_How did the Tetons and Jackson Hole form?_ They are both tilted blocks
of the earth’s crust that behaved like two adjoining giant trapdoors
hinged so that they would swing in opposite directions. The block on the
west, which forms the Teton Range, was hinged along the Idaho-Wyoming
State line; the eastern edge was uplifted along a fault (a fracture
along which displacement has occurred). This is why the highest peaks
and steepest faces are near the east margin of the range. The hinge line
of the eastern block, which forms Jackson Hole, was in the highlands to
the east. The western edge of the block is downdropped along the fault
at the base of the Teton Range. As a consequence, the floor of Jackson
Hole tilts westward toward the Tetons (see cross section inside back
cover).
_When did the Tetons and Jackson Hole develop the spectacular scenery we
see today?_ The Tetons are the youngest of all the mountain ranges in
the Rocky Mountain chain. Most other mountains in the region are at
least 50 million years old but the Tetons are less than 10 million and
are still rising. Jackson Hole is of the same age and is still sinking.
The Teton landscape is the product of many earth processes, the most
recent of which is cutting by water and ice. Within the last 15,000
years, ice sculpturing of peaks and canyons and impounding of glacial
lakes have added finishing touches to the scenic beauty.
_Why did the Tetons rise and Jackson Hole sink?_ For thousands of years
men have wondered about the origin of mountains and their speculations
have filled many books. Two of the more popular theories are: (1)
continental drift (such as South America moving away from Africa), with
the upper lighter layer of the earth’s crust moving over the lower
denser layer and wrinkling along belts of weakness; and (2) convection
currents within the earth, caused by heat transfer, resulting in linear
zones of wrinkling, uplift, and collapse.
These concepts were developed to explain the origin of mountainous areas
hundreds or thousands of miles long but they do not answer directly the
question of why the Tetons rose and Jackson Hole sank. As is discussed
in the chapter on mountains, it is probable that semifluid rock far
below the surface of Jackson Hole flowed north into the Yellowstone
Volcanic Plateau-Absaroka Range volcanic area, perhaps taking the place
of the enormous amount of ash and lava blown out of volcanoes during the
last 50 million years. The origin of the line of weakness that marks the
Teton fault along the east face of the Teton Range may go back to some
unknown inequality in the earth’s composition several billion years ago.
Why it suddenly became active late in the earth’s history is an
unanswered question.
The ultimate source of heat and energy that caused the mountains and
basins to form probably is disintegration of radioactive materials deep
within the earth. The Tetons are a spectacular demonstration that the
enormous energy necessary to create mountains is not declining, even
though our planet is several billion years old.