Final Flashcards
(23 cards)
Where does oceanic crust form, and where
is it destroyed?
formed at mid-ocean ridges, destroyed in subduction zones
Does continental crust behave in the same way?
No. Continental crust is less dense than oceanic crust, and generally does
not subduct because it “floats” on top. There are places where contintental
crust subducts, but this is less common.
The oldest continental rocks are >4 Ga old, the oldest oceanic crust only
~350 Ma. Explain the difference.
Oceanic crust is much younger because it is created and destroyed all the
time. Continental crust is almost never destroyed in subduction, and is
therefore older.
Where do you expect the ocean to be the deepest?
The ocean is deepest near the subduction zones, where the oceanic plate
subducts underneath a neighboring plate. These are the oceanic trenches.
Order these materials by density: oceanic crust, continental crust,
asthenosphere (= weak upper mantle)
Asthenosphere, oceanic crust, continental crust (dense to light)
so-called Magnetite crystals in ocean crust
are like compass needles that record the direction of Earth’s
magnetic field at the time that the crust was formed.
How do you predict the magnetization of the ocean crust looks on
both sides of a mid-ocean ridge? Remember, the Earth’s
magnetic field reverses occasionally.
On both sides of the ridge we find alternating strips of normal
and reversed polarity of the magnetization. These strips on both
sides are symmetrical (mirror image). See Lecture slides for a
figure.
Can you think of a modern way to test the theory of plate
motion?
Today we can detect plate motion using GPS
In the Northern Hemisphere, CO 2 decreases during summer, and increases during winter. What is the cause of this seasonal variation?
Carbon uptake for photosynthesis in spring and summer, carbon
release during fall and winter due to respiration and decomposition.
In the Northern Hemisphere, CO 2 decreases during
summer, and increases during winter. Which two carbon reservoirs are involved in this exchange? Does
the total amount of carbon change?
Exchange between the biosphere (vegetation) and the atmosphere
In the Northern Hemisphere, CO 2 decreases during
summer, and increases during winter. Sketch how the atmospheric δ 13 C of CO 2 changes due to this
exchange (remember, life uses light isotopes). Don’t worry about the
exact numbers, just the trends.
(See study guide slide 9) The biosphere uses light carbon, so the atmospheric reservoir becomes heavier when the biosphere takes up
carbon (i.e. in summer when atmospheric CO 2 goes down).
Which carbon reservoirs are involved in the
production and combustion of fossil fuels?
During production carbon from the atmosphere is
taken up by the biosphere via photosynthesis. As the
organic carbon is buried, it becomes part of the
lithosphere.
Under international agreements, deforestation is often
counted as a CO 2 emission. Why?
Deforestation reduces the amount of carbon in the
biosphere (vegetation). As this organic matter
decomposes, the carbon is released to the atmosphere as
CO 2 .
How long would it take for this added 14 C to decay to 25% of the peak
value? ( 14 C half life is 5730 yr.)
This would take 2 half-lives, so 11460 years
Nuclear tests doubled 14 C in the atmosphere! Out of ~80 added 14 C atoms, only 20 were left in 1985. (See study guide slide 11)
How long did it take for 14C to decay to 25% of peek value in the observations?
In the observations it took roughly from 1963 to 1985, so about 22 years
Nuclear tests doubled 14 C in the atmosphere! Out of ~80 added 14 C atoms, only 20 were left in 1985. (See study guide slide 11)
According to C14’s half-life, It should take 11460 years to decay to 25% of its peak value.
The observations it decayed to 25% within 22 years in the atmosphere.
Can you explain the difference?
The reduction in 14 C during this time is not due to radioactive decay.
Instead, the radioactive carbon is taken up by the ocean and by vegetation,
lowering 14 C in the atmosphere.
What is the fate of the ocean floor and its
sediments on tectonic time scales?
The ocean floor and much of the sediments are
subducted and return to the mantle
Which carbon reservoirs are involved in the chemical
weathering of rocks?
Carbon from the atmosphere dissolves into water,
ends up in the ocean. From the ocean it mineralizes, and
eventually becomes part of the lithosphere after
subduction.
Chemical weathering would remove all CO 2 from the
atmosphere within ~300,000 years or so. What
mechanism resupplies CO 2 to the atmosphere?
volcanism
If the atmospheric concentration of CO 2
increases, does chemical weathering increase or decrease?
it will increase, because there is more CO 2 available for
chemical weathering.
Assume that volcanic degassing happens at a more or less
constant rate. Is chemical weathering a positive or negative
feedback on changes in CO 2 concentration?
Negative. See slide 14-15
At higher temperatures, chemical weathering rates increase.
Is this a positive or negative feedback?
Negative. See slide 14-15
What are the two types of plate boundaries
where most of the volcanic degassing occurs?
mid-ocean ridges and subduction zones (also known as divergent
and convergent margins, respectively)
What controls the rate of volcanic degassing at these locations? Is
the control the same for both locations?
controlled by the rate of spreading / subduction.
