Geography Reference
In-Depth Information
ton , meaning “builder”) build up the Earth's crust. The pressures involved here are mighty enough
to literally make mountains out of molehills, and also cause earthquakes to occur and volcanoes to
erupt. Tectonic force has been modifying the crust for as long as crust has existed and will continue
to do so for billions of years to come. Thus, I can say with complete confidence that the force will be
with you always.
On the other hand, gradational forces wear down the crust. Given enough time, they can transform
today's mountains into tomorrow's molehills. Gradational forces are the subjects of Chapter 7.
Starting at the Bottom: Inside Earth
The source of power for tectonic force lies deep within the Earth. For that reason, tectonic
forces are sometimes called endogenous forces. This comes from the Greek endon , meaning
within, and another old word that is the source of “genesis.” So endogenous forces have their
genesis, or origin, within the Earth.
It would be great if you could go there and see what's going on, but that's impossible. The av-
erage distance from Earth's surface to the center is 3,960 miles, and no human has ever come
close. Several topics and movies have portrayed such fanciful feats, but the truth is that people
have barely penetrated the crust. Miners in South Africa have gone down about two miles, and
if that's not the record, then the real one can't be much farther. So instead of going on a fant-
astic journey, you must settle for a diagram (as shown in Figure 6-1). Looking at it may cause
you to wonder, “Well, if nobody's ever been down there, then how do you know what it looks
like?” Great question! And the answer is, it's based on informed speculation. Check out the
“How do we know what's down there?” sidebar for details.
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