ees dynamic earth study slides

8/13/2019 EES Dynamic Earth Study Slides

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Introducing Geology and anOverview of Important Concepts

Physical Geology: Earth Revealed 6/e,

Chapter 1


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Geology in Today’s World • Geology - The scientific study of the Earth

– Physical Geology is the study of Earth’s materials,

changes of the surface and interior of the Earth, and the

forces that cause those changes

– The study of Geology includes these subsystems of Earth: – Atmosphere

– Biosphere

– Hydrosphere

– Lithosphere

– Mantle

– Core


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Specialties of Geology related to the subsystems of Earth include:

GeochronologyPaleontologyGeochemistryMineralogy

Planetary GeologyEnvironmental GeologyHydrogeologyGeophysicsPetroleum Geology

Structural GeologyStratigraphySeismologyOceanography


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Practical Aspects of Geology

• Natural Resources

– All manufactured objects

depend on Earth’s resources

– Localized concentrations ofuseful geological resources

are mined or extracted

– If it can’t be grown, it must

be mined

– Most resources are limited in

quantity and non-renewable


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Resource Extraction andEnvironmental Protection

• Coal Mining – Careless mining can release

acids into groundwater

• Petroleum Resources

– Removal, transportation and

waste disposal can damage the

environment

• Dwindling resources canencourage disregard forecological damage caused byextraction activities

Alaska pipeline


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Geologic Hazards • Earthquakes

– Shaking can damage buildings and break

utility lines (electric, gas,

water, sewer)

• Volcanoes – Ash flows and mudflows

can overwhelm populated

areas

• Landslides, floods, andwave erosion


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Physical Geology Concepts

• Earth’s Heat Engines

– External (energy from the Sun)

• Primary driver of atmospheric (weather) and

hydrospheric circulation

• Controls weathering of rocks at Earth’s surface

– Internal (heat moving from hot interior to

cooler exterior)

• Primary driver of most geospheric phenomena

(volcanism, magmatism, tectonism)


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Earth’s Interior

• Compositional Layers

– Crust (~3-70 km thick)

• Very thin outer rocky shell of Earth

– Continental crust - thicker and less dense

– Oceanic crust - thinner and more dense

– Mantle (~2900 km thick)

• Hot solid that flows slowly over time;

Fe-, Mg-, Si-rich minerals

– Core (~3400 km radius)

• Outer core - metallic liquid;mostly iron

• Inner core - metallic solid; mostly iron


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Earth’s Interior

• Mechanical Layers

– Lithosphere (~100 km thick)

• Rigid/brittle outer shell of Earth• Composed of both crust and

uppermost mantle

• Makes up Earth’s tectonic “plates”

– Asthenosphere

• Plastic (capable of flow) zone on

which the lithosphere “floats”


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Theory of Plate Tectonics

• Continental Drift Hypothesis – Originally proposed in early 20th century to

explain the “fit of continents”, common rock

types and fossils across ocean basins, etc.

– Insufficient evidence found for driving

mechanism; hypothesis initially rejected

• Plate Tectonics Theory – Originally proposed in the late 1960s

– Included new understanding of the seafloor

and explanation of driving force – Describes lithosphere as being broken into

plates that are in motion

– Explains origin and locations of such things as

volcanoes, fault zones and mountain belts


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Distribution of Major “Ring

of Fire” Volcanoes


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Tectonic Plate Boundaries

• Divergent boundaries – Plates move apart

– Magma rises, cools and forms new lithosphere

– Typically expressed as mid-oceanic ridges

• Transform boundaries – Plates slide past one another

– Fault zones and earthquakes mark boundary

– San Andreas fault in California

• Convergent boundaries – Plates move toward each other

– Mountain belts and volcanoes common

– Oceanic plates may sink into mantle along a subduction

zone, typically marked by a deep ocean trench


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THE “BIG 3” GROUPS OF ROCKS

IGNEOUS – form from the cooling,crystallization & solidification of magma

METAMORPHIC – form as a result ofincreases in P & T and the interaction of fluidsresulting in mineralogical changes

SEDIMENTARY – form from the breakdown ofpre-existing rocks or as chemical precipitatesfrom a fluid


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ROCK CYCLE - Relates Igneous, Metamorphic, andSedimentary rocks to one another and to the processes

which „recycle‟ earth materials.

Internal processes – magmatism & metamorphism

External processes – weathering, transportation, deposition


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THE ROCK CYCLE

Igneous rocks

basalt, granite

Sedimentary rocks

limestone, conglomerate

Metamorphic rocks

gneiss, quartzite


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PLATE TECTONICS AND THE ROCK CYCLE

How are the RockCycle and PlateTectonics related?

