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Respond to the following scenario:
o Your younger brother is in 5th grade. He comes home and tells you to look at an
animation his teacher showed him in geology class.
o Review the Continents of the Past animation on this week’s aXcess page.
o After viewing the animation, your brother has a look of disbelief on his face, “C’mon,
really? Our whole continent moves? That’s bogus!” Instead of blowing him off like you
usually do, you decide to sit down and educate him about plate tectonics. First, you
give him an overview about two historical figures that contributed to the plate tectonic
theory. Then, you decide to show him how the plates move by boiling a pot of water
(shown on p. 108) and relating it to mantle convection and plate movement.
· Write a 200- to 300-word summary about the two historical figures and the process of
mantle convection and plate movement

Convection Cell
Water cools
108 CHAPTER 4 Plate Tectonics
Although virtually all geologists accept the basic theory
of plate tectonics, some questions remain. What, exactly,
drives plate motion? How does the mantle interact with
the crust? What initiates subduction? Scientists have a
basic understanding of these processes, but the details
have not been completely worked out. Thermal motion
in the mantle is at least partially responsible for the motion
of plates. This thermal motion, in turn, results
from the release of heat from Earth’s interior. Let’s take
a closer look at some of the complexities of Earth’s
heat-releasing processes.
Earth’s internal heat Earth gives off heat for two
main reasons. First, it is slowly cooling off from its initial
formation processes, including formation of a molten
iron core. Second, heat is constantly being generated by
the decay of radioactive elements in the interior, primarily
uranium, potassium, and thorium. If Earth did not release
heat into outer space, the entire interior would
eventually melt.
Some of Earth’s heat is released through conduction.
This is a gentle and slow process similar to what you
feel when you hold a cup of hot coffee in your hands.
The heat moves through the wall of the cup by conduction,
a gradual transfer of energy from atom to atom.
When you boil water in a pot on a stove, you will
see the water churning around in big circles called convection
cells (see Figure 4.14A). A mass of hot water
at the bottom is slightly less dense than the cooler water
at the top, and hence it will rise. When it reaches the surface,
it sheds its heat, moves sideways as it cools, and then
sinks back down to the bottom, where it is reheated. This
mechanism of heat transfer, is more efficient than conduction.
The convection cells act like couriers, carrying
Cold rock
Cold rock
Heat source
within the Earth
Ocean floor Ocean floor
Convection inside Earth drives the lithospheric plates on the surface.
A An everyday example of convection can be seen when you boil a pot of water. The water
closest to the burners is hotter than the rest of the water. As it heats up it becomes less dense
and rises to the top. At the surface it cools down and moves sideways to make room for the
hot water rising beneath it. As the water at the surface cools, it becomes denser and sinks.
B The same process happens in
Earth’s mantle on a much grander
scale and over a much longer time.
Hot rock rises slowly and plastically
from deep inside Earth, then cools,
flows sideways, and sinks. The relation
between convection cells and
lithospheric plates is far more complex
than what we see, for example,
in a pot of boiling water.

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