Geology Reference
In-Depth Information
Understanding Why Density Matters:
Isostasy
The different densities of oceanic and continental crust affect the way each one inter-
acts with the mantle material below it. The mantle material beneath the earth's crust is
solid rock, but it moves a little via plastic flow, which I describe in Chapter 13. The result
of this semi-fluid characteristic of the mantle rock is that the solid lithosphere above
sinks into it slightly.
The density of a floating object relative to the material it floats in determines
how much of the object sinks into, or displaces, the liquid. Because density is the
relationship between mass and volume, the point at which the volume of dis-
placed liquid has the same mass as the object that is displacing it is marked by
the
equilibrium line.
This is illustrated in Figure 9-1.
Figure 9-1:
The
equilibrium line for
continental and
oceanic crust.
Therefore, the density of the crustal plate determines how deeply it sinks into the
mantle rock. The more dense oceanic crust sinks more of its volume into the mantle
than the less dense continental plates.
However, continental crust is usually much thicker than oceanic crust. Figure 9-1 illus-
trates oceanic crust and continental crust above earth's mantle. Because the continental
crust is so much thicker than the ocean crust, it may appear that much more of it is sub-
merged into the mantle materials. But if you compare the relative proportions or per-
