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CHAPTER 7
INTERIOR MODIFICATIONS
This chapter seeks to explain what happened after the segregation of planets from the
solar nebula. It is a given that planets are not homogenous mixtures of matter; they have
structure and are differentiated into layers with distinct compositions. In this section we
find out how we know and how it happened. Ultimately it all comes down to melting.
Earth Structure
Existence of Iron Core:
-In Chapter 4 we learned that the surface composition is not representative of the earth as
a whole given Lord Cavendish’s famous experiment calculating the density of the earth
close to the actual 5.25 gm/cc.
-Also, we know that the dense material is concentrated towards the center given that the
“moment of inertia” for our planet is only 20% less than that of a perfect sphere (if
weight were evenly distributed, our spinning planet would become more elliptical).
-Earth must have a core and density must be around 11 gm/cconly precious metals are
that heavy! Many scientists didn’t take into account the fact that solids are compressible
-Furthermore, meteorites are often homogenous having necessarily gone through a
process of differentiation.
Seismic Evidence:
-Waves produced by earthquakes travel throughout the globe and can be measured at any
location. The timing of the arrival gives us information about the velocity of the waves
which in turn reveals information about the density of the medium.
a. Three types of waves: compression, shear and surface. Surface wave are
irrelevant,
and shear waves (side to side) can not travel through liquid, not enough
elasticity, and the
fact that certain areas consistently received only compression
waves after earthquakes
indicated that part of the center was liquid. (so-called
“shadow zones” led to our knowledge of the liquid outer-core.)
-Conclusion, 4 layers. crust (35km / 6 below ocean), mantle, (35 – 2900 km), outer core
(2900 -5000 km), and finally the solid, inner core.
Chemical Composition of layers:
-Earth’s core made primarily of Fe and Ni with a small % of some lighter elements
-Crust composed primarily of minerals, namely quartz and feldspar, with small amounts
of pyroxenes and amphiboles. Oceanic crust has no quartz and is ~1/2 feldspar (thus it’s
more dense)
-Mantle composed of peridotite rocks made almost exclusively from the mafic minerals
olivine and pyroxenes. Hardest to determine and outlines some evidence, mostly analysis
of magma @ oceanic ridges
*note, it is possible for a liquid layer to exist between two solid layers if the temperature
increases at the boundary or if metallic iron melts at a lower temperature than peridotite
under extreme pressure!
*note, it is possible to determine the mass of each layer and thus their compositions. For
example, the earth is 30% iron! [see chapter for more explanation, seems unimportant]
Chemical Affinities of the Elements
-Earth separated into four fundamental layers: core, mantle, crust and the outer-envelope
of the oceans and atmosphere. As expressed earlier, elements are not distributed evenly
among the layers, but can be divided into four groups because of their affinity for certain
minerals and states of matter
a. lithophile elements are those that prefer to be in silicate rocks, exist in the
mantle and
crust (ex. Si, Mg, O, Ca, Al…etc.)
*note, subset of large lithophiles (Rb, Cs, U, Th…etc.) that need special attention.
These elements are so large that they do not fit readily into silicate minerals and tend to
concentrate into the liquid phase when a rock melts, thus they are concentrated in
the
crust having risen with magmacan be called magmaphile!
b. siderophile elements are those that prefer the metallic state, exist in the core.
(ex. Ag,
Au, Fe, Ni, Pt…etc)
c. chalcophile elements are sulfur-loving and occur in sulfur bearing minerals (ex.
Pb, Zn,
many metals…etc.). much overlap between chalcophile and siderophile.
d. atmophile elements are those that are very volatile and tend to occur as a gas or
liquid molecule, exist in atmosphere and ocean (ex. noble gases, H2O, CO2…etc.).
The combination of moment of inertia, seismology, geological observations,
experimentation, geochemistry and cosmochemistry thus provides substantial knowledge
of the composition of earth’s layers. Multi-disciplinary synthesis is characteristic of
modern earth science.
Origin of the Earth’s Layers
Separation of Core from Mantle
-Critical question is whether the layers were created as the planet formed (heterogeneous
accretion) or whether metal and silicate were all mixed together initially and later
segregated into layers (homogeneous accretion)
-High density of metal relative to silicate means the metal would sink through the silicate
under the force of gravity. This is possible because metal and silicate liquids are
immiscible (do not mix).
-Heterogeneous accretion requires that metal be the first material to precipitate from the
nebular gas, followed at a lower temperature by silicate, however experiments and
theoretical calculations with regards to carbonaceous chondrites imply that silicates
formed first, and furthermore, astronomers studying interstellar clouds have found
evidence for the simultaneous existence of both metals and silicates. Thus, homogeneous
accretion seems more likely.
*note, later volatiles may have been added heterogeneously in separate impact events.