Plate Tectonic

processes result inthe formation ofcertain rock types in particular areas.

These rocks can berecycled in bothcontinental andoceanic areas


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• The perspective of geologic time

requires a shift in our usual way ofthinking.

• “Deep” Time – Most geologic processes occur gradually over millions

of years – Changes typically imperceptible over the span of a

human lifetime

– Current best estimate for age of Earth is ~4.55 billion

years

Geologic Time

The geologic time scale is the result ofthe collaboration of many earth scientistsworking together to construct a chronologyof events on Earth.


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James Hutton’s Principle of Uniformitarianism holds thatpresent day processes have operated throughout Earth’s

history. Therefore, we can better understand past events by

studying modern processes.

"The present is the key to the past"

Uniformitarianism


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Earth’s Interior andGeophysical Properties

Chapter 2

S


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Earth's Internal Structure

Compositional

Differences:

1.Crust – oceanic (basalt) &

continental (~granite)

2. Mantle (peridotite)

3. Core - inner & outer (Fe-Ni)

Differences in physical

properties-behavior:

1. Lithosphere –

crust & uppermantle (brittle)

2. Asthenosphere – upper

mantle (plastic)


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Introduction • Deep parts of Earth must be

studied indirectly

– Direct access is only available to

crustal rocks and small upper

mantle fragments brought up by

volcanic eruptions or slapped

onto continents from subductingoceanic plates

– Deepest hole ever drilled is 12

km deep and did not reach the

mantle• Geophysics is the branch of

geology that studies the interior

of the Earth


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Evidence from Seismic Waves

• Seismic waves or vibrations from a

large earthquake (or nuclear bomb

blast) will pass through the entire

Earth

• Seismic reflection is the return of

some waves to the surface after

bouncing off a rock boundary

– Two materials of different densities

separated by a sharp boundary will lead

to reflection of seismic waves off the boundary

• Seismic refraction is the bending of

seismic waves as they pass from one

material to another with different

seismic wave velocities


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Earth’s Internal Structure

• Seismic waves have been usedto determine the three main

zones within the Earth: the

crust , mantle and core

• The crust is the outer layer of

rock that forms a thin skin on

Earth’s surface

• The mantle is a thick shell of

dense rock that separates the

crust above from the core below

• The core is the metallic central

zone of the Earth


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The Crust • Seismic wave studies have indicated that

the crust is thinner and denser beneath

the oceans than on the continents

• Different seismic wave velocities in

oceanic (7 km/sec) vs. continental (~6

km/sec) crustal rocks are indicative of

different compositions

• Oceanic crust is mafic, composed

primarily of basalt and gabbro

• Continental crust is felsic, with an

average composition similar to granite

Table 2.1


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The Mantle • Seismic wave studies have indicated that

the mantle, like the crust is made of solid

rock with only isolated pockets of magma

• Higher seismic wave velocity (8 km/sec)

of mantle vs. crustal rocks indicative of

denser, ultramafic composition

• Crust and upper mantle together form thelithosphere, the brittle outer shell of the Earth that

makes up the tectonic plates

– Lithosphere averages about 70 km thick

beneath oceans and 125-250 km thick beneath

continents

• Just beneath the lithosphere, seismic wave speeds

abruptly decrease in a plastic low-velocity zone

called the asthenosphere


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The Core • Seismic wave studies have provided primary

evidence for existence and nature of Earth’s core

• Specific areas on the opposite side of the Earthfrom large earthquakes do not receive seismic

waves, resulting in seismic shadow zones

Th C


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The Core • P-wave shadow zone (103°-142° from epicenter)explained by refraction of waves encountering

core-mantle boundary

• S-wave shadow zone (≥103° from epicenter)

suggests outer core is a liquid

• Careful observations of P-wave refraction patterns

indicate inner core is solid


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The Core

• Core composition is inferred from the

calculated density, physical and electro-

magnetic properties, and composition of

meteorites

– Iron metal (liquid in outer core and solid ininner core) best fits observed properties

– Iron is the only metal common in meteorites

• Core-mantle boundary (D” layer) is

marked by great changes in seismic

velocity, density and temperature

– Hot core may melt lowermost mantle or react

chemically to form iron silicates in this seismic

ultralow-velocity zone (ULVZ)


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Gravity Measurements • Gravitational force between two

objects varies with the masses of theobjects and the distance between

them

• Gravity meters are extremely

sensitive instruments that detectchanges in gravity at the Earth’s

surface related to total mass beneath

any given point

– Gravity is slightly higher ( positive

gravity anomaly) over dense materials(metallic ore bodies, mafic rocks) and

slightly lower (negative gravity

anomaly) over less dense materials

(caves, water, magma, sediments, felsic

rocks)

E h’ M i Fi ld


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Earth’s Magnetic Field • A region of magnetic force - a magnetic field -

surrounds Earth – Field has north and south magnetic poles – Earth’s magnetic field is what a compass detects

– Recorded by magnetic minerals (e.g., magnetite) in

igneous rocks as they cool below their Curie point

• Magnetic reversals are times when the polesof Earths magnetic field switch – Switches in the magnetism recorded in magnetic minerals

– Have occurred many times in the past; timing appears

chaotic

– After the next reversal, a compass needle will pointtowards the south magnetic pole

• Paleomagnetism, the study of ancient

magnetic fields in rocks, allows reconstruction

of plate motions over time

Magnetic Anomalies


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Magnetic Anomalies

• Local increases or decreases in the

Earth’s magnetic field strength are

known as magnetic anomalies – Positive and negative magnetic anomalies

represent larger and smaller than average local

magnetic field strengths, respectively

• Magnetometers are used to measure

local magnetic field strength – Used as metal detectors in airports

– Can detect metallic ore deposits, igneous rocks

(positive anomalies), and thick layers of non-

magnetic sediments (negative anomaly) beneathEarth’s surface

H t Withi th E th


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Heat Within the Earth • The temperature increase with depth into the Earth

is called the geothermal gradient

– Tapers off sharply beneath lithosphere – Due to steady pressure increase with depth,

increased temperatures produce little melt

(mostly within Asthenosphere) other than in

the outer core

• Heat flow is the gradual loss of heat throughEarth’s surface

– Heat sources include original heat (from

accretion and compression as Earth formed)

and radioactive decay within the Earth

– Locally higher where magma is near surface – Same magnitude, but with different sources, in

the oceanic (from mantle) and continental crust

(radioactive decay within the crust)


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End of Chapter 2


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The Sea Floor

Physical Geology: Earth Revealed

Chapter 3


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The Water Planet

• Over 70% of the surface of the

Earth is covered by the oceans

• Until the second half of the 20th

century, very little was knownabout the floor of the open ocean

• Oceans originated primarily from

volcanic degassing of water vapor

from the interior of the primordial Earth

– Small additional amount of water may have

come from comets impacting Earth


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Studying the Sea Floor

• Although sea floor rocksare widespread, they are

difficult to study

• Sea floor rocks and

sediments can be sampledusing rock dredges,

seafloor drilling , or

submersibles

• Indirect observations of

the sea floor are also made

with sonar and similar

systems

F t f th S Fl


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Features of the Sea Floor • Passive continental margins have a continental shelf , continental

slope, and continental rise descending to the extremely flat deep

ocean floor of the abyssal plain • Active continental margins, which are associated with numerous

earthquakes and active volcanoes, have continental shelves and

slopes, but the slope extends down into a deep oceanic trench

• A mid-oceanic ridge system encircles the globe, typicallyrunning down the center of an ocean, and numerous conical

seamounts rise from the ocean floor

Continental Shelves and Slopes


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Continental Shelves and Slopes • Continental shelves are gently seaward-

sloping (0.1°) shallow submarine platforms

at the edges of continents – Range in width from a few km to > 500 km

– Typically covered with young sediments

• Continental slopes are relatively steep

slopes (typically 4-5°, but locally may be

much steeper) that extend down from the

edge of the continental shelf to the deep

sea floor

• Submarine canyons are V-shaped valleys

that run across continental shelves and

down continental slopes

– Deliver continental sediments to abyssal fans on

deep sea floor, sometimes by turbidity currents

25x vertical exaggeration

No vertical exaggeration

C ti t l Ri d Ab l Pl i


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Continental Rises and Abyssal Plains

• Continental rises are gently seaward-

sloping (0.5°) wedges of sedimentsextending from the base of the

continental slope to the deep sea floor – Sediment deposited by turbidity and contour currents

– Typically end in abyssal plains at depth of about 5 km

– Lie upon oceanic crust

• Abyssal plains are extremely flat layers

of sediment burying more rugged

oceanic crust – Flattest features on Earth, some with slopes


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Oceanic Trenches • An oceanic trench is a narrow,

deep trough parallel to the edgeof a continent or an island arc – Deepest parts of the oceans

– Benioff zone earthquake foci begin at

trenches and dip landward under continents

or island arcs – Volcanoes found above upper part of Benioff

zone are arranged in long belts parallel to

trenches

– Marked by very low heat flow and large

negative gravity anomalies

Mid O i Rid


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Mid-Oceanic Ridges

• The mid-oceanic ridge is a giant

undersea mountain range thatextends around the world like the

seams on a baseball

– Made mostly of young basalt flows

– More than 80,000 km long, 1,500-2,500km wide, and rises 2-3 km above ocean

floor

– A rift valley, 1-2 km deep, runs down the

crest of the ridge

– Shallow focus earthquakes common – Extremely high heat flow

– Often marked by line of hot springs,

supporting unique biological communities

– Offset along fracture zones


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Seamounts Guyots and Reefs


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Seamounts, Guyots, and Reefs • Conical undersea mountains that

rise ≥1000 m above the seafloor are

called seamounts – Isolated basaltic volcanoes along mid-

oceanic ridges and out in abyssal plains

– Chains of seamounts form aseismic

ridges • Guyots are flat-topped seamounts,

apparently cut by wave action, and

commonly capped with coral reefs

– Reefs are wave-resistant ridges of coral

and other calcareous organisms that may

encircle islands ( fringing reefs), parallel

coastlines (barrier reefs), or rim circular

lagoons (atolls)


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fringing reef barrier reef atoll

Reefs are wave-resistant ridges of coral and other

calcareous organisms that may encircle islands


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C iti f th O C t


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Composition of the Ocean Crust

• Seismic surveys suggest oceanic crust is

~7 km thick and comprised of threelayers – First layer is marine sediment of various composition

and thickness

– Second layer is pillow basalt overlying basaltic dikes

(extensively sampled)

– Third layer is thought to be composed of sill-like

gabbro intrusions (not directly sampled)

• Ophiolites are rock sequences in

mountain chains on land that are thought

to represent slivers of ocean crust anduppermost mantle – Composed of layers 1-3 overlying ultramafic rock

Oceanic crust Ophiolite sequence


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Age of the Sea Floor and the

Theory of Plate Tectonics • All rocks and sediments of the deep sea floor

are less than 200 million years old

– In contrast, continents preserve rocks up to 4

billion years in age

• Explanation of the young age and formation

mechanisms of oceanic crust is a crucial part

of the Theory of Plate Tectonics


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End of Chapter 3


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Plate Tectonics

Physical Geology: Earth Revealed 6/e,

Chapter 4

Plate Tectonics


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Plate Tectonics • Basic idea of plate tectonics theory is that Earth’s

surface is divided into a few large, plates that move

slowly and change in size

• Intense geologic activity is concentrated at plate

boundaries, where plates move away, toward or past

each other• Theory born in late 1960s by combining hypotheses of

continental drift and seafloor spreading

Early Case for Continental Drift


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Early Case for Continental Drift • Puzzle-piece fit of coastlines

(Africa and South America) has

long been noticed • In the early 1900s, Alfred Wegener

noted that South America, Africa,

India, Antarctica, and Australia

(Gondwanaland) have almostidentical late Paleozoic rocks and

fossils

– Glossopteris (plant), Lystrosaurus

and Cynognathus (animals) fossils

found on all five continents

– Mesosaurus (reptile) fossils found in

Brazil and South Africa only

E l C f C ti t l D ift


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Early Case for Continental Drift • Wegener reassembled continents into

the supercontinent Pangea • Pangea initially separated into

Laurasia and Gondwanaland

– Laurasia - northern supercontinent

containing North America and Asia,

minus India

– Gondwanaland - southern supercontinent

containing South America, Africa, India,

Antarctica, and Australia

• Late Paleozoic glaciation patterns onsouthern continents best explained by

their reconstruction into

Gondwanaland



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• Coal beds of North America and

Europe support reconstruction intoLaurasia

• Reconstructed paleoclimate belts

indicated polar wandering, potential

evidence for continental drift over

time

• Continental drift hypothesis

initially rejected because Wegener

could not come up with a viable

driving force or mechanism for drift

– Could not plow continentsthrough the sea floor rocks, as

he proposed

Paleomagnetism and


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Paleomagnetism andContinental Drift Revived

