next up previous
Next: Question 3 - 20 Up: ES 10 HW Previous: Question 1 - 20

Question 2 - 20 points

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:

  1. Radiogenic Heat. Radioactive decay of elements such as uranium releases heat into the surrounding rock.

  2. 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).

  3. 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.

  4. 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 up previous
Next: Question 3 - 20 Up: ES 10 HW Previous: Question 1 - 20

Back to Top
Back to Homework Page
Back to Home Page

Greg Anderson
Mon Feb 17 15:24:05 PST 1997