Timing of Core Formation
-Consideration of heat sources in the early earth make it clear that substantial melting
must have occurred during formation (sources include impacts and radio-nuclides like
26Al).
a. Good evidence also exists for lunar melting, and if the moon melted it is
probable that earth melted as well (the earth retains heat much more efficiently)
-Melting of substantial portions of earth in the first few tens of millions of years would
have led to very rapid and efficient segregation of core from mantle.
a. Hafnium-tungsten system is perfect given the different affinities of the parent /
daugher
elements and an appropriate half-life. Since most of the W has been
demonstrated to
have merged with the core, it is likely that the differentiation
occurred within the first 30 million years!
b. This agrees with evidence that demonstrates smaller planetary objects, such as
asteroids, were able to form and differentiate into layers in as little as five million
years.
Origin of the Atmosphere and Ocean
-Homogeneous and heterogeneous accretion debate exists here as well:
a. Most volatile elements can reside in the solid state in the minerals that make up
rocks, and could accreted originally and formed the atmosphere during a degassing
associated
with the initial melting.
b. However, some believe that accretion may have consisted of only volatiledepleted
materials, and the atmophilic elements arrived afterwards as a result of
impacts of volatile
rich comets.
-Similar experimentation with radioactive series involving different affinities, namely I /
Xe, also suggests a period of about 30 million years. Therefore it appears that formation
of all the major layers happened as one large planetary differentiation.
Origin of the Crust
-Crustal rocks have been extensively reprocessed and no longer contain a direct record of
the primary differentiation of the early earth. (oceanic crust is especially young with
rocks typically aging no more than 150 years).
-Additions to the earth’s crust occur through igneous activity that transfers materials form
the mantle to the crust.
a. This takes the form of volcanoes or plutons. Plutons are large crystals that
result from
the slow cooling of magma at depth (over ½ of all crust is accreted from
below in this way!)
Partial Melting and Crust Formation
-Geochemists have come to understand the essential aspects of melting that leads to
formation and eruption of magmas which can be quite complex.
a. For mixtures of minerals, melting occurs over a range of temperatures,
beginning at the
solidus and ending at the liquidus allowing for a partial melt that
separates liquid from solid and has a different composition form the bulk composition.
b. Higher pressures cause the denser solid phases to become stabilized relative to
the
lighter liquids, and therefore it takes higher temperatures to cause melting to
occur.
-The mantle layer of the earth is almost entirely solid, but it is a solid due to the very high
temperatures and pressures of the mantle, however, it is still able to slowly flow.
Ex. of silly putty which breaks if hit suddenly, but flows. Similar to lead and
copper…
-As a given parcel of mantle flow upwards, there is increasingly less weight of rock
above it, and the pressure decreases ultimately causing the mantle to cross the
temperature at which melting begins, which only makes the sample lighter still
continuing the process.
*note, elements that are larger or of opposite charge than elements within other molecules
tend to avoid substitution in this process and are thus more likely to be in the liquid state,
so even a small degree of melting can cause the almost complete extraction of elements
with large ionsmany of these “large-ion lithophiles” reside in the crust.
-These principles apply well to the formation of the oceanic crust today given our
understanding of spreading ridges at which up-wells of magma form new crust.
Continental Crust
-We can understand the content, but we do not understand the processes by which
continental crust was formed.
a. Perhaps processes have changed through earth history, and a hotter mantle
melted recycled basalt materials at depth to give rise to the continents
b. Perhaps a basaltic layer was formed, and then upon re-heating from below,
partially
melted again forming granite.
c. Perhaps weathering is as important a process as igneous activity in controlling
composition.
-Oldest rocks do not cover the first 600 million years of earth history, but indicate that
crustal processes were active in the earliest earth. We can only infer from the moon that
there was massive volcanism.
Summary
Hence we see that a family of different processes has led to the gradual differentiation of
the earth into the layers whose extent and composition can be very well determined
today, sorted by density. The innermost layer is a solid core of iron, nickel and a small
proportion of lighter elements. The liquid outer core is also metallic. The mantle is
dominantly solid solutions of ferro-magnesian silicates. Melting of the mantle leads to
out-gassing to create ocean and atmosphere, and eruption of silicate magmas to form the
crust. Additional volatiles might have been contributed to the earth y the impact of
comets. The continental crust is the least dense and is the final product of multiple stages
of melting and melt extraction. Core and atmosphere must have segregated very early in
earth history and remain separated today. Ocean crust that we see today is a very recent
phenomenon, but we believe that similar rocks have been created constantly since the
beginning of earth history. Continental crust has a vast range of ages. Even the oldest
continental rocks show evidence of a long previous history, and hence the earliest crustal
differentiation events are hidden from view. The atmophile elements are mostly in the
atmosphere, the siderophile elements in the core, and the large ion lithopile elements
have been concentrated into the continents.
CHAPTER 9
ESTABLISHING THE CIRCULATION
Quick Summary:
Alfred Wegener proposes continental drift theory in the early 20th century (quack). After
WWII, explorations of the oceans began a series of observations that proved continental
drift and formulated the theory of plate tectonics:
a) symmetry to the Atlantic ocean (depth of ocean and thickness of oceanic crust
increased regularly from ridge to continents),
b) pattern of symmetrical magnetic anomalies (caused by periodic reversals in Earth’s
magnetic field),
c) sea floor spreading (new ocean crust at ridges),
d) global seismology proves ocean crust recycling down into mantle at trenches (can be
mapped by earthquake distribution),
e) GPS confirm speeds of spreading (confirmation of magnetic anomalies).
