Mountain Building

Teton Range at Sunset

GRTE/Adams

 

Today, the Teton Range towers above the Jackson Hole valley. To the north is the Yellowstone Plateau, to the northeast is the Absaroka Mountains, to the east and southeast rise the Gros Ventre Range, and to the south is the Snake River Range. This area is ringed by mountains, each with a different past and a different form of mountain building.

How many mountain ranges have been here before? Geoscientists, based on geologic evidence, believe that up to a dozen mountain ranges have been built up and eroded away in the past three billion years of geologic history.

 

Ancient Past / Mountain Core

Roughly three billion years ago, two continents collided much like India and Asia are colliding today. These extreme tectonic forces uplifted a mountain range similar to the Himalayas. As collision progressed, the existing rocks were pushed skyward or downward, crumpling under the forces and being metamorphosed by heat and pressure. Based on geologic data, the metamorphic rocks found throughout today's range were buried up to 18 miles below the surface – the distance from Moose to Moran!

About 2.5 billion years ago, molten magma squeezed into cracks in the metamorphic rocks cooling and crystallizing to form igneous granite. Look for light streaks of igneous granite cutting across darker swaths of metamorphic gneiss.

As tectonic forces shift through time and space, what was once collision may become extension. Geologists have no rock evidence from 2.5 billion years to about 0.8 billion years ago. But around 775 million years ago this region was stretched – what today is north to south – vertical cracks formed, and basaltic magma squeezed up into those cracks. The magma cooled and crystallized to form igneous diabase, known locally as the "black dikes."

These events represent at least three mountain building events in the distant past!

 

Fast Forward

Beginning about 540 million years ago, this region was near sealevel. For the next several hundred million years, layer upon layer of sediment was laid down on the ancient core rocks. When these sediments compressed into rocks, the reigon was blanketed by thousands of feet of sedimentary rocks such as sandstone, limestone, and shale.

Off the west coast of North America, tectonic forces began to change more than 100 million years ago. The Farallon Plate began to subduct underneath the North American Plate. Subduction results from compression where one tectonic plate is pushed under another tectonic plate. The subduction underneath crumpled the overlying plate generating mountains beginning on the West Coast and migrating eastward.