Geography Reference
In-Depth Information
Folding the crust
When plates collide head-on, a couple of outcomes are possible. One is folding, in which the
crust buckles in response to the compression, and may eventually assume a rather wave-like
appearance, as you can see in Figure 6-3. You can crudely simulate that sequence by doing
the following: Put a piece of paper on the surface of a desk or table and place your right and
left fingertips on opposite edges. Then very slowly move your hands together. Hopefully, the
paper will assume a wave-like form as your fingers approach each other. Remember, however,
that the Earth area represented by that paper contains hundreds of square miles, and that the
convergence of your finger tips mimic plate movements that span millions of years.
Figure 6-3:
A fol-
ded landscape.
Former grandeur
The Appalachian Mountains of the eastern United States are a prime example of a folded mountain
range, as illustrated by Figure 6-4. They do not, however, coincide today with a plate boundary. This
tells geographers that plates and plate boundaries come and go over the broad expanse of geologic
time. In some cases, therefore, mountains mark an active plate boundary where mountain making is
in progress. In other cases, mountain ranges mark ancient (extinct) plate boundaries, and are them-
selves mere eroded remnants of what used to be. Today, the highest peaks in the Appalachians are
in the 6,000 feet range. But orientation of various rock layers suggests to geologists that in their an-
cient heyday, the Appalachians towered 30,000 feet and more above sea level. That's higher that Mt.
Everest, the tallest mountain on Earth today.