Plates consist of brittle lithosphere floating on top of mobile asthenosphere.
Continents float on top of plates and are too light to be recycled.
Mountain belts occur when plates collide, earthquakes and volcanoes occur at spreading
centers, subduction zones or fractional zones.
Introduction
- 4.5 billion years ago, Earth stratified into layers (because of differential volatility and
density) as olivine rich silicates separate from metals, volatiles degassed (creating
atmospheric envelope).
- this happened in all terrestrial planets (Venus, Mars) but the Earth’s gravitational
attraction and total mass meant we retained atmosphere (so did Venus).
- Earth’s ocean and atmosphere are dynamic (moving vigorously), and Earth’s
surface and interior are too movements permit circulation, recycling and exchange
between the layers and is critical for habitability.
- to understand Earth’s past, we must now turn to current processes.
The Static Earth Viewpoint:
- uniformitarianism (guiding principle of geology since beginning): processes that we
can observe today, acting over vast reaches of time, can create all the features of Earth
that we now observe.
- but did not answer these questions (based on observations):
a) Why are there mountain belts, and why located only on some continental
margins, and sometimes in continental interiors?
b) Why is E’s surface divided into continents and oceans with extreme differences in
elevation?
c) Why do earthquakes/volcanoes occur where they do?
d) Why do Africa and S. America fit together but N. and S. America don’t?
e) Why are the sediments in the oceans so thin? (If they are fixed, accumulation
rates over billions of years would make them thicker)
f) Why do oceans deepen away from ridges?
g) Why are some animals similar to others on other continents while other
continents like Australia have very unique species?
Continental Drift Theory:
- Early 20th C: Alfred Wegener (meteorologist) collected observations, announced
“continental drift theory” in 1912:
a) submerged continental margins fit better than coastlines
b) fossil populations of Africa and S. America similar (fossil/rock formations, and
fossils typical of tropical areas like ferns are far north of tropics today)
c) glacial deposits in Africa and S. America match (single continental glacier)
- fundamental flaw with theory:
- lack of mechanism by which continents move: Wegener said plowed
through ocean but there was no sign of movement, no forces to cause
continents to move.
New Data from the Ocean Floor:
- post-WWII, the Lamont group (Columbia Univ.) used the ship Vema to collect ocean
floor info:
a) sound waves determined depth
b) a magenetomer measured magnetic field
c) explosives to look at reflection of seismic waves
d) recovered a sample every 18 hours.
- determines:
a) existence of mid-Atlantic ridge rift valley
b) depths increase progressively from ridge to continental margins (and rate of
increase largest near the ridge)
c) ridge extended around the globe (baseball idea)
d) fresh volcanic rocks gathered in rift valley (basalts) vs. older sediment further
away (also, ocean drilling program determined age of sediment from fossils in the
basaltic basin underneath). Clearly new crust is being formed at ridges by
magmatism.
e) Thickening of sediment/ocean crust further away from ridges.
f) Bands of higher/lower intensity magnetism symmetrically aligned to ridge axes.
g) symmetry to the topography or bathymetry of the ocean floor
h) ocean trenches surround Pacific Ocean, associated with young volcanoes on land
(Mt. Fuji, Mt. St. Helens, Mt. Ranier) while young volcanism at ocean ridges.
- smoking gun = magnetic reversals:
- Earth undergoes magnetic reversals around every 1 million years (switches polarity
because of changes in the flow of Earth’s liquid outer core-- iron). Rocks contain
magnetite (a magnetic mineral) that acts as small compasses. Measuring sequences of
sedimentary/volcanic rocks showed “bar code” of differing intervals for “normal” or
“reversed” magnetism. The changing magnetic intensity of ocean floor sediment matched
(higher intensity corresponds to when minerals pointing in same direction as current, and
less when opposite).
- changes in magnetic intensity provide time scale, and could calculate the “spreading
rate” of ridges (1 to 20 cm/year).
Global Distribution of Seismicity:
- Earthquakes occur along faults: 2 large portions of E’s crust move in
different directions.
- Movement creates a wave and size of wave depends on size of crustal
section and how far it is displaced.
- Waves propagate within the Earth and along surface from place of origin
at speeds of 6 km/second (so earthquake locations can be determined from
timing of arrival of waves).
- Sense of motion along fault can be figured out from whether the wave is
an up or down motion when arrives.
- Locations of earthquakes show where tectonic plates on Earth are
interacting.
- small, shallow earthquakes occur along ridge seam of earth caused by active volcanism
and tectonism at spreading ridges. (shallowest large features of ocean basin)
- much larger earthquakes at deep ocean trenches, b/c of inclined plane extending from
trench into mantle (called Wadati-Benioff zones) during subduction (at convergent
margins or subduction zones). Very regular volcanoes in these areas 110 km above the
Benioff zone, occurring in the overriding plate. (at deepest regions).
- some earthquakes (medium sized) at transform faults in large fracture zones midocean plate (like San Andreas Fault in CA). These faults can connect two offset ridges or
a ridge and a subduction zone. Earthquakes occur because plates slide by each other.
