velocity forms on the inside of the curve.


VARIATIONS IN STREAM VELOCITY

STREAM FLOW PATTERN


3) Stream gradient – the slope of the stream channel. The gradient is steepest in the

headwaters and decreases downslope. The longitudinal profile (a cross section of a stream

from its headwaters to its mouth) is a smooth, concave upward curve that becomes very flat

at the lower end of the stream.

4) Sediment load – the amount of suspended and dissolved matter that the river transports.

The capacity of a stream to transport sediment increase to a 3rd or 4th poweer of its velocity,

i.e., doubling the velocity will increase the transporting capacity of the stream by 8 to 16

times. Sediment is transported in 3 ways:

a) Suspended load – generally the largest fraction of material moved by a river, mostly

consisting of silt and clay, or particles that remain in suspension most of the time and move

downward at the velocity of flowing water.

b) Bed load – particles of sediment too large to remain in suspension, such that they

collect on the stream bottom. Thee particles move by sliding, rolling and saltation

transportation of particles by wind or water through a series of bouncing movements). The

bed load moves only when there is sufficient velocity to move the large particles.

c) Dissolved load – matter transported in the form of chemical ions and is essentially

invisible. The most abundant materials in solution are calcium and bicarbonate ions, but

also includes Na, Mg, chloride, ferric and sulfate ions. Organic acids are also present.


Movement of sediment load


Base level – the lowest level to which the stream can erode its channel. It is, in

effect, the elevation of the stream’s mouth where the stream enters the ocean,

or lake, or another stream.

PROCESSES OF STREAM EROSION

River systems erode the landscape by three main processes:

1) Removal of regolith – loose rock debris is washed downslope into the

drainage systemand is transported as sediment load in streams and rivers.

Soluble material is carried in solution.

2) Downcutting of stream channels – this process is accomplished by abrasion

(mechanical wearing away of rock by friction, rubbing, scraping, or grinding). Of

the channel floor by sand and gravel as they are swept downstream by the

flowing water. Sometimes, the rotational movement of sand, gravel and

boulders acts like a drill and cuts deep holes called potholes.

3) Headward erosion – streams have a universal tendency to erode

headward, or upslope, to increase the length of their valleys, until they

reach the divide.


With headward erosion, the following can be accomplished:

a) Stream piracy or capture – occurs when the tributaries of one stream extends

upslope and intersects the middle course of another stream, thus diverting the

headwater of one stream to the other.


b) Superposed stream – a stream with a course originally established on a

cover of rock now removed by erosion, so that the stream or drainange system is

independent of the newly exposed rocks and structures. Consider the following

example:

1) Initially, a dendritic pattern on

horizontal sedimentary rocks covering

older folded strata is established.

2) Regional uplift causes erosionto remove

the horizontal sediments, so that older,

folded rocks are exposed at the surface.

The dendritic drainage pattern is then

superposed upon the folded rocks


3) Streams cut across resistant and non-

resistant rocks alike

4) Rapid headward erosion along

exposurtes of weak rocks results in stream

capture and modification of the original

dendritic pattern to a trellis pattern.


PROCESSES OF STREAM DEPOSITION

1) Floodplain – the flat, occasionally flooded area bordering a stream. It is usually covered with

large quantities of sediments. Certain features are developed:


a) Meanders and pointbars

Meanders are broad, looping bends in a river.

Point bar – a crescent-shaped accumulation of sand and gravel depoisted on the inside of a

meander bend.

Oxbow lake – a lake formed in the channel of an abandoned meander.

b) Natural levees – a broad, low embankment built up along the banks of a river channel

during floods. Flooding significantly reduces stream velocity, causing deposition of some

suspended sediments. The coarsest sediments are deposited close to the channel, creating

natural levees.


c) Backswamp – the marshy area of a floodplain at some distance beyond and lower

than the natural levees that confine the river. It is swampy because it is poorly

drained. Tributary streams in the backswamp are unable to flow upslope the natural

levees, so they are forced to empty into the backswamp or to flow as yazoo streams,

which flows parallel to the main stream for a considerable distance before joining it.


d) Braided stream – a stream with a

complex of diverging and converging

channels separated by bars or islands. They

form where more sediment is available than

can be removed by the discharge of the

stream.


