The Geology P.A.G.E.: Geology of the National Parks in Pictures

My next post about the Geology of the National Parks Through Pictures is from our move across the country from Utah to New York. Along the way we visited 13 National Parks as well as some other sites. This was the first National Park along the way.

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.

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Grand Teton National Park

Obligatory entrance sign photo. This specific photo however is from our first trip there in 2010.

Geological stratigraphic column of the Teton Range within Grand Teton National Park. Image courtesy of the USGS.

The rocks that form the Teton Range, the mountains that make up the central focus of Grand Teton NP, date back hundreds of millions to billions of years. The oldest rocks within the part, at the bottom of the stratigraphic column, are 2.7 billion year old (Ga) gneiss. These were formed from the metamorphism of seafloor sediments and volcanic debris caught within a continental collision.

View of the mountains towards the southern entrance near Jackson Hole, WY.

After the metamorphism of the gneiss, the rocks were infiltrated by magma bodies 2.5 billion years ago. These magma bodies slowly cooled forming the granites that top many of the mountains within the park including Grand Teton, Middle Teton, and Mount Owen. These mountains (pictured below and maybe above if the clouds weren't covering the peaks) form the middle part of Grand Teton National Park.

Me in front of Grand Teton (the prominent point in the middle), Middle Teton (I believe the sheltered point to the left), and Mount Owen (the short point to the right of Grand Teton) along the Taggart Lake Trail.

The last of the basement rocks formed 775 million years ago (Ma) when the region started to stretch, resulting in cracks running through the gneiss and granites. Basaltic magma flowed upwards through the cracks and cooled forming dikes of an igneous rock known as diabase.

Along the shores of Jenny Lake.

After the formation of the igneous and metamorphic bedrocks, sea level rose and allowed for the deposition of a suite of sedimentary rocks starting with beach sands (the Flathead Sandstone at 510 Ma), then deeper water mudstones and limestones. While that was the origin of the rocks within the mountains, we now get to the formation of the mountains themselves.

This great geological history diagram of the Tetons is located along the shores of Jenny Lake.

As shown in the geological diagram above, the region that became the Teton Mountains started to expand yet again. This expansion started around 10 million years ago and created the Teton Fault, a normal fault.

A normal fault

This relatively young age for the mountains makes them far younger than the nearby Rockies, which started to be formed 50 to 80 million years ago. Like the nearby Wasatch Mountains in Utah, the extensional forces that formed the Teton Mountains are related to the extensional pressures from the Basin and Range expansion.

Extensional activity in the Basin and Range Province. Image courtesy of Miracosta.edu.

What happens in this type of expansion is that while the blocks of crust are being pulled apart, they start to rotate. This rotation is what produces the mountain peaks. Think of a row of blocks all sitting next to each other, then rotate each of them on their corner so that you have a corner sticking up, that is your mountains. The "V" that is formed between the mountains is your valleys, or basins, which begin to fill up with eroded sediment from the new mountains. These areas become nice flat plains in relation to the nearby jagged peaks. The mountains are still being uplifted through this process. Overall, the blocks have moved almost 30,000 feet along the Teton Fault and continue to move with each earthquake.

Glacial boulders in Taggart Creek along the Taggart Lake Trail.

While the mountains are built up, erosion wears them down. There is erosion through water activity, like rain and streams, but also glacial activity. While water erosion can carve out phenomenal features over time, it has a tendency to smooth over landscape features. Glaciers on the other hand have a tendency to produce jagged, awe-inspiring landscape features. Many of the striking landscapes were formed during the last few glacial events over the previous 2 million years. Glacial erosion carved out many of the valleys within the Tetons from their "V" shaped stream profile to a "U" shaped valley, and also dug out many of the lakes within the park. Jackson Lake, by far the largest lake in the park, is over 400 feet deep carved out during the Pinedale glacial period 50,000 to 14,000 years ago.

Jackson Lake

After the glaciers eroded the mountains and carried their debris down, the material was deposited along the flanks of the mountains in piles known as moraines. Hiking up the lower slopes of the Tetons brings you through these glacial deposits where you can see many boulders such as seen along the Taggart Lake Trail above.

Glacial pebbles

Glaciers also tend to mix up sediments from a wide variety of sources, so you end up with the awesome amalgamation of colorful pebbles in the surrounding lakes. While glaciers formed many of the lakes and moraines within the park in the geological past, there actual glaciers still within the park, although much smaller than in the geological past and shrinking ever faster with a warming climate.