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Geology: the study of solid-surface planetary processes

GoalsLearn some of the language of geologists.

Understand three geological processes on Earth that contribute to its being habitable.

Learn the Earth’s history.

The history of big impacts on the Earth.

Climate stability, climate change.

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Three rock types

Igneous rock…Arises from molten material, and so is intimately connected to volcanic activity.

Above-ground, molten rock is lava. Below-ground, it is magma.

Granite BasaltLight colored; Darker;A major component of the Earth’s crust; Found on the Moon & other planets;Quartz, feldspar, mica, hornblende; Often undersea; Often coarse-grained. Fine-grained.

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Three rock types

Sedimentary rock…

Arises from the consolidation of sediments such as silt, mud, sand, gravel, etc. Often fossil-bearing.

Shale—derived from fine-grained material such as mud and clay.

Sandstone—derived from sand.

Limestone—grains of skeletal remains of organisms.

Conglomerate—large rocks, cemented together.

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Three rock typesRock that has been transformed (chemically or structurally) by very high temperatures and pressure, but unlike volcanic rock, it did not become molten.

Quartzite—metamorphosed sandstone. Slate—metamorphosed shale.

Marble—metamorphosed limestone. Gneiss—often banded, many progenitors

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Igneous

Sedimentary Metamorphic

Mel

ting→

←Er

osio

n an

d de

posi

tion

Temperature and pressure→

←Erosion and deposition

Temperature and pressure→

←M

elting

Also: igneous → igneous

sedimentary → sedimentarymetamorphic → metamorphic

The rock cycle

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A digression: seismic waves

Compression (longitudinal) and shear (transverse) wavesVsound = 330m/s in air, 5000m/s in granite;Many shear waves cannot travel through liquids or gas.

P-wavesLongitudinal or compression;Travel at the speed of sound;Low amplitude (not destructive).

S-wavesA shear wave that can only travel in solids;They travel at about 60% the speed of P-waves;Larger in amplitude.

The different waves allow us to probe the Earth’s interior!

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The Earth’s (nongaseous) anatomyCrust

Continental crust is 30-50 km thick, light rock;Oceanic crust is 5-10 km thick, denser rock;Some consider oceanic crust to be mantle;Hotter by 30ºC/km inwards.

Mantle Inner CoreRock denser than the crust; Nickle-iron;3200 km thick; Solid;Solid/plastic; Also around 6100ºC.500-900ºC near the crust;4000ºC near the core.

Outer coreNickle-iron;Molten;4400ºC near the mantle;6100ºC near the inner core.

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Planetary heat budgets

Why is the Earth hot inside?

Terrestrial heat sources – Radioactivity (critically important);– Differentiation (perhaps important long ago);– Impacts (perhaps important long ago);– Solar radiation (negligible).

Terrestrial heat loss is exclusively via radiation.

All else being held equal, smaller planets cool faster!

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Process #1: volcanismNow that we understand the basics, let us look at three geological processes with significant connection to life on Earth.

1. Volcanism;2. Plate tectonics;3. Global magnetic fields.

Vapors emitted by volcanoes...– Water vapor (H2O)—60%– Carbon dioxide (CO2)—10-40% or more– Nitrogen (N2)– Sulfur gases (H2S, SO2)– Hydrogen (H2)

Volcanism’s relationship to life– It is an energy source for developing life.– It releases gases (CO2, N2) essential for forming an

atmosphere.– It releases H2O for the formation of oceans.

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Process #2: plate tectonics

Convection currents result from heat flow– Mantle material moves at only about 10 cm/yr;– Material takes about 2×108 yr for a full cycle;– Comparatively light rock (lithosphere) rides on top.

Subduction zones– Seafloor crust slides under continental crust (subducts); – Oceanic trenches form as oceanic mantle is subducted;– Often indicates the edge of the continental shelf;– Often associated with vulcanism.

Continental plates– As a consequence of these stresses, the surface of the Earth has

been snapped into separate continental masses.

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Process #2: plate tectonics (contd.)The crust is not only broken into continental plates…these plates are slowly dragged this way or that, moving with considerable momentum.

This is called plate tectonics. (Be cool—don’t say “continental drift.”)

Features of plate tectonics– Speeds are 1-9 cm/yr;– Drift speeds are measurable with GPS; – Effects of the motions are significant over 108 yr timescales;– This explains why continents can fit together like puzzles;– Lateral slippage can create earthquakes at fault lines.

Supercontinents (form every ~350-500 million years or so)– Pangaea (300 MYA)—broke into Gondwana and Laurasia;– Pannotia (600 MYA);– Rodinia (1 BYA);– Columbia (1.7 BYA);– Kenorland (2.3 BYA);– Ur & Valbarra supercontinents were even earlier—3 BYA or so.

