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Many people had trouble with this question, and in particular, the difference
between this question and Question 3. I assure you that they are in
fact different, but I agree that the difference is subtle.
While the exact source of energy for the driving force for plate tectonics is
not fully understood, I think most geoscientists would agree that the best
candidate is heat in the Earth's interior, which comes from four primary
sources:
- Radiogenic Heat. Radioactive decay of elements such as
uranium releases heat into the surrounding rock.
- Gravitational Heat. As Earth grew, it became heavier, and
began to compress its inner layers more and more. As they
compressed, they became hot (think about Lisa's lecture examples
of pumping up a bike tire -- the air in the pump gets hot as you
compress it into the tire).
- Bombardment Heat. As Earth formed, large chunks of rock
left over from earlier stages of the formation of the Solar System
slammed into the planet. The impacts of these planetesimals
converted energy of motion into heat energy.
- Freezing of the Inner Core. Without going into all the
gory details, some scientists have argued that a considerable
amount of heat is released by the formation of the solid inner
core. These scientists believe that the solid inner core freezes
out of the liquid outer core, which releases heat.
Anyone who told me that the energy source was heat, and gave me one or more
of the heat sources above, got full credit. Anyone who told me that the energy
source was unknown, but gave a theory as to what it might be, also got full
credit -- because this reflects good science. Anyone who just told me
``heat'', but not where it comes from, got 12 points.
A significant number of people wrote that the source of energy was convection
-- not so. Convection occurs when (among other things) the bottom
of a fluid body is hotter than the top. This makes the hot fluid at the bottom
less dense than the fluid above it, which makes the hot fluid buoyant relative
to the fluid above it, and so the hot fluid rises. When the hot fluid reaches
the top, it cools and sinks -- a classic convection cell. However,
convection doesn't just happen. It still requires that there be
heating involved. So the fundamental source of energy is still heat, not
convection. Anyone who told me that the source of energy was convection got
8 points.
You can read more about this problem in Chapters 1 and 19 of your textbook,
particularly figure 1.6.
Next: Question 3 - 20
Up: ES 10 HW
Previous: Question 1 - 20
Greg Anderson
ganderson@ucsd.edu
Mon Feb 17 15:24:05 PST 1997