• Studies of rock magnetism allowed

determination of magnetic pole

locations (close to geographic poles)

through time

• Paleomagnetism uses mineralmagnetic alignment direction and

dip angle to determine the direction

and distance to the magnetic pole

– Steeper dip angles (inclination) indicaterocks formed closer to the north

magnetic pole (90° at poles, 0° at

equator) - indicates latitude

Paleomagnetism and Continental Drift


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gRevived

• Apparent polar wandering curves for

different continents suggest different

north pole positions for the same

time relative to one another

• Reconstruction of super- continentsusing paleomagnetic information fits

Africa and South America like puzzle

pieces

– Improved fit results in rock units (and

glacial ice flow directions) precisely

matching up across continent margins

Seafloor Spreading


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Seafloor Spreading • In 1962, Harry Hess

proposed seafloorspreading – Seafloor moves away from the

mid-oceanic ridge (MOR) due to

mantle convection

– Convection is circulation driven byrising hot material and/or sinking

cooler material

– Evidence: thickness of sediments,

seamounts, guyots, trenches

• Hot mantle rock rises under

MOR – Ridge elevation, high heat flow,

and abundant basaltic volcanism

are evidence for this

Seafloor Spreading


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p g• Seafloor rocks, and mantle rocks beneath them (lithosphere), cool and become

more dense with distance from MOR

• When sufficiently cool and dense, these rocks sink back into the mantle at

subduction zones

– Junction between subducting cold oceanic rocks and overlying less dense

plate forms oceanic trenches (very low heat flow), earthquakes (Benioff

zones) and generation of magmas associated with subduction zones

• Explains overall young age of sea floor (


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Plates and Plate Motion

• Tectonic plates are composed of

the relatively rigid lithosphere

– Lithospheric thickness and age of

seafloor increase with distance

from mid-oceanic ridge

• Plates “float” upon ductile or plastic

asthenosphere

• Plates interact at their boundaries

• Classified by relative plate motion

– Plates move apart at divergent boundaries,

together at convergent boundaries, and slide

past one another at transform boundaries

Evidence of Plate Motion• Marine magnetic anomalies -


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Evidence of Plate Motion Marine magnetic anomalies -bands of stronger and weaker

than average magnetic field

strength – Parallel MOR

– Field strength related to basalts

magnetized with same and

opposite polarities of current

(normal polarity) global magneticfield

– Symmetric “bar-code” anomaly

pattern reflects plate motion away

from ridge coupled with magnetic

field reversals• Seafloor age increases with distance

from MOR

– Rate of plate motion - distance

from ridge divided by age

Evidence of Plate Motion


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Evidence of Plate Motion • Mid-oceanic ridges are offset

along fracture zones

– The segment of the fracture zonebetween the offset ridge crests is a

seismically active transform fault

– Relative motion along fault is result

of seafloor spreading from adjacent

ridges• Plate motion can be directly

measured using satellites, radar,

lasers and global positioning

systems – Measurements accurate to within 1cm

– Motion rates closely match those

predicted using seafloor magnetic

anomalies


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Divergent Plate Boundaries


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g• At divergent plate boundaries,

plates move away from each

other – Can occur in the middle of the ocean

or within a continent

– Divergent motion eventually creates a

new ocean basin

• Marked by rifting , basaltic

volcanism, and uplift – During rifting, crust is stretched and

thinned

– Graben valleys mark rift zones

– Volcanism common as magma rises

through thinner crust along normal

faults

– Uplift is due to thermal expansion of

hot rock

Transform Plate Boundaries


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Transform Plate Boundaries • At transform plate boundaries,

plates slide horizontally past one

another

– Marked by transform faults

– Transform offsets of the MOR

allow a series of straight-line

segments to approximate the

curved boundaries required by

a spheroidal Earth

Convergent PlateB d i


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gBoundaries

• At convergent plate boundaries, plates move

toward one another

• Nature of boundary depends on plates

involved (oceanic vs. continental)

– Ocean-ocean plate convergence

• Marked by ocean trench, Benioff

zone, and volcanic island arc – Ocean-continent plate convergence

• Marked by ocean trench, Benioff

zone, volcanic arc, and mountain belt

– Continent-Continent plate convergence

• Marked by mountain belts and thrust

faults

Movement of Plate


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Boundaries • Plate boundaries move over time

– MOR crests can migrate toward or awayfrom subduction zones or abruptly jump

to new positions

– Convergent boundaries can migrate if

subduction angle steepens or overlying

plate has a trenchward motion

• Back-arc spreading may occur, but is

poorly understood

– Transform boundaries can shift as slivers

of plate shear off

• San Andreas fault shifted eastward

about 5 m.y. ago and may do so again

What Causes Plate Motions?


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What Causes Plate Motions?

• Causes of plate motion not yet fully

understood, but any proposed

mechanism must explain why:

– MOR are hot and elevated, while

trenches are cold and deep

– Ridge crests have tensional cracks

– The leading edges of some platesare subducting sea floor, while

others are continents (which

cannot subduct)

• Mantle convection may be the cause

or an effect of circulation set up by

ridge-push and/or slab-pull


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Mantle Plumes and Hot Spots


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p – Large mantle plumes may dome

up and break or rift apart the

overlying plate• 3 rifts form at 120° angles, 2

continue to rift and 1 fails (the

failed rift arm is called an

aulacogen; ex. East African Rift

Zone)• Characterized by flood basalt

eruptions

• If rifting continues, seas may form

(ex: Red Sea and Gulf of Aden)

• Rifting apart of continental landmasses can occur

– New divergent boundaries may form if

several hot spots link up and break

apart continents


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Plate Tectonics and Ore Deposits


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p• Metallic ore deposits are often located near

plate boundaries

– Commonly associated with igneous activity

• Divergent plate boundaries often marked by

lines of hot springs on sea floor

– Mineral-rich hot springs (black smokers)

deposit metal ores on sea floor after hittingcold water

• Subducting plates at convergent boundaries

may produce metal-rich magmatic fluids

– Different metallic ores originate atdifferent depths along the subducting plate


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End of Chapter 4


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Mountain Belts and theContinental Crust

Chapter 5

Mountain Belts and Earth’s Subsystems


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• Mountain belts - chains of mountain

ranges - 1000s of km long

– Commonly located at or near theedges of continental landmasses

– Composed of multiple mountain

ranges

• Mountain belts are part of the geosphere

– Form and grow, by tectonic and

volcanic processes, over tens of

millions of years

• As mountains grow higher, erosion by

running water and ice (hydrosphere)occur at higher rates

• Air (atmosphere) rising over mountain

ranges directly results in precipitation

and erosion


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Rock Patterns in Mountain Belts


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• Mountain belts typically contain thick

sequences of folded and faulted sedimentary

rocks, often of marine origin. May also

contain great thicknesses of volcanic rock

• Fold and thrust belts are common, indicating

large amounts of crustal shortening and

thickening has taken place under

compressional forces

– Mountain belts are common at

convergent boundaries

– May contain large amounts of

metamorphic rock

• Erosion-resistant batholiths may be leftbehind as mountain ranges after long

periods of erosion

• Intensity of deformation, folding, faulting,

metamorphism & plutonism increases as

move across the mt belt to the craton



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• Erosion-resistant batholiths may be left behind as mountain ranges

after long periods of erosion

• Localized tension in uplifting mountain belts can result in normal

faulting

– Horsts and grabens can produce mountains and valleys,

respectively

• Earthquakes common along faults in mountain ranges

Evolution of Mountain Belts


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• Rocks (sedimentary and volcanic) that

will later be uplifted into mountains are

deposited during accumulation stage

– Typically occurs in marine

environment along continental

margins, and convergent plate

boundaries

• Mountains are uplifted at convergent

boundaries during the orogenic stage

– May be the result of ocean-continent ,

arc-continent , or continent-continent convergence



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• After convergence stops, a long period

of erosion, uplift and block-faulting

occurs

– As erosion removes overlying rock,

the crustal root of a mountain

range rises by isostatic adjustment

– Tension in uplifting and spreadingcrust results in normal faulting

and production of fault-block

mountain ranges (Teton Mts, WY)

Growth of Continents


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• Continents grow larger as mountain belts

evolve along their margins

– Accumulation and igneous activity(volcanic arcs added to continents

during convergence) add new

continental crust beyond old

coastlines – New accreted terranes can be added

with each episode of convergence

• Western North America (especially

Alaska) comprised of many terranes

– Numerous terranes, of gradually

decreasing age, surround older

cratons that form the cores of the

continents



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• Basin-and-Range province of western

North America may be the result of

delamination

– Overthickened mantle lithosphere

beneath old orogenic mountain belt

may break off and sink ( founder ) into

asthenosphere

– Resulting inflow of hot asthenosphere

can stretch and thin overlying crust,

producing normal faults under tension


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End of Chapter 5


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Geologic StructuresPhysical Geology: Earth Revealed

Chapter 6

Geologic Structures


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Geologic Structures • Geologic structures are dynamically-produced patterns or arrangements of rock

or sediment that result from forces within the Earth

– Rocks change shape and orientation (strain) in response to applied stress

– Structural geology is the study of the shapes, arrangement, and

interrelationships of bedrock units and the forces that cause them

St d St i


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Stress and Strain

• Stress is a force per unit area – The three basic types of stress are

compressive, tensional and shear

• Strain is a change in size or

shape in response to stress – Structures produced are examples

of strain that are indicative of the

type of stress and its rate of

application, as well the physical properties of the rock or sediment

being stressed

Rock Responses to Stress and Strain


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Rock Responses to Stress and Strain