The Theory of Plate Tectonics
- Earth’s crust is rigid, can crack, and be subject to earthquakes.
-
-
-
-
-
-
Interior of Earth is at high temperatures and pressures and so solid mantle
flows like a viscous fluid. The mantle moves in slow but steady
convection.
The outer rigid zone of mantle is the lithosphere which is divided into
plates (includes the outer rigid mantle and the oceanic and continental
crust on top of it), interior zone of mantle underneath is the
asthenosphere.
Entire surface of E is divided into different plates, the relative motions of
these plates conserve the total surface area.
Oceans: At the spreading center, the plate is formed by creation of new
crust (thickness approx. 6 km), so the very hot asthenosphere (1300C) is
very close to the surface a very steep thermal gradient.
The cool blanket of seawater cools the plate as it ages (and because the
thickness of the cold layer increases as the square root of age), the
density increases, and therefore b/c of isostasy, the plate subsides as it
cools and thickens.
Continents have low density and float on top of the mantle lithosphere
and cannot be subducted. When plate movement brings 2 continents
together  massive collision, creation of mountain ranges (Himalayas).
Plates tectonics drive continental drift (the force that Wegener lacked).
Fundamental Controls on Earth’s Topography:
- the eleveation of continents and oceans continually adjusts by isostatic
compensation as the mantle beneath flows to account for pressure
differences caused by the differences in density and thickness in the
overlying layers.
- Isostasy: principle that governs topographic variations.
- Pratt model: differences in elevation results from differences in density.
Ocean Floor.
- Airy model: differences in elevation results from differences in thickness.
Continents.
- When supporting material can flow, it adjusts to the weight of an object
placed upon it. When you put wood in water, the depth that the wood sinks
to is where the pressure in the water becomes the same throughout (depth
of compensation).
- When 2 pieces of wood of different sizes (same density, different
thickness) are put in water, the larger one rises higher and sinks lower.
The depth of compensation is the bottom line of the thicker (deeper) piece.
We know that continental rocks are all the same density so they must
conform to this model (Airy model).
- When a piece of wood and a piece of metal of same thickness are added to
water, the piece of wood floats higher than the metal (different densities),
and depth of compensation is an even line (because both sink to same
level). Lighter materials float higher than denser materials (so, because
basaltic rocks cool and become more dense as you move from the ridges,
they are less elevated than the ridge rocks). (Pratt model).
-
The reason why the ocean floor is lower than the continental crust is
because a) basaltic rocks are more dense than crustal rocks and b) because
the oceanic crust is less thick (both models).
We know that the Earth’s mantle flows because mountains do not stick up because they
are extra mass piled on a rigid floor (this would increase the gravity in this area because
increasing the total mass of column extending from the earth’s surface to core—but
gravity is the same everywhere).
The Earth’s Interior is not rigid because otherwise isostatic compensation would not
occur. The fact that compensation occurs rapidly (continents rise and sink in
response to the load of the glacial ice sheet), proves that the mantle flows.
Results of plate tectonics: universally accepted over 5 year period in 1960s.
- Revolutionized study of Earth Science (can’t understand African fossils
without looking at S. American ones).
- Earth became an interconnected whole.
- Answered our questions that uniformitarianism couldn’t:
- 1) mountain belts are built by processes at convergent margins (interior
mountains during collisions). Volcanic mountain ranges (Andes) caused
by subduction.
- 2) oceans are deep b/c partial melting of E’s mantle, generating basalts
that are dense. Ocean gets deeper as plate ages and cools. Continents float
on top of plates.
- 3) earthquakes/volcanoes caused by movements of the plates and
convection of the mantle.
- 4) continents around Atlantic fit together b/c once together but then
separated from sea-floor spreading.
- 5) sediments in oceans are thin because oceanic plates have limited
lifetime.
- 6) animals differ from continent to continent in relationship to time the
continents have been separated.
Random Facts about plates:
- the average plate moves about as fast as our hair grows.
- The ocean floor never gets much older than 150 million years (b/c its
recycled). The oceanic crust only provides a record of the last 4% of Earth
history.
- Continents contain much older rocks than the oceans. Erosion and
mountain building are making older rocks increasingly rare. Additionally,
the continental record is more difficult to interpret.
CHAPTERS 10 AND 11
CONNECTING THE PARTS TOGETHER
Langmuir tries to tell the Earth’s story in the form of mysteries that were uncovered by
scientists. It is easier if information is presented in the form of a question that Langmuir
answers in the text.
Why does the mantle flow?
There are two theories of why the mantle flows. The first, plate-driven convection, argues
that the mantle’s flow is passively driven by the movement of plates in the crust above.
The second, mantle-driven convection, argues that the mantle’s flow is driven my active
convection.
The conclusion that Langmuir arrives at is a combination of both theories. First, the upper
mantle’s flow is driven by the passive movement of plates (discussed below). Second, the
inner mantle’s flow is actively driven by convection (discussed below).
Plate-Driven Flow
This theory argues that the upper mantle’s flow is driven by the passive movement of
plates in the crust. The plates move by two processes— ridge push (spreading) and slab
pull (subduction).