2) Alluvial valleys Streams may fill part of their valleys with sediment, and then cut through the

sediment fill, creating alluvial valleys. This is accompanied by the formation of

stream terraces (a series of level surfaces in a stream valley representing the

dissected remnants of an abandoned floodplain, stream bed of valley floor produced

in a previous stage of erosion or deposition). Stream terraces develop as follows:

i) A stream cuts a valley by normal

downcutting and headward erosion

processes


ii) Changes in climate, base level,

denudation or other factors that reduce flow

energy cause the stream to partially fill its

valley with sediments, forming a broad, flat

floor.

iii) An increase in flow energy causes the

stream to erode through the previously

deposited alluvium.

iv) The stream shifts laterally and forms

lower terraces as subsequent changes cause

it to erode though the older valley fill.


3) Delta – a large, roughly triangular body of sediment deposited at the mouth of a

river. As a river enters a lake or ocean, its velocity suddenly diminished and most of

its sediment load is deposited to from the delta.

2 major processes are fundamental to the formation of a delta:

a) The splitting of a stream into a distributary channel system, which extends into

the open water in a branching pattern

b) The development of local breaks, called crevasses, in natural levees, through

which sediment is diverted and deposited as splays in the area between

distributaries.


Shoreline systems

WAVES

Wave generation

As wind moves over the open ocean, the turbulent air distorts the surface of the

water.

Gusts of wind depress the surface where they move downward, and as they move

upward, they cause a decrease in pressure, elevating the water surface.

These changes in pressure produce irregular, wavy surface in the ocean and transfer

part of the wind’s energy to the water.

Wave motion in water

Wavelength – the horizontal distance between adjacent wave crests (the highest

part of a wave) or adjacent wave troughs (the lowest part of the trough between

successive crests).


Wave height – the vertical distance between wave crest and wave trough.

Wave period – also known as frequency, is the time between the passage of two

successive crests.

Wave motion has a circular orbit. A floating object move forward as the crest of a

wave approaches and then sink back into the following trough.

The morphology of a wave


Beneath the surface this orbital motion dies out rapidly,

becoming negligible at a depth equal to about ½ the

wavelength. This level is known as the wave base.

Fig. 16.3

The energy of a wave depends on its length and height –

the greater the wave height, the greater the size of the orbit,

and the deeper is the wave base.


Breakers

As a wave approaches shallow water:

1) the wavelength decreases because the wave base touches the ocean bottom, and the resulting

friction gradually slows down the wave.

2) The wave height increases as the column of water encounters the sea floor and is pushed up.

3) The wave height continues to increase, while the velocity decreases, and acritical point is

reached at which the wave crest extends beyond the support range of the underlying column

of water, and the wave collapses or breaks

4) At this point, all the water in the column moves forward, releasing its energy as a wall of

moving, turbulent surf called a breaker.

Swash – the rush of water up onto a beach after a wave breaks; it causes the landward movement

of sand and gravel.

Backwash – the return sheet flow down a beach after a wave is spent.; some of the water,

however, seeps into the sand and gravel


WAVE REFRACTION

Waves approaching a shore are bent, or refracted, so that energy is concentrated on

headlands (an extension of land – promontory, cape or peninsula – seaward from

the general trend of the coast) and dispersed in bays (wide, curving recesses or inlet

between two headlands). Refraction occurs because the part of a wave in shallow

water slows down, while segments in deeper water move forward at normal velocity.


LONGSHORE DRIFT

Development of longshore drift

1) As a wave strikes the shore at an angle of <90º, water and sediment are transported

obliquely up the beach in the direction of the wave’s advance.

2) When the energy of the wave is spent, the water and sediment return with the backwash

directly down the beach, perpendicular to the shore. This process is known as beach drift.

3) A similar process that develops in the breaker zone is called a longshore current, which

transports material in suspension and by saltation.

4) The combined action of 2) and 3) is called longshore drift.

If the wave is constant, longshore drift occurs in one direction only. It can be reversed when

there are seasonal changes in the angle of the wave’s approach to the shore.

Longshore currents can pile significant volumes of water on the beach, which return seaward

through the breaker zone as a narrow rip current, which can be dangerous to swimmers.


COASTAL EROSION

Erosion along coasts results from the abrasive action of sand and gravel moved by

waves and currents and, to a lesser extent, from solution and hydraulic action. Coastal

erosion produces certain landforms:

Sea cliff – or wave-cut cliff, produced where steeply sloping land descends beneath

the water, and waves cut a notch into the bedrock at sea level. The cliff ultimately

collapses, and fallen debris is removed by wave action, after which the process is

repeated.


