Chapter 8 Plate Tectonics

202 CHAPTER 8: PLATE TECTONICS
Figure 8-6 Slow-moving convection currents within Earth carry heat energy toward
the surface. Material rises and creates new oceanic crust at the ocean ridges, and it
moves back into Earth at ocean trenches. This circular pattern is called a convection
cell. The continents are carried by this motion like passive rafts floating on water.
Convection is driven by density differences. Most materials
expand when they are heated. Expansion increases volume
and makes matter less dense. In places where a fluid is
heated, it becomes less dense and tends to rise. Cooling usually
causes matter to become more dense. This makes the
substance heavier than an equal volume of the surrounding
material. When fluids are cooled they usually sink. A heat
source in one place, such as Earth’s interior, and cooling in
another place, such as Earth’s surface, can set up a circular
motion known as a convection cell as shown in Figure 8-6. In
convection, the motion of the fluid occurs when heated material
moves away from the source of heat. This is what is happening
in Earth’s asthenosphere.
WHAT IS EARTH’S INTERNAL STRUCTURE?
In Chapter 7, you read about the four layers within Earth as
determined by seismic waves: crust, mantle, outer core, and
inner core. Motion of the plates leads us to consider a different
way to divide the outer layers of Earth into lithosphere,
asthenosphere, and stiffer mantle. Note that Figure 8-7 is