[Illustration: A.—Normal fault (tensional)]
[Illustration: B.—Reverse fault (compressional)]
[Illustration: 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.
The site of the Teton block no doubt reflects hidden inequalities at
depth. We cannot see these, nor in this area can we drill below the
outer layer of the earth; nevertheless, measurements of gravity and of
the earth’s magnetic field clearly show that they exist.
We know that the Tetons rose at the time Jackson Hole collapsed but the
volume of the uplifted block is considerably less than that of the
downdropped block. This, then, was not just a simple case in which all
the subcrustal material displaced by the sinking block was squeezed
under the rising block (the way a hydraulic jack works). What happened
to the rest of the material that once was under Jackson Hole? It could
not be compressed so it had to go somewhere.
As you look northward from the top of the Grand Teton or Mount Moran, or
from the main highway at the north edge of Grand Teton National Park,
you see the great smooth sweep of the volcanic plateau in Yellowstone
National Park. Farther off to the northeast are the strikingly layered
volcanic rocks of the Absaroka Range (fig. 52). For these two areas, an
estimate of the volume of volcanic rock that reached the surface and
flowed out, or was blown out and spread far and wide by wind and water,
is considerably in excess of 10,000 cubic miles. On the other hand, this
volume is many times more than that displaced by the sagging and
downfaulting of Jackson Hole.
Where did the rest of the volcanic material come from? Is it pertinent
to our story? Teton Basin, on the west side of the Teton Range, and the
broad Snake River downwarp farther to the northwest (fig. 1) are
sufficiently large to have furnished the remainder of the volcanic
debris. As it was blown out of vents in the Yellowstone-Absaroka area,
its place could have been taken deep underground by material that moved
laterally from below all three downdropped areas. The movement may have
been caused by slow convection currents within the earth, or perhaps by
some other, as yet unknown, force. The sagging of the earth’s crust on
both sides of the Teton Range as well as the long-continued volcanism
are certainly directly related to the geologic history of the park.
In summary, we theorize as to how the Tetons rose and Jackson Hole sank
but are not sure why the range is located at this particular place, why
it trends north, why it rose so high, or why this one, of all the
mountain ranges surrounding the Yellowstone-Absaroka volcanic area, had
such a unique history of uplift. These are problems to challenge the
minds of generations of earth scientists yet to come.