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Geology: the study of solid-surface planetary processes
Goals
Learn 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
Light colored;
A major component of the Earth’s crust;
Quartz, feldspar, mica, hornblende;
Often coarse-grained.
Basalt
Darker;
Found on the Moon & other planets;
Often undersea;
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 types
Metamorphic rock…
Rock that has been transformed (chemically or structurally) by very
high temperatures and pressured; different from volcanic because it
did not reach a molten state.
Slate—metamorphosed shale.
Quartzite—metamorphosed sandstone.
Marble—metamorphosed from limestone.
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The rock cycle
Igneous
Sedimentary
Temperature and pressure→
←Erosion and deposition
Metamorphic
Also:
igneous → igneous
sedimentary → sedimentary
metamorphic → metamorphic
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A digression: seismic waves
Compression (longitudinal) and shear (transverse) waves
Vsound = 330m/s in air, 5000m/s in granite;
Many shear waves cannot travel through liquids or gas.
P-waves
Longitudinal or compression;
Travel at the speed of sound;
Low amplitude (not destructive).
S-waves
A 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) anatomy
Crust
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
Rock denser than the crust;
3200 km thick;
Solid/plastic;
500-900ºC near the crust;
4000ºC near the core.
Inner Core
Nickle-iron;
Solid;
Also around 6100ºC.
Outer core
Nickle-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: volcanism
Now that we understand the basics, let us look at three geological
processes with significant connection to life on Earth.
1.
2.
3.
Volcanism;
Plate tectonics;
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.
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A tour of the Earth: sea floor spreading and rift valleys
Starting at Sewell
The Midatlantic;
descend to the Midatlantic ridge
See marine vulcanism
Northern Europe;
descend to the Rift
valley (Lk. Baikal)
Hawai’i’s hot
plume migrates 51
km/106 years
(5.1cm/yr);
descend to the
Lo’ihi seamount
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 Moon
These worlds are very small (0.38RE, 0.27RE), so they cooled too
quickly. Geologically, they are mostly or completely “dead.”
Mars
Mars has enormous volcanoes and fault-like cracks, but lacks
continents. It is smaller than the Earth (0.53RE), and so it cooled
too fast.
Venus
Venus 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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Process #3: magnetic fields
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!
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Process #: Force fields (contd.)
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.
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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?
Mars
Being small, it cooled rapidly, lost its magnetic field, and had its
atmosphere blown away.
Venus
Has 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.
2.
3.
4.
An atom’s nucleus consists of neutrons and protons.
Neutrons are slightly more massive than protons.
The number of protons in a nucleus determines the element.
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
Processes
1. By emitting an electron, a neutron can transform itself into a proton...
n → p+ + eIf 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
→
(92p, 146n)
234Th
+
(90p, 144n)
4He
(2p, 2n)
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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
238Uranium
40Potassium
14Carbon
→
→
→
→
87Strontium
206Lead
40Argon
14Nitrogen
τ = 49.4 BY
τ = 4.47 BY
τ = 1.25 BY
τ = 5730 Y
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Estimating the Earth’s age
Theme
Look 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
(meteorites)
-
4.5×109 y =
(zircon)
0.07×109 y
=
7x107 y
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 eon
4.6 – 3.8 BYA (17%) [midnight to 4:05a.m.]
More or less, pre-life.
(A rather poorly defined eon.)
Archean eon
3.8 – 2.5 BYA (28%) [- 10:48a.m.]
Indications of single-celled life.
Proterozoic eon
2.5 BYA – 540 million years ago (43%) [- 9:07p.m.]
Oxygen atmosphere develops, multicellular life evolves.
Phanerozoic eon
540 million years ago – present (12%) [midnight]
Life visible to naked eye.
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Atmospherics: the greenhouse effect
Greenhouse gases
Molecules 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 gases
H2O—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 effect
Frosts 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.
2.
3.
4.
5.
6.
Atmospheric CO2 is dissolved into rainwater;
Acidic CO2 water dissolves rock (such as calcium);
The dissolved calcium and CO2 flows to the oceans;
The CO2 is deposited as limestone or reefs (CaCO3);
The limestone is subducted underground by plate tectonics;
Ultimately, the CO2 is released by volcanic outgassing;
…and repeat
The process takes hundreds of thousands of years to complete.
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The CO2 cycle as a stabilizing feedback loop
Plate tectonics, the greenhouse effect, and radiation from the Sun work together to
stabilize our climate!
Suppose you increased the concentrations of atmospheric CO2 …
 More CO2 means the Earth would become warmer via the greenhouse effect.
 A warmer Earth would have more storms, and more rainfall.
 Increased rainfall means more CO2 is rained out of the atmosphere.
 CO2 is sequestered in rocks at an increased rate.
 Meanwhile, volcanism would not be able to compensate for atmospheric CO2
losses.
  The concentrations of atmospheric CO2 return to normal.
Suppose you decreased the concentrations of atmospheric CO2 …
 Less CO2 means the Earth would become cooler via the greenhouse effect.
 A cooler Earth would have fewer storms, and less rainfall.
 Decreased rainfall means more CO2 is rained out of the atmosphere.
 CO2 is sequestered in rocks at an decreased rate.
 Meanwhile, volcanism would bring the atmospheric CO2 levels to higher values.
  The concentrations of atmospheric CO2 return to normal.
Plate tectonics helps stabilize our climate over 400,000 year timescales!
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The Earth’s CO2
Where is the Earth’s CO2?
Where is Venus’s CO2?
For 1 molecule of CO2 in the Earth’s
atmosphere…
For 1 molecule of CO2 in the
Earth’s atmosphere…
60 molecules are in the Earth’s oceans.
0 molecules are in Venus’s oceans.
170,000 molecules are in the Earth’s
rocks.
0 molecules are in Venus’s rocks.
200,000 molecules are in Venus’s
atmosphere!
The bad news for Venus
Even 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 hypothesis
Glaciers 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 phase
What 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.
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