• Rocks behave as elastic, ductile or brittlematerials depending on:

– amount and rate of stress application

– type of rock

– temperature and pressure

• If deformed materials return to original shapeafter stress removal, they are behaving

elastically

• However, once the stress exceeds the elastic

limit of a rock, it deforms permanently

– ductile deformation involves bending

plastically

– brittle deformation involves fracturing

Structures and Geologic Maps


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• Rock structures are determinedon the ground by geologists

observing rock outcrops

– Outcrops are places where bedrock

is exposed at the surface

• Geologic maps use standardized

symbols and patterns to represent

rock types and geologic structures,

such as tilted beds, joints, faults

and folds

Orientation of Geologic Structures


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• Geologic structures are most

obvious in sedimentary rocks when

stresses have altered their

originally horizontal orientation

• Tilted beds, joints, and faults are

planar features whose orientation is

described by their strike and dip – Strike is the compass direction of a

line formed by the intersection of an

inclined plane with a horizontal plane

– Dip is the direction and angle

downward from a horizontal plane to

an inclined plane

Types of Geologic Structures F ld b d i l d k


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• Folds are bends in layered rock

– Represent rock strained in a ductile

manner, under compression stress

• The axial plane divides a fold into its two limbs

– The surface trace of an axial plane is

called the hinge line (or axis) of the

fold

• Anticlines are upward-arching folds – oldest

rocks in the center of fold

• Synclines are downward-arching folds –

youngest rocks in the center of fold

Types of Folds


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yp

• Plunging folds are folds in which the hinge line is not horizontal

– Where surfaces have been leveled by erosion, plunging folds form V- or

horseshoe-shaped patterns of exposed rock layers (beds)

• Open folds have limbs that dip gently, whereas isoclinal folds have parallel

limbs • Overturned folds have limbs that dip in the same directions, and recumbent

folds are overturned to the point of being horizontal

Structural Domes and Basins


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• Domes are structures in which the

beds dip away from a central point.

Oldest beds are in the center of the

dome.

– Sometimes called doubly

plunging anticlines

• Basins are structures in which the

beds dip toward a central point.

Youngest beds are in the center of a

basin.

– Sometimes called doubly

plunging synclines

Structural Dome

Structural Basin

Domenear

Casper,

WY

Fractures in Rock


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• Joints are fractures or cracks in

bedrock along which essentiallyno movement has occurred – Multiple parallel joints are called joint

sets

• Faults are fractures in bedrockalong which movement has

occurred

– Considered “active” if movement has occurredalong them within the last 11,000 years (since

the last ice age)

– Categorized by type of movement as dip-slip,

strike-slip, or oblique-slip

Types of Faults Dip slip faults have movement


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Dip-slip faults have movement

parallel to the dip of the fault plane

– Most common types are normal andreverse

• In normal faults, the hanging-wall block has

moved down relative to the footwall block

• In reverse faults, the hanging-wall block has

moved up relative to the footwall block

– Fault blocks, bounded by normal faults,

that drop down or are uplifted are

known as grabens and horsts,

respectively• Grabens associated with divergent plate

boundaries are called rifts

Types of Faults


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Types of Faults – Thrust faults are reverse faults with dip

angles less than 30° from horizontal


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End of Chapter 6


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EarthquakesPhysical Geology: Earth Revealed, 6/e

Chapter 7

Earthquakes • An earthquake is a trembling or shaking of the


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• An earthquake is a trembling or shaking of the

ground caused by the sudden release of elastic

energy stored in the rocks beneath Earth’s surface

– Tectonic forces within the Earth produce

stresses on rocks that eventually exceed their

elastic limits, resulting in brittle failure

• Energy is released during earthquakes in the form

of seismic waves – Released from a position along a break

between two rock masses (fault)

• Elastic rebound theory explains the occurrence of

earthquakes as a sudden release of strain

progressively stored in rocks that bend until they

finally break and move along a fault releasing the

stored elastic energy

Seismic Waves Th i t ithi th E th h i i


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• The point within the Earth where seismic

waves originate is called the focus (or

hypocenter ) of the earthquake, and is the point of initial breakage and movement

along a fault

• The point on the Earth’s surface directly

above the focus is known as the epicenter

• Two types of waves are produced during

earthquakes: body waves and surface

waves

– Body waves are seismic waves that travel

outward from the focus in all directionsthrough Earth’s interior

– Surface waves are seismic waves that travel

along Earth’s surface away from the epicenter

Body Waves


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• P waves are compressional (or longitudinal)

body waves in which rock vibrates back and

forth parallel to the direction of wave propagation

– Fast (4 to 7 kilometers per second) wave that is the

first or primary wave to arrive at a recording station

following an earthquake

– Can pass through solids and fluids (liquids or gases)