Evidence 1: Ridge Push
Ridge Push: The spreading movement of ocean ridges creates a vertical gap. This reduces
pressure on the mantle beneath, causing mantle upwelling, which rises to fill the gap. The
intrusion of magma into the spreading ridge provides an additional force (a passive “ridge
push”) to propel and maintain plate movement. The plate movement that results from
ridge push creates flow in the upper mantle beneath it.
Evidence 2: Slab Pull
Slab Pull: At higher depths, basalt becomes denser. Because the spreading center (one
end of the plate) is much higher than the subducting end, the subducting end is denser
and heavier, dragging the rest of the plate down. Gravity pulls the denser end down into
the subduction zone, dragging the rest of the plate with it, creating “slab pull”. The plate
movement that results from slab pull creates flow in the upper mantle beneath it.
Mantle Convection-Driven Flow
Convection is the motion that arises from density differences, lighter material rises and
denser material sinks. Active convection is responsible for flow in the inner mantle, as
evidenced by the Raleigh number and plumes.
Evidence 1: Raleigh Number
The Raleigh number takes the terms that enhance convection:
Temperature differences – Increases density contrasts.
Thermal expansion – Increases density contrasts.
Distance – Makes it difficult for heat to diffuse away (density contrasts remain).
and divides them by those that inhibit convection:
Diffusivity – Makes it easy for heat to diffuse away (density contrasts eliminated).
Viscosity – Makes it harder for material to flow (limiting convection).
In the Earth, the mantle viscosity (measured by glacial rebound) is large but distance is
large, diffusivity is low, and temperature differences are high. Once all the numbers are
plugged into the Raleigh equation, the Raleigh number exceeds the threshold for
convection, indicating that there is active convection in the mantle.
Evidence 2: Plumes
Volcanoes in Hawaii are found in the middle of plates. Dating of rocks from Hawaiin
volcanoes show a simple age progression along the chain of Hawaiin islands, as though
the Pacific plate was moving over a fixed “hot spot” in the mantle that created volcanic
activity.
This type of volcanism requires an upwelling of mantle. But since the hot spots occurred
in the middle of the Pacific plate, the upwelling could not have been produced by plate
motion but had to have come from mantle convection.
Because the plumes have a vertical motion and a fixed position, they are believed to be
generated by convection in the inner mantle. They could not have been formed in the
outer mantle, because the plume’s movement is vertical, while the outer mantle’s has
lateral flow driven by the movement of the plates. Plumes only form in hot boundary
layers, suggesting that there is some sort of sharp temperature gradient at the core/mantle
boundary.
Conclusion
First, the upper mantle’s flow is driven by the passive movement of plates (which occurs
because of ridge push and slab pull). Second, the inner mantle’s flow is actively driven
by convection (which we know exists because of the Raleigh number and plume activity
in fixed hot spots).
What is the role of ocean ridges?
Using SONAR, scientists have been able to map most of the global ridges. In fastspreading ridges, the crust is continually spreading, the ocean ridge consists of linked
volcanically active segments, rather than the tall conal volcanoes we find on land. In
intermediate-spreading ridges, the basalt can accumulate into deep trenches. In slowspreading ridges, there is not enough volcanic activity to form conic arrangements.
Ocean ridges are part of many global systems. The best way to understand them is by
discussing their roles in each.
Ocean Crust Formation
How does ocean crust form?
Ocean ridges are responsible for creating two-thirds of the Earth’s crust and 80% of
volcanic activity. As the plates along the ocean ridges spread, pressure on the mantle is
reduced, causing the melting temperature to fall, the mantle to melt and upwell into the
ridge gap. The ocean crust is formed from the extracted melt.
Temperature of the mantle affects the thickness of the crust.
As the ridges move along the upper mantle, the temperature of the underlying mantle
varies. The temperature of the underlying mantle effects the depth at which melting
begins. Therefore, the thickness of the ocean crust is directly affected by the temperature
of the underlying mantle. Where the mantle is hotter, the mantle starts to melt deeper and
melts more, creating thicker ocean crust and a shallow ocean floor. Where the mantle is
colder, it melts less, forming thin ocean crust and deep ridges.
Temperature of the mantle is responsible for forming ocean islands.
Ocean islands are formed when a hot spot occurs directly beneath an ocean ridge,
because the underlying mantle is extremely hot and the crust becomes thick enough to
form an island.
Mantle convection transports heat form the core and mantle to the surface, influencing
the depths of the ocean floor.
Hydrothermal Vent Habitats
Is there life at the ocean floor?
Dives by the submarine Alvin found chimneys (hydrothermal vents) and clouds of black
smoke with temperatures above 400 degrees C. Higher pressure in the ocean floor results
in a higher boiling temperature of water, allowing such high temperatures.
In this habitat, they found complex life (e.g. tall white tube worms) and densities of life
that are comparable to tropical rainforests. The entire ecosystem lived off the sulfides and
reduced metals in the hydrothermal gas. They found bacteria thriving at temperatures
above 100 degrees C, something that was previously considered impossible.
Organisms in hydrothermal vents are important for three reasons:
1. They form a base level in the ocean’s food web (fish and octopus consume the
ocean ridge organisms).
