where did we come from and how did we get here? · pdf filewhere did we come from and how did...

Where did we come from and how did we get here?

The Universe formed about 14 billion years ago

The Solar System and Earth formed about 4.6 billion years ago

We are a planet, revolving around a star that is one of about four hundred billion stars in a galaxy (The Milky Way), that is one of more than 80 billion galaxies in the observable Universe.

Our Star, the Sun, like most stars is composed of mostly Hydrogen, with some Helium

The Earth is one of four ‘terrestrial’ planets (Mercury, Venus, Earth and Mars) in the inner Solar System, and like those planets, is composed largely of a silicate (SiO2) and Iron (Fe)

The outer planets (Jupiter, Saturn, Neptune and Uranus) are gas giants (H and He)

Unlike most of the other ‘terrestrial’ planets, the Earth is a dynamic planet, with a constantly changing lithosphere (rocks), atmosphere (wind), hydrosphere (water), and biosphere (life).

That is why the surface of the Earth is largely free of meteorite impact craters.

Much of this dynamism is due to heat-driven convection

~14 GA (GigaAnnum, i.e,

Billion Years)

today

But first things first

Typical spiral galaxy. Similar to ‘our’ Milk Way Galaxy’

We are not alone.

About 80 billion galaxies in the observable universe.

About 400 billion stars in the Milky Way galaxy (but that may be a bit larger than average)

Many (most?) of those probably have planets.

How many of those planets are terrestrial (Earth-like?)

How many have life?

The Crab Nebula in Lyra

Remnants of a supernova

The Surface of our Sun ( a very close star)

SUN Rocky inner planets

The giant Gas planets of the outer solar system

Hydrogen, Helium, methane, water, ammonia

Silicates with Iron/Nickel cores

Hyd

rog

en (7

4%

), so

me h

elium

(24

%) plus small icy planets like Titan

Gaseous Outer Giant

Mars

‘terrestrial planets’

The surface of Mars – close up

‘terrestrial planets’

The surface of Idaho – close up

‘terrestrial planets’

Earth

Our moon: Luna

Close up of Tycho

Earth’s Outermost Layers

• The most dynamic portion of the Earth– Atmosphere

• Thin gaseous envelope surrounding Earth

– Hydrosphere

• Water layer dominated by the oceans

– Biosphere

• All living things on the planet

– Lithosphere

• Rocky outer shell

Heat driven convection

1. Bottom water is warmed

2. It expands an is therefore less dense

3. It rises to the surface and then spreads out

4. Cooler water at the sides descends to fill the void

A convective thunderstorm

Atoms and Minerals

What are we (the Earth) made of?

All matter is composed of atoms, which consist of a nucleus with protons and neutrons, and electrons which ‘orbit’ the nucleus

Bonds are formed between the valence electrons of atoms to form molecules

Minerals are ‘naturally occurring inorganic solid that has an exact (or clearly defined range) chemical composition with an orderly internal arrangement of atoms generally formed by inorganic processes’.

The nature of the bonds results in the physical properties of minerals, including crystal form, cleavage, fracture, hardness, density, color, luster, streak, etc.

Rocks are formed of minerals

The rock-forming minerals include silicates, carbonates, evaporites and secondary minerals such as clays

Rocks are formed of minerals

Most rocks are silicates and are composed of cations linked by silicate tetrahedra, chains, sheets and solids

Matter

• Atoms– The smallest unit of an element that

retain its properties• Molecules - a small orderly group of atoms

that possess specific properties - H2O

– Small nucleus surrounded by a cloud of electrons

– The nucleus contains protons and neutrons

Bonding

• Atoms are stable when their outmost electron shell is filled

–Atoms lose, gain or share electrons to achieve a noble gas structure

• Types or bonds

– Ionic Covalent Metallic

The Nature of Minerals

• Mineral

–A naturally occurring inorganic solid that has an exact (or clearly defined range) chemical composition with an orderly internal arrangement of atoms generallyformed by inorganic processes.

Physical Properties

Crystal Form

Cleavage and Fracture

Hardness

Density

Color

Luster: Metallic vs Non-metallic

Streak

Taste, magnetism, etc.

