Geology Reference
In-Depth Information
Running in Circles: Models of Mantle Con-
vection
All three of the current models describing plate movement rely on
convection
as their
driving force in one way or another. Convection is, quite simply, the movement of heated
materials. You probably know it as the concept that “heat rises.” The trouble with that
phrase is that
heat
itself doesn't actually go anywhere. Instead, the
material
that is
heated — whether it is solid, liquid, or gas — starts moving.
When matter is heated, the molecules start moving due to the increase in energy. As
they get moving, they occupy more space; they spread out and become less dense. The
number of molecules stays the same, but they take up more space or
volume.
Think of a dance floor: During a slow song, all the couples embrace and sway slowly to-
gether, and you can really pack the floor with couples! Then the deejay switches to a
dance tune, and people start to boogie. Suddenly, with all that energy and movement,
people are taking up more space, and some of them have to move off the dance floor
and dance between the tables because there just isn't enough room anymore. You still
have the same number of people, but you add some energy and they take up more
space; the dancing crowd becomes less dense and takes up more volume.
In a similar way, the rock material in the
mantle
(the layer between earth's
crust and core, as described in Chapter 4) is heated and begins to move. When
the mantle rock is near the core, it heats up, becoming less dense. This heated
mantle rock then moves up, toward the earth's crust and away from the core, for-
cing the cooler and more dense mantle rock near the crust down toward the core.
This circular rotation of material creates what's called a
convection cell.
Some scientists think that only part of the earth's mantle moves through convection
cells; they think that only the very top portion of the mantle — the part right beneath
the crust, called the
asthenosphere
— has convection cells. Other scientists think that
the entire mantle rotates this way.
Each of the three convection models I explore in this section proposes a different per-
spective on the role that internal heat and the crustal plates play in the system of plate
tectonics. Figure 10-1 illustrates all three models, which I explain in turn.