2. They have enzymes that function in extreme conditions which have been used
in the biotech sector for a variety of uses.
3. They expand our understanding of what kind of environments can support life
(similar to environments in early Venus and Mars and on Jupiter’s moon
Europa; Europe may have volcanism beneath an ocean like on Earth). And
what the earliest life looked like (found them to be close to the original
lifeforms).
Using sensors that detect the black smoke (precipitated metals) coming from the
hydrothermal vents, scientists found that the vents are ubiquitous outcomes of sea floor
spreading. They are found in virtually every region of every ocean ridge.
Volcanism at Subduction Zones
Why is there melting at subduction zones?
Until the discovery of hydrothermal vents, it was unclear why subduction was associated
with such melting activity. While ocean ridges have volcanic activity because the
spreading reduces mantle pressure and causes mantle upwelling, in subduction, the
descending plate cools the mantle below, inhibiting melting. With the cold plate leading
to lower mantle temperatures, and downward flow giving mantle a higher melting
temperature, why is there so much volcanic activity?
Water in the ocean crust leads to mantle melting at subduction zones.
As the water-rich plate descends into the subduction zone, high temperatures and
pressures dehydrate and decarbonate the rocks, releasing water and carbon dioxide into
the overlying mantle. The 5% or more of water content is enough to lower the mantle
melting temperatures and create melting at subduction zones. Whereas melting at ridges
is a result of reduced pressure on hot magma, melting at convergent magmas is caused by
lowering melting temperatures due to the addition of water.
Water and sediment add water to the plate over time.
When the crust is first formed in the ocean ridges, there is very little water (<0.2%). As it
spreads across the ocean floor toward a subduction zone, it becomes hydrated from a
number of sources:
1. Interaction with salt water transforms some of the dry minerals to hydrous
ones. Some of the rock in the crust changes from black basalt to a greenschist
or amphibole that contains water.
2. Huge amount of hydration occurs near the hydrothermal vent.
3. Sediments from continental weathering (carried by rivers or wind).
4. Sediments from biological processes in the ocean.
5. The mantle beneath the crust also becomes somewhat hydrated.
Subduction of the water-rich plate lowers the melting point of the overlying mantle.
Over the course of 100 million years, the composition of the crust and underlying mantle
changes from its original magma (deprived of water and carbon dioxide) to a package of
hydrated mantle, hydrated basalt, and sediments that moves down into the subduction
zone.
As it moves down into the subduction zone, the high temperatures and pressures cause
the hydrous minerals in the plate to dehydrate, releasing water. Thus, as the water-rich
plate is subducted, the temperature and pressure cause dehydration and decarbonation,
releasing water and carbon dioxide into the overlying mantle.
The released water lowers the melting point of the overlying mantle (water acts as a
“flux”). This allows melt to form which is released out of the volcanoe.
Volcanic release of subducted ocean crust allows continental crust to float.
The water-rich magma from the subducted ocean plate are released through volcanic
activity. They differentiate upon cooling to compositions that are rich in silica, creating
low density materials that allow the continental crust to float above sea level.
Sediments from the top layer of the subducted slab are also recycled onto the continental
crust through volcanic activity. (Scientists found the isotope 10Be on the surface layer of
the sediments being subducted and also in erupted convergent magmas, indicating that
the 10Be from the uppermost sediments is subducted and released out of the volcano onto
the continental crust. 10Be is only created on the surface of rocks from solar rays. If its
half life is 1.6 million years, then the fact that we find 10Be in volcanic melt then the
whole cycle takes place in less than 10 million years.)
Seawater Composition
Why is seawater not overly mineral-rich?
The water cycle should make the ocean extremely mineral-rich. When rainwater washes
over the continental crust, it accumulates mineral. More mineral-rich river water is
recycled back into the ocean. The mineral content of seawater should steadily increase
with time.
However, water in the ocean is not overly mineral-rich. The Na content in seawater
remains below saturation and levels of radiogenic isotopes are lower than that of river
water. How is such a steady state maintained?
Hydrothermal vents remove minerals from seawater.
Seawater penetrates the ocean crust through hydrothermal vents, removing some minerals
and adding others, maintaining the ocean’s steady-state. The total hydrothermal system is
vast enough to allow the entire ocean to circulate.
CHAPTERS 12
MAKING IT COMFORTABLE
How has the Earth maintained a water supply?
Requirements:
1. Planet must have captured enough water for a sizable ocean.
2. Water must have migrated from the planet’s interior to the surface.
3. Must not have been lost to space.
4. Must have been largely in liquid form.
How did the Earth capture enough water?
We do not know. Maybe carbonaceous chondrites or comets brought water to the young
Earth.
How did water migrate from the planet’s interior to the surface?
At the high temperatures during the formation of Earth’s iron core, water would have
been in a gaseous form. As iron migrated to the core, water should have migrated to the
surface.
How did water not escape to space?
The likelihood that a molecule will escape to space depends on the strength of the
planet’s gravity and the mass of the molecule itself. Water molecules are too heavy to
escape Earth’s atmosphere.
