A.—Normal fault (tensional)
B.—Reverse fault (compressional)
C.—Thrust fault (compressional)
Why are mountains here?
Why did the Tetons form where they are?
At the beginning of this booklet we discussed briefly the two most common theories of origin of mountains: continental drift and convection currents. The question of why mountains are where they are and more specifically why the Tetons are here remains a continuing scientific challenge regardless of the wealth of data already accumulated in our storehouse of knowledge.
The mobility of the earth’s crust is an established fact. Despite its apparent rigidity, laboratory experiments demonstrate that rocks flow when subjected to extremely high pressures and temperatures. If the stress exceeds the strength at a given pressure and temperature, the rock breaks. Flowing and fracturing are two of the ways by which rocks adjust to the changing environments at various levels in the earth’s crust. These acquired characteristics, some of which can be duplicated in the laboratory, are guides by which we interpret the geologic history of rocks that once were deep within the earth.