As the sea cliff retreats, a wave-cut platform is produced at its base.

Sediment derived from the erosion of the cliff and transported by longshore drift

may be deposited in deeper water to form wave-built terrace.

Stream valleys that formerly reached the coast at sea level are shortened and left as

hanging valleys when the cliff recedes.

As the platform is enlarged, the waves break progressively farther from the shore,

wave action on the cliff is reduced, and beaches can then develop at the base of the

cliff.


Sea caves, sea arches and sea stacks

Sea cave – cave formed by wave action, usually from the erosion of joint systems and

fault planes in the rock.

Sea arch – an arch formed by the erosion from two opposite sides of a headland.

Sea stack – an isolated pinnacle that remains after a sea stack collapses.


DEPOSITION ALONG COASTS

Sediment transported along the shore is deposited in areas of low energy and

produces a variety of landforms

Beach – a shore built of unconsolidated sediment. Sand is the most common

material, but some beaches are composed of cobbles and boulders, and others of fine

silt and clay. Beaches composed of fine-grained material are generally flatter.

Spit – a sandy bar projecting from the mainland into the open water, formed by

deposition of sediment by longshore drift. It usually forms where a straight shoreline

is indented by bays or estuaries (bays at the mouths of rivers formed by subsidence

of the sand or rise in sea level, and where fresh and seawater mixes.


Tombolo – a beach or bar connecting an island to the mainland, formed by the

island’s effect on wave refraction and longshore drift.


Barrier island – long, low offshore islands of

sediment trending parallel to the shore. They are

typically separated from the mainland by a shallow

lagoon.


Tectonic uplift may elevate sea cliffs and wave-cut platforms, resulting in elevated

marine terraces.


REEFS

A reef is a solid structure built of shells and other secretions of marine organisms,

particularly coral. Most grow and thrive only in the warm, shallow waters of

semitropical and tropical regions.


Reef ecology

The marine life that forms a reef can flourish only under strict conditions of

temperature, salinity and water depth.

1) Most modern coral (a bottom-dwelling marine invertebrate organism of the class

Anthozoa) reefs occur in warm tropical waters between the limits of 30º south and

north latitudes.

2) Colonial reefs cannot live in water deeper than 76 m, because they need sunlight

3) They survive only if the salinity of the water ranges from 27-40 ppt.

Thus, corals are good indicators of past climatic, geographic and tectonic conditions.

Types of reefs

Fringing reefs – generally ranging from 0.5 to 1 km wide, are attached to shores of

volcanic islands. The corals grow seaward toward their food supply. They are usually

absent near deltas and mouths of rivers where the waters are muddy.

Barrier reefs – are separated from the mainland by a lagoon, which can be >20 km

wide.


Atolls – roughly circular reefs that rise from deep water, enclosing a shallow lagoon in

which there is noe exposed landmass. They are the most common type of coral reef.

Over 330 are known. Drilling into the coral reefs confirm Darwin;s theory on the

origin of atolls.

Platform reefs – grow in isolated patches in warm, shallow water on the continental

shelf.


TIDES

On most shorelines, the sea advances and retreats in a regular rhythm twice in

approximately 24 hours. These changes are called tides. The origin of tides was not

known until Newton showed how tides arise from the gravitational attraction of the

Moon and the Earth.


The gravitational force exerted by the moon tends to pull the oceans facing the Moon

into a bulge.another tidal bulge, on the opposite side of the Earth, is caused by

centrifugal force. Earth and Moon lies at the same center of gravity, about 4500 km

from the center of the Earth. The eccentric motion of the Earth as it revolves

around this center of gravity creates a large centrifugal force, which forms the second

tidal bulge. Earth rotates beneath the bulges, so the tides rise and fall twice every 24

hours.

TSUNAMI

Movement of the ocean floor by earthquakes, volcanic eruptions or submarine

landslides frequently produces an unusual wave called a tsunami (from the Japanese

word for “harbor wave”), which has a long wavelength and travels across the ocean at

great speeds. As tsunami approach the shore, wavelengths decrease and their wave

heights increase. Therefore, tsunami can be formidable agents of destruction along

shorelines.


physical geology02

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