A tour of the Earth: sea floor spreading and rift valleys

See marine vulcanism

Hawai’i’s hot plume migrates 51 km/106 years (5.1cm/yr);

descend to the Lo’ihi seamount

Starting at Sewell

The Midatlantic;

descend to the Mid-atlantic ridge

Northern Europe;

descend to the Rift valley (Lk. Baikal)

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Process #2: plate tectonics (contd.)

What does plate tectonics do for life on Earth?1. Cycles surface rocks with deeper layers, regulating CO2.

2. Redistributes landmasses, which adjusts climate.

3. Shifting landmasses changes evolutionary processes.

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Only the Earth has tectonics

Mercury & the MoonThese worlds are very small (0.38RE, 0.27RE), so they cooled too

quickly. Geologically, they are mostly or completely “dead.”

MarsMars has enormous volcanoes and fault-like cracks, but lacks continents. It is smaller than the Earth (0.53RE), and so it cooled too fast.

VenusVenus is extremely hot and dry, and we think water has been cooked out of the crust. The hard, dry crust resists tectonics. Even so, primitive continents are present.

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Gravitational and electrical force fields– Gravitational fields are created by matter;– Electrical fields are generated by electric charges;– Lines of force are directly towards (or away) from the sources.

Magnetic fields– Created by moving electrical charges;– Charged particles do not cross the “lines of force”;– The Earth’s magnetic field is very much like a bar magnet’s field!

Process #3: magnetic fields

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The source of the Earth’s magnetic field– Convection currents exist in the Earth’s electrically conducting core;

…and…– The Earth rotates.

As a result, the electrical charges are dragged around….and this generates a magnetic field.

For some reason, the magnetic field reverses itself every few hundred thousand to million years.

We can see a record of this in the iron deposits in the oceanic rock.

We don’t know why the magnetic fields flip.

It has been about a million years since the last flip.

Process #: Force fields (contd.)

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Process #3: …and the atmosphere (contd.)

The Sun produces the solar wind—a constant (but irregular) flow of charged particles.

Over time, this solar wind would blow away our atmosphere, in a mechanism called solar stripping.

Our magnetosphere shields us.

What will happen when our magnetic field flips?

MarsBeing small, it cooled rapidly, lost its magnetic field, and had its atmosphere blown away.

VenusHas suffered some stripping, but its huge atmosphere has not been too damaged.

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Determining timescales and the Earth’s history

Let us understand the tool of radiometric dating

Recall:1. An atom’s nucleus consists of neutrons and protons.

2. Neutrons are slightly more massive than protons.

3. The number of protons in a nucleus determines the element.

4. The numbers of neutrons and number of protons in a nucleus determine its mass.

Nu-cle-us! Nu-cle-ar!

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Radioactivity

Processes1. By emitting an electron, a neutron can transform itself into a proton...

If this happens inside an atom, the transformation of the neutron into a proton means the atom becomes positively charged, without losing much mass. The element’s atomic number increases by 1.

14C → 14N + e-

(6p, 8n) (7p, 7n)

2. By emitting a He nucleus (2p, 2n), a nucleus can lose mass and charge.

238U → 234Th + 4He(92p, 146n) (90p, 144n) (2p, 2n)

n → p+ + e-

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Half-life

The amount of time during which it is 50% likely that a specific radioactive process will occur.

Conversely, in 1 half-life, 50% of the atoms in a radioactive compound will experience the process.

100% → 50% → 25% → 12.5% → 6.25%

87Rubidium → 87Strontium τ = 49.4 BY238Uranium → 206Lead τ = 4.47 BY40Potassium → 40Argon τ = 1.25 BY14Carbon → 14Nitrogen τ = 5730 Y

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Estimating the Earth’s age

ThemeLook for the oldest rock that you can find—the Earth is at least as old as those ancient rocks.

Zircon grain radiometric dating (Uranium, Hafnium)– They are 4.4×109 yrs old.– Since they formed as part of established continents, the crust solidified about

4.5×109 yrs old.

Moon rocks subjected to little activity– They are 4.4x109 yrs old.

Meteoric material represents the solar system’s building blocks– They are 4.57×109 yrs old.– The Earth might have started to agglomerate this long ago.

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Early Earth: bombardment

4.57×109 y - 4.5×109 y = 0.07×109 y = 7x107 y(meteorites) (zircon)

Therefore… The transition from the era of heavy bombardment to a solid Earth was speedy; The Earth might have been habitable within 108 years after the end of the era of

heavy bombardment.

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The late heavy bombardment

– The Moon and the Earth have experienced the same eras of bombardment (a reasonable assumption).

– Scientists studying the ages and distributions of craters on the moon have discovered that the era of bombardment decreased 4.5×109 years ago.

– They also see evidence that it resumed 3.9×109 years ago, for a short time.

– This defines the end of the Hadean eon.

– Why this “late heavy bombardment” at the end of the Hadean eon?