However, UV light can break water molecules apart, creating free H atoms. The reason
why this hasn’t been an issue on Earth is because most of the Earth’s water is in the
ocean, sediments, and ice.
How has the Earth’s water remained in liquid form?
The Earth has maintained a temperature that has allowed water to remain in liquid form.
A planet’s temperature depends on the amount of sunlight it receives, the reflectivity of
its surface, and the content of greenhouse gases in its atmosphere. The carbon cycle keeps
the Earth’s temperature in a steady state as discussed below.
How has the Earth maintained its temperature?
Earth’s temperature has managed to remain in a steady state between 0 and 100 degrees
Celsius for most of its history.
Why has the Earth never become greenhouse Earth?
Carbon dioxide is kept from creating a runaway greenhouse effect by the carbon cycle.
Carbon dioxide leads to more acidic waters, higher temperatures, and thus higher rock
weathering rates. Rock weathering releases calcium into the runoff. Organisms in the
ocean use calcium and carbon dioxide to form calcium carbonate. These shells later
become sediments which accumulate on the ocean crust. The sediments are subducted
into the mantle where calcite reacts with sulfides to release carbon dioxide gas.
SiO2 + CaCO3  CaSiO3 + CO2
This negative feedback keeps the Earth in a steady state. Low carbon dioxide levels result
in a cooler surface and thus less weathering. Less weathering means less calcium in the
ocean and thus less carbon dioxide removal by ocean organisms that make calcium
carbonate.
High carbon dioxide levels cause a hotter surface and more weathering. More weathering
means more calcium runoff and more removal of carbon dioxide by ocean organisms to
make calcium carbonate. This continues until carbon dioxide levels return to the steady
state, where the calcite production rate would cancel out the excess carbon dioxide rate.
Carbon dioxide would remain at a constant level in the atmosphere.
Why has the Earth never become icehouse Earth?
If the Earth were to freeze, there would be no organisms to make calcite and carbon
dioxide would accumulate in the atmosphere. Because carbon dioxide is released by the
Earth’s interior, it is insensitive to surface temperature and would continue to accumulate
in the atmosphere. The greenhouse effect would accelerate by carbon dioxide
accumulation until the planet became warm enough to melt the ice.
Exceptions (Snowball Earth)
During certain episodes of glaciation, the Earth did become frozen. The evidence for this
is that glacial deposits were found near the equator, indicating that the entire Earth was
frozen.
Without enough greenhouse gas, the Earth became colder. As more water became snow
and ice, the reflectivity increased and the Earth became even colder. The oceans froze but
carbon dioxide continued to escape from the interior until the greenhouse gas content
became high enough to melt the Earth.
Ice Ages
Over the last million years, there have been major climate cycles every 20,000, 40,000
and 100,000 years that were characterized by a gradual growth of ice caps. Each cycle
was terminated by an abrupt warming which brought it back to its original condition.
Oxygen Isotopes
In evaporation, O16 will be released. In an ice age, more O16 turns into ice. So O18/O16
levels in the ocean can provide evidence of ice levels. So during ice ages, O18/O16 levels
are higher.
What caused the ice age cycles?
The 20,000 year cycle is caused by precession when the Northern Hemisphere oscillates
in distance between the Earth at its farthest and closest points to the sun.
The 40,000 year cycle is caused by angle changes in the tilt of the Earth’s axis.
The 100,000 year cycle is caused by changes in the elliptical orbit of the Earth around the
Sun.
CHAPTERS 13
MANKIND AT THE HELM
Humans hold the largest influence on the planet’s fate – what do we do about global
warming then?
History and “Fun” Facts
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As the population of humans increases so does the extent of our interference on
the planet
Dumb shit we do:
o Kill animals
o Pollute water
o Pollute the atmosphere (with CO2, CFCs, SO2, soot, etc…)
o Erosion (think the Hamptons beaches, which are now microscopic)
o Effects on the tropical rainforests and coral reefs)
2 factors of huge population increase:
o Poor people have more kids as a sort of “old age insurance”
o Advances in medical technology have increased the average life span
significantly

Result: disproportionate amount of young people so the population will continue
to increase even though many industrialized countries are now below replacement
rate in terms of birth rate
How Do We Fix This? Looking at the Quest for Energy:
Fossil Fuels
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Currently 85% of energy comes from burning oil, coal, and natural gas
o Petroleum will run short in about 50 years (wtf?!)
o Can get gas from tar sands and oil shales (or coal) but need to figure out
how to safely and economically recover the methane trapped in clathrates
(look down for definition)
o Problem: by burning fuels we release CO2 into the atmosphere which
threatens the climate
 In the atmosphere, CO2 is chemically inert so it just accumulates
 Pre 1800: CO2 level in atmosphere – avg 280 ppm (parts per
million)
 1958: 316 ppm
 2000: 365 ppm
 If fuel remains our primary energy source then around 2100: 840
ppm (WTF?!?!)
