The rugged grandeur of the Tetons is a product of four geologic factors: the tough hard rocks in the core (precambrian gneiss and granite), the amount of vertical uplift, the recency of the mountain-making movement, and the dynamic forces of destruction.
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.
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.
How rapidly did the Tetons rise? Can the rate be measured?
We know that in less than 9 million years (and probably in less than 7 million years) there has been 25,000 to 30,000 feet of displacement on the Teton fault. This is an average of about 1 foot in 300-400 years. The movement probably was not continuous but came as a series of jerks accompanied by violent earthquakes. One fault on the floor of Jackson Hole near the southern boundary of the park moved 150 feet in the last 15,000 years, an average of 1 foot per 100 years.
Why did the Tetons rise?
I have not found this to be entirely clear as the mechanisms referred to do not distinguish why the rise was so much more rapid than the Laramide Orogeny that caused the other parts of the Rocky Mountains to uplift. The Teton fault extends 40 miles along the base of the Teton Range. About ten million years ago, this region began to stretch and the Earth’s crust cracked forming faults. Each time the crust broke; an earthquake up to magnitude 7.5 shook the land tilting the mountains skyward and dropping the valley floor. These sporadic bursts of energy created the abrupt front of the Teton Range as it towers above Jackson Hole.
It is conceivable that the expulsion of magma during one of the volcanic eruptions added to the subsidence of Jackson Hole area which may have accelerated the uprise of the Tetons.
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