– Possibly “planetary migration?” The solar system is dynamic!

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Bombardment and outgassing

Consider those incoming objects that collided into the Earth to contribute matter.

– They brought gases in addition to the solid matter;– Some of these gas was deposited directly into the atmosphere;– Some of the gas was locked in planetary rock, later to be

released via volcanic eruptions (outgassing).

To be explored in this course: How did the early CO2 atmosphere evolve into our current N2 (78%)and O2 (21%) atmosphere?

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Sterilizing Impacts

A sufficiently large asteroid or comet would be so devastating, it would destroy all life on Earth.

Such events are called sterilizing impacts.

An impacting object 350-400km across would:

– Vaporize the Earth’s oceans;

– Raise the Earth’s surface temperature to 2000ºC (3600ºF);

– This probably happened several to a dozen times in ancient times;

– Did life evolve long, long ago, just to be wiped out?

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How did we get the Moon?

Evidence to explain– The Moon’s composition is basically similar to the Earth– The Moon does not have volatiles– The Moon is lower density than the Earth

Don’t even talk about the capture model!

Giant impact model– There was a major glancing impact between a proto-Earth and

proto-Moon.– This exchanged a great deal of material between the two;– The lunar material was heated, and volatiles were driven off;– Much matter from the Earth’s mantle (but not core) was given to

the Moon.

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The geological time scale

Hadean eon4.6 – 3.8 BYA (17%) [midnight to 4:05a.m.]More or less, pre-life.(A rather poorly defined eon.)

Archean eon3.8 – 2.5 BYA (28%) [- 10:48a.m.]Indications of single-celled life.

Proterozoic eon2.5 BYA – 540 million years ago (43%) [- 9:07p.m.]Oxygen atmosphere develops, multicellular life evolves.

Phanerozoic eon540 million years ago – present (12%) [midnight]Life visible to naked eye.

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Atmospherics: the greenhouse effect

Greenhouse gasesMolecules which are transparent to incoming solar (visible) radiation, but which trap the re-radiated infrared radiation.

The greenhouse effect is a good thing for us!

Natural greenhouse gasesH2O—water vapor (~more than 60% of the effect)

CO2—carbon dioxide (~more than 20% of the effect)

CH4—methane (~5-10% of the effect)

O3—ozone (~5%)

The greenhouse effectFrosts are more common on clear nights than cloudy nights;

The Earth is, on average, about 14ºC (57ºF);

Without the effect, the Earth would be -18ºC (0ºF)!

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Atmospherics: the CO2 cycle

Our climate is intimately related to the amount of CO2 in our atmosphere.

Watch what happens to atmospheric CO2 over time…

1. Atmospheric CO2 is dissolved into rainwater;

2. Acidic CO2 water dissolves rock (such as calcium);

3. The dissolved calcium and CO2 flows to the oceans;

4. The CO2 is deposited as limestone or reefs (CaCO3);5. The limestone is subducted underground by plate tectonics;

6. Ultimately, the CO2 is released by volcanic outgassing;

…and repeat

Plate tectonics helps stabilize our climate over 400,000 year timescales.

The Earth’s CO2

Where is the Earth’s CO2?

For 1 molecule of CO2 in the Earth’s atmosphere…

60 molecules are in the Earth’s oceans.

170,000 molecules are in the Earth’s rocks.

Where is Venus’s CO2?

For 1 molecule of CO2 in the Earth’s atmosphere…

0 molecules are in Venus’s oceans.

0 molecules are in Venus’s rocks.

200,000 molecules are in Venus’s atmosphere!

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The bad news for VenusEven though Venus is almost exactly the same size as the Earth, and even though it is just a little closer to the Sun….

– It has an atmosphere 90× the Earth’s, and it is mostly CO2.– It’s surface temperature is 470ºC (900ºF)!– For comparison, Mercury is only 420ºC, and it receives 300% the radiation!

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The snowball Earth hypothesisGlaciers have reached the equator in the past…

As ice built up on the Earth, → It reflected more sunlight;→ With less heat from the Sun there was more cooling!

Positive feedback→ T=-50ºC (-58ºF);→ Equatorial regions would be

as cold as Antarctica is today;→ The ocean freezes 1 km thick;→ The ocean may have even

frozen completely!

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Breaking out of a snowball Earth phaseWhat happens when the Earth in in a snowball Earth phase?

– There is no rainfall to leach the CO2 out of the atmosphere;– Continued volcanism results in the build up of CO2 levels;– Atmospheric CO2 concentrations rise;– Concentrations become critical in about 10 million years;– The temperature rises dramatically up to about 50ºC (122ºF)!

Models suggest that the time for the temperature change is only a few centuries.

With great changes comes evolutionary opportunity? Some evidence suggests that a snowball Earth phase preceded a time called the Cambrian Explosion (about 540 MYA), when the life on Earth increased enormously in both diversity and complexity.