Rock-Forming Minerals

• About 20 common minerals make up most rocks

– Silicates dominate– Quartz, Feldspars, Mica, Amphiboles, Pyroxenes

–Carbonates are common

– Evaporite minerals

– Secondary minerals formed during weathering

Silicate Minerals

• Silica tetrahedron may polymerize to form a variety of geometric structures, alone or in combination with other cations

• Isolated tetrahedron

• Single chains

• Double chains

• 2-D sheet

• 3-D frameworks

Silica Tetrahedron

Isolated

Silicate Structures

Single chain Double chain

Solid

Sheet

Nonsilicate Minerals

– Carbonates (biologic)

• Calcite - Ca CO3

• Dolomite - CaMg(CO3)2

– Evaporite Minerals (seawater evaporation)

• Gypsum - CaSO4-2H2O

• Halite – NaCl

– Clays and Oxides (rust and weathering)

• Hematite

• Bentonite, Kaolinite

Rocks

Imagine the first rock and the cycles that it has been through.

Igneous Rocks

Igneous Rocks

• Form from Magma (hot, liquid rock)

• Cool and solidify underground (plutonic) or as lavas above ground (volcanic)

• Most properties are controlled by silica (SiO2) content: classification, melting point, minerals, appearance, etc.

• Viscosity of magma is controlled by temperature, silica content, and to a lesser extent, water.

• Silica-rich magmas are more likely to erupt explosively than are mafic magmas, which are runny

• Texture (size and shape of xtals) is controlled by the rate cooling history of the rock.

Igneous Rocks (cont)

• Faster cooling results in finer-grained crystals

• Common textures include aphanitic (fine-grained), phaneritic(coarse-grained), porphyritic (big xtals in a fine-grained matrix), pyroclastic (explosive) and glassy

• The kind of volcanism depends upon the viscosity of magma

• Plutonic bodies include plutons, batholiths, sills, dikes etc.

• Magmas originate in the upper Mantle

• Magmas differentiate (change composition) through mixing, melting of country rock, and partial melting

• The Bowen’s Reaction series describes the order in which silicate minerals solidify in a magma

Mafic (Fe,Mg –rich) Magmas

• Silica content of ~ 50%

• High concentrations of Fe, Mg and Ca

• High temperature of molten magma

–1000o to 1200oC

• Major minerals

–Olivine - Ca-rich Plagioclase

–Pyroxene

Felsic (Si,Al-rich) Magma

• Silica content of 65-77%

• High concentrations of Al, Na and K

• Lower temperature magmas

– Less than 850oC

• Major minerals

– Feldspars - Micas

–Quartz

Magma Viscosity

• Controlled by silica temperature

• As magma cools, silica tetrahedron form links– Similar to polymers - e.g., nylon

• Increasing linkages– Higher silica & lower temp

• Linkages increase viscosity

Note: this is just like oils, fats and other organic compounds used in the household

Igneous Textures

• Texture - the size, shape and relationship of mineral crystals in the rock

• Reflects cooling history of the magma or lava

• Slow cooling rate >> Big crystals• Fast cooling rate >> Small crystals

• Very fast cooling rate >> glass

Glassy texture in obsidian

Aphanitic Texture

• Fine grained texture

• Few crystals visible in hand specimen

• Relatively rapid rate of cooling

Aphanitic texture in rhyolite

Phaneritic Texture

• Coarse grained texture

• Relatively slow rate of cooling

• Equigranular, interlocking crystals

• Slow cooling = crystallization at depth

• Pegmatites - very coarse grained texture

Phaneritic texture in granite

Porphyritic Texture

• Well formed crystals (phenocrysts)

• Fine grained matrix (groundmass)

• Complex cooling history

– Initial stage of slow cooling

• Large, well formed crystals form

– Later stage of rapid cooling

• Remaining magma crystallizes more rapidly

Porphyritic igneous rock:

Big xtals in a fine grain matrix

Pyroclastic Texture

• Produced by explosive volcanic eruptions

• May appear porphyritic with visible crystals

– Crystals show breakage or distortion

• Matrix may be dominated by glassy fragments

– Fragments also show distortion

– Hot fragments may “weld” together

Concept Art, p. 105

Fine grained

Coarse grained

Classification of common igneous rocks

Volcanic Eruptions

• Basaltic eruptions are runny

• Low Silica + High T = Low Viscosity

• Produce

– Lava Flows - Pahoehoe or Aa

– Flood basalts

– Shield Volcanoes

– Pillow lavas

Fig. 4-1, p. 102

Flood basalts with several thick and thin layers. Each layer represents a separate eruption.