Main concern: the accumulation of CO2 in the atmosphere impedes the return of
the energy from the sun into space (aka global warming)
o How global warming works:
 The Earth’s surface responds immediately to the accumulation of
CO2 by getting warmer
 Then Earth emits more infrared light to balance for the lack of
energy released into space
 So, if the levels get to 840 ppm, it is estimated the average surface
temperature of the Earth will increase by 3-5 degrees centigrade
and probably even more inside the Earth (this is approximately
equal to the jump between the last period of glaciation and our
current interglaciation period)
 There are problems with the simulations though which contribute
to people’s hesitation to address the problem:
They oversimplify the role of clouds (reflectivity is very important in determining
temperature)
They oversimplify the interaction between atmosphere and ocean (an increase in
CO2 concentration can alter the ocean conveyor circulation – or even shut it down
– because a warmer Earth has more evaporation and precipitation, which
decreases salinity and density of sea water in areas like the Northern Atlantic
which alters the amount of deep water formed. If the decrease is large enough,
this could shut down deep water formation altogether)
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Can we switch to fossil fuels? It would be ideal but it entails a total switch in
energy sources to nuclear, wind, hydro, solar, geothermal, etc. sources and right
now no combination would produce enough
o Nuclear: dangerous because of terrorism
o Solar: too expensive
o Hydro and geothermal: too limited
o Wind: economically good but would take 10-20% of energy carried by
surface wind which would have huge environmental results
So what do we do? Couple CO2 capture with long-term storage is one method
o Instead of burning coal in atmospheric oxygen, use steam producing CO
and H2 (so coal + H2O  H2 + CO) and then CO is oxidized to CO2 with
H fed into electricity-generated fuel cell – this is cheap capture of CO2 in a
plant
o But we would need other ways too because 2/3 of CO2 is produced in
small venues like homes, cars, etc.
o Powering cars with batteries: no batter is powerful enough
o Therefore, combine wind energy with CO2 storage is a very possible
option. There are 4 methods of CO2 storage:
o Deep-sea storage: liquid CO2 is very compressible so at 3500 m
depth the density of sea water = density of liquid CO2 and would
actually combine with water to create clathrate (6 H20-CO2) which
would pile on the sea floor (because so cold it turns to solid)
o Storage in polar ice caps: pump liquid CO2 into lakes below
Antartica’s ice cap which would form clathrates but need to couple
with CO2 extraction from air (this could happen anywhere, and
would be necessary to balance out costs)
o Storage in saline aquifers: Pump liquid CO2 into sedimentary
basins of very salty waters  CO2 would remain liquid because
these aquifers are warmer (which is a good thing) – in fact, Statoil
in Norway is already doing this
o Convert to MgCO3: would permenantly immobilize CO2 by
grinding up olivine and reacting parts of it with CO2:
o Mg2SiO4 + 2CO2  2MgCO3 + SiO2
o Need to construct large electric plants and air
extraction facilities at surface outcrops of olivine
o All storage options have environmental impacts though:
o Deep-sea has bad potential effects on ocean life (Green Peace
disapproves)
o Antarctica is controversial mining site
o Pumping to saline aquifers could screw with earthquakes and
people’s nearby homes
o MgCO3 options require a lot of mining and could increase the
release of asbestos into the atmosphere
o The hope is that environmental damage from a solution would be
significantly less than the damage from the problem
o We would probably wind up producing approximately 64 km3/year of
liquid CO2
o This would increase the cost of fossil fuel energy 25 (+/- 10) %, which
translates into a few % for global GNP so some will ask do the pros really
outweigh the cost?
o Other problems:
o Need to develop technology mostly from scratch
o Payment plan needed
o Need 180 nations on board – will take about 40 years to really get
everyone involved
o Need to convince a skeptical public
Atomic Energy
o Developed the fusion bomb in the 1940s leading the Cold War weapons race and
also the goal of harnessing fission to produce cheap electrical power (historical
background)
o Problems with fission power:
o Only 235U is fissionable but it is a lot less abundant than 238U (a 1:138
ratio!) and we need to raise it from the <1% abundancy to at least 3%
abundancy
o 235U can be used to:
 Make atomic weapons
 Power nuclear reactors
 Operate breeder reactors
o All of these require managing neutrons given off from fission reactions
 In a bomb, the 3 n are used to start the chain reaction
 In a reactor, 2 of 3 n are captured by Boron control rods
 In the breeder, convert the non-fissionable 238U or 232Th to
fissionable plutonium
o Advocates see the breeder as the answer because it uses the more abundant
isotopes (rather than waste the 235U which would only power the world for
about 100 years)
o Problems:
 Dramatic increase in potential for nuclear disasters
 Nuclear plants are excellent terrorist targets
Energy from the Sun
o 2 Routes we could take:
1. Photovoltaic cells: These have a high cost and would need to be
coupled with a storage system for nighttime and cloudy days, so unless
there’s a miracle (his words not mine!) then it’s pointless
2. Biofuels: This is already being pursued. Basically, you take
agricultural products like sugar cane (Brazil) or ethanol from corn
(US?!) and blend it with fuel to make more fuel. The ultimate
limitation is that the amount of land on which suitable plants could be
grown (called arable land) necessary to power the world, when taking
population growth and world hunger into consideration, is
problematic. It could probably work, but considering we can’t
distribute our food equally now (aka we’re incompetent) it’s hard to
see how we could manage distributing land to produce our energy