Page 327 - ABCTE Study Guide_Neat

Plate Tectonics

There are six major plates (and more than a dozen minor plates) of lithosphere that are about 50 miles
thick and almost float on top of the asthenosphere, which is believed to be a very viscous liquid-like layer
of the Earth. These plates grind and push against each other at some margins while they pull apart at
others. These compressional forces are believed to cause earthquakes, volcanoes, and mountain ranges.

Continental and Oceanic Crust

The surface of the earth is covered by what is labeled the crust. This is divided into two types: the
continental and oceanic crusts. Because the density of the water is less than the density of the continent,
the water floats on top of the continental crust. The oceanic crust is crust covered by ocean by virtue of its
elevation. The oceanic crust is a little denser than continental crust as well as much thinner.

The continental crust is formed as volcanic magma cools on the surface. Most of this volcanic activity is
found in subduction zones where one plate is being pushed under another plate. As this liquid rock
erupts, it spreads and repeatedly covers over the surface in layers, and more continental crust is built up.
Over time, particular materials are carried by the wind or water and deposited on the surface along with
seeds dropped by birds or carried by the wind or water. With time, plants grow and stabilize the surface.

In areas where there is little stabilization from living inhabitants, weathering processes occur, including
temperature changes that expand and contract the rocks, and rain which can freeze and crack the rocks
or dissolve more soluble materials to expose and weaken the overall structure of the rocks. Eventually,
over long periods of exposure, much of the rocks are either dissolved or deposited by erosion into the