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Carbon
Cycle As a Thermostat for Earth |
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Chemical
Weathering and Continental Drift |
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Why No
Similar Thermostat on Venus and Mars |
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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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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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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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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 six 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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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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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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Mars is
too small in size |
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Mars had no large internal heat |
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Mars lost all the internal heat quickly |
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No tectonic activity on Mars |
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Carbon can not be injected back to the
atmosphere |
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Little greenhouse effect |
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A very cold Mars!! |
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Venus is
too close to the Sun |
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Venus
temperature is very high |
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Very
difficult for Venus’s atmosphere to get
saturated |
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Evaporation keep on bringing water vapor into Venus’s
atmosphere |
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Greenhouse effect is very large |
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A “run
away” greenhouse happened on Venus |
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Water
vapor is dissociated into hydrogen and oxygen |
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Hydrogen
then escaped to space and oxygen reacted with carbon to form carbon dioxide |
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No water
left on Venus (and no more chemical weathering) |
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