The Teton Range towers over the valley of Jackson Hole, providing dramatic alpine scenery and drawing millions of visitors to Grand Teton National Park each year. Over billions of years, natural forces including earthquakes, glaciers and erosion have shaped this magnificent landscape.
Metamorphic & Igneous Rocks - Ancient Core
The Teton Range contains some of the oldest rocks in North America, similar to those exposed in other major mountain ranges of the western U.S., including the Bighorns, the Gros Ventre and Wind River ranges.
A 2.7 billion-year old metamorphic rock called gneiss makes up much of the Teton Range. These rocks were formed when sea floor sediments and volcanic debris were buried up to 18 miles deep as two tectonic plates collided. The intense heat and pressure at these great depths changed or metamorphosed the sediments into today's rocks, separating different minerals into lighter and darker layers. Watch for the zebra-striped layers as you step over rocks on your hike.
Molten magma began squeezing into cracks in the gneiss 2.5 billion years ago, and it cooled and crystallized to form igneous granite. This speckled rock with its interlocking crystals is harder than gneiss, and it forms the highest peaks in the central Teton Range, including the Grand Teton and Middle Teton. Other peaks, such as Teewinot and Mount Moran, show stripes of darker and lighter gray where the granite cross-cuts the gneiss.
Roughly 775 million years ago, the region stretched north to south, cracking the deeply buried gneiss and granite and forming a series of vertical, east-west trending cracks. Basaltic magma squirted into these cracks and cooled to form dikes of an igneous rock called diabase. One of these dikes that slices through the face of Mount Moran is 150 feet wide—if the exposed part of the dike melted, the magma would fill Jenny Lake three times over! Black diabase dikes are also visible from the Teton Park Road on the east face of the Middle Teton and the southeast flank of the Grand Teton.
Sedimentary Rocks - Drape the Core
Above the igneous and metamorphic core are layers of sedimentary rocks that wrap around the Teton Range and are eroding from the eastern side. For example, a small island of remnant sandstone tops the summit of Mount Moran! These sediments were laid down when global sea levels rose, first depositing beach sands (the Flathead Sandstone that caps Mount Moran) and continuing with limestones deposited in shallow seas and mudstones that were a deeper ocean floor. These seafloor rocks preserve a variety of fossils, including trilobites, stromatolites, corals, and shells of other organisms.
The Tetons are one of the youngest mountain ranges in North America. They have been uplifting for less than 10 million years, making them "adolescent" mountains, as compared to the "middle-aged" Rockies (60-80 million years old) or the "elderly" Appalachians (more than 300 million years old). Erosion has had much less time to work in the Tetons, comparatively, so their jagged peaks remain standing high.
The mountains uplift one earthquake at a time along the 40-mile long Teton fault, a north-south trending crack in the earth's crust. As the region stretches in an east-west direction, this stress builds to a breaking point when it generates an earthquake, lifting the mountain block skyward while dropping the valley floor. On average, the fault moves 10 feet in each earthquake: six to eight feet up and two to four feet down. In the 10 million years since the fault began moving, the total offset is approaching 30,000 feet. As a quick estimate, the Flathead sandstone on top of Mount Moran is 6,000 feet above the valley and that same layer is buried roughly 24,000 feet below the valley floor on the east side of the fault.
The Teton fault is still active and capable of generating a magnitude 7.0-7.5 earthquake. The best place to view the fault is the Cathedral Group Turnout on the Jenny Lake Scenic Loop. The fault scarp is along the mountain front, visible as a steeper slope that breaks through more gently sloping glacial deposits. This offset represents up to a dozen earthquakes occurring since glacial ice melted from the valley about 20,000 years ago. Geoscientists have dated the two most recent earthquakes at 4,800 and less than 8,000 years ago.
Pleistocene Ice Age glaciers sculpted the Teton landscape over multiple glacial advances, which widened V-shaped river canyons into U-shaped glacial canyons, bulldozed depressions filled by lakes today, sharpened high peaks, and filled the valley floor with outwash debris. The most recent advance, known as the Pinedale, lasted from roughly 50,000 to 12,000 years ago. Many hikers access the park's high country by traveling up glacial canyons such as Cascade, Paintbrush, Death Canyon and Granite. Look for features like glacially polished bedrock as you hike through these areas.
In an earlier glacial advance, about 200,000 years ago, larger ice sheets flowed off the Yellowstone Plateau through the Jackson Hole valley, almost reaching Hoback Junction and burying the valley with more than 2,000 feet of ice. These glaciers left behind outwash plains of gravel and cobbles that are now covered in sagebrush. Glacial meltwater carried away the fine sands and silts that would have retained moisture and allowed topsoil to form; sagebrush is well adapted to dry soils, so it now covers these outwash plains. Conversely, conifers cover moraines, which mark the borders of former glaciers. Glaciers carry boulders, cobbles and gravel as they flow and grind up rocky material to the consistency of flour. As glaciers melt constantly at their terminus, they deposit this rocky debris as ridges called moraines. The fine-grained material help soils form and retains moisture, allowing trees to grow. Look for moraines ringing glacial lakes such as Jenny, Taggart and Jackson, as well as some isolated moraines such as Timbered Island.
Other Park Resources:To learn more, visit our other websites on glacial features and current glacier research, as well as the park's geology brochure. Watch four video podcasts: Geology Overview, Mount Moran, Teton Fault and Glaciers; and a geology animation to experience the forces that shaped this landscape.
Discover Grand Teton allows viewers to explore the geology of the region through a timeline, earthquakes, rocks and forces.
What's shaking in the Intermountain West (Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona and New Mexico)? The site provides local magnitude, times of occurrence and proximity to local towns and cities.
Last updated: December 8, 2016