Fig. 5-12d, p. 145

Intermediate & Silicic Eruptions

• Higher Silica + Lower T = Higher Viscosity

–Composite or Stratovolcanos

– Lava Domes

–Ash Flow Calderas

Concept Art, p. 155

Mt Fuji: Stratovolcano

Caldera Explosions: Super volcanoes

Fig. 5-9b, p. 142

Fig. 5-9c, p. 142

Fig. 5-9d, p. 142

Fig. 5-9e, p. 142

Basalt

River Gravels

Rhyolite

Basalt

Fig. 5-21c, p. 157

Concept Art, p. 104

Plutonic Rocks

• Less dense magmas rise through the crust

• Intrusions form as magma solidifies beneath the surface

Figure 4.18. Types of magmatic intrusions

Half Dome; part of the Sierra Nevada batholith

Sill; parallels layers in the country rock

Dike; cuts across layers in the country rock

Origin of Magmas

• Solid rock is at equilibrium with its surrounding

• Changes in the surroundings may cause solid rock magma

–Raising T

– Lowering P

–Changing composition

Magma Differentiation

• Magmas, and the resulting igneous rocks, show a wide range of compositions

• Source Rock

– variations cause major and minor variations in the magma

• Magma Mixing

• Assimilation

Bowen’s Reaction Series

Metamorphic Rocks• Rocks can be metamorphosed (changed) into other rocks when subjected to

high temperatures and pressures.

• The presence of fluids increases the rate of metamorphism

• Metamorphic changes occur in the solid state

• The three kinds of metamorphism are Regional, Contact and Hydrothermal

• Regional metamorphism involves large scale pressures and temperatures

caused by collision of plates in subduction zones or continental collisions

• Contact metamorphism involves baking of adjacent rocks by hot magma

intrusions

• Hydrothermal alteration involves alteration of minerals through percolation of

hot, mineral-rich fluids through the rock

• The ‘Parent’ rock is an important control on the type of metamorphic rock

formed

• Index minerals form at specific temperatures and pressures and thus record the

T and P ‘experienced’ by the rock

• Metamorphic rock textures are either foliated (layered due to directional

pressure) or non-foliated

Metamorphic Rocks

• The transformation of rock by

temperature and pressure

• Alters igneous, sedimentary and even

other metamorphic rocks

What causes metamorphism?

• Heat• Most important agent

• Heat drives recrystallization - creates new, stable minerals

• Pressure (stress)• Increases with depth

• Pressure can be applied equally in all directions or differentially,

i.e. directed

• Fluids• The flow of hot mineral-rich water through the rock can have a

big impact on metamorphism

• Referred to as hydrothermal alteration and creates specific easily

identified minerals

Main factor affecting metamorphism

• Parent rock• Metamorphic rocks typically have the same

chemical composition as the parent rock.

• They contain different minerals, but the same chemicals; just rearranged.

• Exception: at sometimes gases like carbon dioxide (CO2) and water (H2O) are released

• Examples: – Quartz SandstoneQuartzite

– ShaleSlate Schist Gneiss

– GraniteGranite, though minerals might align

Source of Heat

Source of Fluids

Ocean-Continent convergence

Regional Metamorphism:Subduction zones …..

High PLow T

High TLow P

High THigh P

Fig. 6.15.RegionalMetamorphicGradients

Why it is called regional

Colors represent different levels of Temperature and

Pressure as recorded in the minerals.

This regional pattern was caused by the

collision of two continents

Metamorphic Index Minerals

Other minerals behave similarly

Index Minerals in metamorphic rocks

Each of these minerals is an index of T and P

Metamorphic textures

• Foliation

• Foliation can form in various ways:

– Rotation of platy or elongated minerals

– Recrystallization of minerals in a preferred

orientation

– Changing the shape of equidimensional

grains into elongated and aligned shapes

Development of foliation due to directed pressure

Change in metamorphic grade with depth

Shale

Progressive metamorphism of a shale

Schist

Progressive metamorphism of a shale

Progressive metamorphism of a shale

Gneiss

Common metamorphic rocks

• Nonfoliated rocks

• Quartzite

– Formed from a parent rock of quartz-rich

sandstone

– Quartz grains are fused together

– Forms in intermediate T, P conditions

Sample of

quartzite

Thin section

of quartzite

Marble (Random fabric = annealing; nonfoliated)