• S waves are shearing (or transverse) body

waves in which rock vibrates back and forth

perpendicular to the direction of wave

propagation – Slower (2 to 5 kilometers per second) wave that is

the secondary wave to arrive at a recording station

following an earthquake

– Can pass only through solids

Surface Waves


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• Slowest type of seismic waves

set off by earthquakes • Love waves involve only side-to-

side motion of the ground surface

– Can’t travel through fluids

• Rayleigh waves behave like

ocean waves, and cause the

ground to move in an elliptical

path opposite the direction ofwave motion

– Extremely destructive to buildings


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Locating Earthquakes


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• P- and S-waves start out from the

focus of an earthquake at sametime but separate because they

move with different velocities

• Faster P-waves get farther ahead

of slower S-waves with distance

and time from the earthquake

• Travel-time curves can be used to

determine the distance to thefocus based on the time gap

between first P- and S-wave

arrivals

Locating Earthquakes


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• Travel-time curve can be used

to determine the distance to

the focus based on the timegap between first P- and S-

wave arrivals

• Plotting distances from 3stations on a map, as circles

with radii equaling the

distance from the quake, will

show the location of theepicenter

Measuring the “Size” ofE th k


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Earthquakes • Size of earthquakes is measured in two

ways, intensity and magnitude• Intensity is a measure of the damage that

an earthquake causes to people and

buildings

– Modified Mercalli scale

• Magnitude is a measure of the amount ofenergy released by an earthquake

– Richter scale (log scale)

• Moment magnitude is a more objective

way of measuring energy released by a

major earthquake

– Uses rock strength, surface area of

fault rupture, and amount of

movement

– Smaller quakes more common than

larger ones


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Effects of Earthquakes


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• Earthquakes produce several types of

effects, all of which can cause loss of property and human life

– Ground motion is the familiar trembling and

shaking of the land during an earthquake

• Can topple buildings and bridges

– Fire is a problem just after earthquakes because of broken gas and water mains and

fallen electrical wires

– Landslides can be triggered by ground shaking,

particularly in larger quakes

– Liquefaction occurs when water-saturated soilor sediment sloshes like a liquid during a quake

– Permanent displacement of the land surface

can also occur, leaving fractures and scarps

Tsunami


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• Very large sea waves, caused by

sudden upward or downwardmovement of the sea floor during

submarine earthquakes, are known

as Tsunami (seismic sea waves)

– Tsunami are generally produced by

magnitude 8+ earthquakes (“great”earthquakes)

– May also be generated by large

undersea landslides or volcanic

explosions

– Travel across open ocean at speeds of

>700 km/hr

– Reach great heights in coastal areas

with gently sloping seafloor and

funnel-shaped bays

World Earthquake Distribution


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• Most earthquakes are concentrated in

narrow geographic belts which mark

the tectonic plate boundaries

• Most important concentrations are in

the circum-Pacific and Mediterranean-

Himalayan belts

• Shallow-focus earthquakes are also

common along the crests of mid-

oceanic ridges

• Nearly all intermediate- and deep-focus

earthquakes occur in Benioff zones (zones of inclined seismic activity

marking location of descending oceanic

plate at subduction zones)

Earthquakes and Plate Tectonics


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• Earthquakes are caused by plate inter-actions

along tectonic plate boundaries

• Plate boundaries are identified and defined

by earthquakes

• Earthquakes occur at each of the three types

of plate boundaries: divergent , transform, and

convergent – At divergent boundaries, tensional forces

produce shallow-focus quakes on normal

faults

– At transform boundaries, shear forces

produce shallow-focus quakes along strike-slip faults

– At convergent boundaries, compressional

forces produce shallow- to deep-focus

quakes along reverse and thrust faults

Earthquake Prediction and SeismicRisk


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Risk • Accurate and consistent short-term

earthquake prediction is not yet possible,three methods assist in determining the

probability that an earthquake will occur:

– Measurement of changes in rock properties,

such as magnetism, electrical resistivity, seismic

velocity, and porosity, which may serve as precursors to earthquakes

– Studies of the slip rate along fault zones

– Paleoseismology studies that determine where

and when earthquakes have occurred and their

size• Average intervals between large earthquakes and

the time since the last one occurred can also be

used to assess the risk (over a given period of

time) that a large quake will occur


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End of Chapter 7

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