 The layer below the crust is the mantle. The mantle has more iron
and magnesium than the crust, making it more dense. The uppermost part
of the mantle is solid and, along with the crust, forms the lithosphere.
The rocky lithosphere is brittle and can fracture. This is the zone where
earthquakes occur. It’s the lithosphere that breaks into the thick, moving
slabs of rock that geologist’s call tectonic plates.
 As we descend into the Earth temperature
rises and we reach part of the mantle that is partially molten, the asthenosphere.
As rock heats up, it becomes pliable or ‘plastic’. Rock here is hot enough
to fold, stretch, compress, and flow very slowly without fracturing. Think
about the behavior of Silly Putty® and you have the general idea. The
plates, made up of the relatively light, rigid rock of the lithosphere
actually ‘float’ on the more dense, flowing asthenosphere! |
