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Climate Role of the Solid Earth |
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Internal Structure of the Solid Earth |
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Theory of Plate Tectonics |
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History of Plate Tectonics |
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The faint young Sun paradox and its possible
explanation. |
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Why was Earth ice-free even at the poles 100 Myr
ago (the Mesozoic Era)? |
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What caused Earth’s climate to cool over the
last 55 Myr (the Cenozoic Era)? |
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Continental
Drifting Theory |
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Alfred Wegener proposed that all the continents were once assembled
into a supercontinent (Pangea) and then broke and slowly drifted to their
current positions. |
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Plate
Tectonics |
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The
branch of tectonics that deals with the processes by which the lithosphere
plates move and interact with each other is called plate tectonics. |
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What can happen to the cold boundary? |
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The lithosphere has broken into a number of
rocky pieces, called plates. |
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There are a few large plates plus a number of
smaller one comprise the Earth’s surface (a total of 20 plates). |
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The plates range from several hundred to several
thousand kilometers in width. |
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Some lithospheric plates are composed primarily
of oceanic crustal material, whereas others are composed primarily of
continental materials. |
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The continents stand, on average, about 4.5 km
above the floor of the ocean basins. |
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Continental crust is relatively light (density
2.7 g/cm3), whereas oceanic crust is relatively heavy (density close to 3.2
g/cm3). |
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There
are two major sources of the heat in Earth’s interior: |
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(1)
Radioactive decay: of potassium, uranium, and thorium. |
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(2)
Residual heat from Earth’s formation: A tremendous amount of energy was
transferred to Earth during the accretion of the planet by collisions with
planetesimals. |
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A major
problem of the continent drifting theory is: How could the continents drift
through the rigid sea floor? |
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This
problem is answered by the seafloor spreading hypothesis: Continents do not
plow through the sea floor. Continents and segments of ocean floor are
connected into plates that continuously move away from one another at
mid-ocean ridges. |
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Interactions between plates occur along their
edges. There are three types of plate margins: |
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Divergent margins |
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form mid-ocean ridges (over oceans) and rift valleys (over lands) |
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(2)
Convergent margins |
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form deep-sea trenches (two oceanic plates or ocean+continental
plates) or high mountains (such as Tibetan Plateau) (two continental
plates). |
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Transform fault margins |
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form earthquake faults |
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Polar
position hypothesis |
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Chemical
Weathering Hypothesis |
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Seafloor
Spreading Hypothesis |
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The polar position hypothesis focused on
latitudinal position as a cause of glaciation of continents. |
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This hypothesis suggested that ice sheets should
appear on continents when they are located at polar or near-polar
latitudes. |
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To explain the occurrence of icehouse intervals,
this hypothesis calls not on worldwide climate changes but simply on the
movements of continents on tectonic plates. |
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This hypothesis can not explain the climate of
the Late Proterozoic Era, when both
continents and glaciers appear to have been situated at relatively
low latitudes. |
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It can not explain the warm Mesozoic Era when
high-latitude continents were present but were almost completely ice-free. |
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Climate in the past 500 million years have
alternated between long periods of warm climate and short periods of cold
climate. |
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During the last 500 million years, major
continent-size ice sheets existed on Earth during three icehouse ear: (1) a
brief interval near 430 Myr ago, (2) a much longer interval from 325 to 240
Myr ago, and (3) the current icehouse era of the last 35 million year. |
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The precipitation process in the atmosphere
dissolve and remove CO2 from the atmosphere. |
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Rocks exposed at Earth’s surface undergo
chemical attack from this rain of dilute acid. |
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This whole process is known as chemical
weathering. |
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The rate of chemical weathering tend to increase
as temperature increases. |
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Weathering requires water as a medium both for
the dissolution of minerals and for the transport of the dissolved
materials to the ocean |
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è The rate
of chemical weathering increases as precipitation increases. |
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The chemical weathering works as a negative
feedback that moderates long-term climate change. |
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This negative feedback mechanism links CO2
level in the atmosphere to the temperature and precipitation of the
atmosphere. |
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A warm and moist climate produces stronger
chemical weathering to remove CO2 out of the atmosphere è smaller
greenhouse effect and colder climate. |
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Chemical weathering acts as Earth’s
thermostat and regulate its
long-term climate. |
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This thermostat mechanism lies in two facts: |
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(1)
the average global rate of chemical weathering depends on the state of
Earth’s climate, |
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(2)
weathering also has the capacity to alter that state by regulating the rate
which CO2 is removed from the atmosphere. |
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Solar luminosity was much weaker (~30%) in the
early part of Earth’s history (a
faint young Sun). |
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If Earth’s albedo and greenhouse effect remained
unchanged at that time, Earth’s mean surface temperature would be well
below the freezing point of water during a large portion of its 4.5 Byr
history. |
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That would result in a “snowball” Earth, which
was not evident in geologic record. |
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During active plate tectonic processes, carbon
cycles constantly between Earth’s interior and its surface. |
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The carbon moves from deep rock reservoirs to
the surface mainly as CO2 gas associated with volcanic activity
along the margins of Earth’s tectonic plates. |
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The centerpiece of the seafloor spreading
hypothesis is the concept that changes in the rate of seafloor spreading
over millions of years control the rate of delivery of CO2 to
the atmosphere from the large rock reservoir of carbon, with the resulting
changes in atmospheric CO2 concentrations controlling Earth’s
climate. |
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The uplifting weathering hypothesis asserts that
the global mean rate of chemical weathering is heavily affected by the
availability of fresh rock and mineral surfaces that the weathering process
can attack. |
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This hypothesis suggests that tectonic uplifting
enhances the exposure of freshly fragmented rock which is an important
factor in the intensity of chemical weathering. |
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This hypothesis looks at chemical weathering as
the active driver of climate change, rather than as a negative feedback
that moderates climate changes. |
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