|
|
click on diagram to enlarge |
What causes the plates to move?
It turns out to be a consequence of the high temperatures
inside Earth. Common experience tells us that heat flows from hot to cold,
so the heat in Earth's deep interior must be flowing somehow to the surface.
The hottest part of Earth's interior is the iron core. The
core heats the bottom of the rocky mantle. The hottest rock near the bottom
of the mantle becomes slightly less dense than the somewhat cooler rock
above it, so buoyancy forces try to push the hottest rocks upward. Although
the rock in the mantle is solid, the pressures and heat are so great that
the rock can deform slowly, like hot wax. So the hot rock creeps upward
through the cooler rock. As the hot rock rises, cooler rock flows downward
to take its place next to the core, where it is heated and becomes buoyant
enough to rise again later. The rising hot
rock comes in contact with cold rocks near the surface of Earth where it
gives off its heat, cools, and sinks again. Most of the rock in the mantle
moves in this broad cyclic flow, indicated by the arrows in the figure.
This zone, where rock is soft enough to flow, is called the asthenosphere
.
The cyclical movement of hot and cold material
is called convection .
Rock near the surface of Earth is so cold and at such low pressures that it cannot flow like mantle rock. So how does heat get through this rigid layer lithosphere, to the surface? It flows through cracks in the earth's surface. Places where liquid rock (lava) flows onto Earth's surface are called volcanoes. The movement of heat by convection in the asthenosphere causes the rock of the mantle to slowly move in huge streams. The solid (but brittle) rock of the lithosphere is resting directly on top of the solid (but soft) rock of the asthenosphere. As the rock of the asthenosphere moves in different directions, it carries parts of the lithosphere along with it. The lithospheric rock can't stretch, so it breaks into pieces--forming the plates. This whole group of observations and ideas describing the motions of the plates and their geologic features is called plate tectonics. [ Locations
of Volcanoes ]
|
 |
 |
 |
 |