Geology Reference
In-Depth Information
centages of submerged continental and oceanic crust, you see that a larger percentage
or proportion of the oceanic crust sinks into the mantle rock. Only 80 percent of contin-
ental crust is submerged into the mantle. (It only appears to be more on first glance be-
cause the continental crust is so thick). A larger percentage (90 percent) of the oceanic
crust is submerged as a result of its higher density.
One way to think about the relationship between the mantle and the contin-
ental crust is to think about an iceberg floating in the ocean. Part of the iceberg is
above the water surface, but a larger percentage of it is below the water where
you can't see it. The portion of the iceberg above and below the water line is de-
termined by the density of the ice. In the case of the iceberg, approximately 10
percent of the overall volume of the iceberg is above water, and 90 percent is be-
low. This is because the iceberg is only slightly less dense (10 percent) than the
water it is floating in.
The position of equilibrium — where the density of the portion of crust sunk into the
mantle is equal to the density of the mantle material displaced — is called
isostatic equi-
librium.
Any time material is moved around the earth's surface, such as when rocks and
sediments are removed from a mountain by erosion, the crust adjusts its position in the
mantle; it finds a new balance between the portion of crust above and below the line of
equilibrium in order to accommodate the material removed in one place and added in
another. This balance of the solid crust floating in the solidly flowing mantle is called
isostasy.
Understanding the different densities and thicknesses of each type of crust, as well as
how the two types of crust sit in the mantle differently, helps you understand what hap-
pens when two plates meet, which I explain in the following sections.
Defining Plate Boundaries by Their Relat-
ive Motion
The crustal plates don't move in a single, specific direction, such as north, south, east,
or west. They just move. When describing how the crustal plates move around the sur-
face of the earth, the best way to define their motions is in relation to one another.
The relative motion of plate boundaries can be narrowed down to three categories:
