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How were the teton mountains formed?

It’s one thing to simply look at the Teton range and take it in – it’s another thing to consider how they got there in the first place. They are part of the Rocky Mountains, but they are also somewhat separated from most of the Rockies – so how were the Tetons formed?

The teton mountain range pictured in front of a river

The geologic history of the Teton Range

The Teton Range has a rich geologic history that spans billions of years. It is composed of a variety of rock formations, each representing a different period in Earth’s history. The oldest rocks in the range are found at the base and date back to the Precambrian era, over 2.5 billion years ago. These rocks are primarily metamorphic and igneous in nature, formed through intense heat and pressure deep within the Earth’s crust.

As we move up the range, we encounter rocks from the Paleozoic era, which began around 540 million years ago. These rocks are sedimentary in nature and were formed in ancient seas that covered the region. Fossils of marine life can be found in these rocks, providing valuable insights into the past environments of the Teton area.

The next layer of rock in the Teton Range is from the Mesozoic era, which spanned from 252 to 66 million years ago. This era is known for the rise of dinosaurs and the formation of the supercontinent Pangaea. The rocks from this era are primarily sedimentary and were formed in a variety of environments, including rivers, lakes, and coastal areas.

The formation of the Teton Mountains

Alongside all of that geologic history, plate tectonics played a crucial role in the formation of the Teton Mountains.

The Teton Range is located near a convergent plate boundary, where two plates – the North American and Pacific plates – are colliding. The Pacific Plate is being subducted beneath the North American Plate, and has been for millions of years, causing intense pressure and heat to build up in the Earth’s crust.

This pressure and heat led to the formation of the Teton Mountains through a process called uplift. As the Pacific Plate was pushed beneath the North American Plate, it caused the crust to buckle and fold, resulting in the formation of the Teton Range around 9 million years ago. This process took millions of years and continues to this day, albeit at a much slower rate. 

Because of this, you will find the exact same types of rock (and even fossils) in the Teton Range as you will buried deep beneath the Jackson Hole basin – they used to be one and the same. For example, there is a layer of sandstone found on top of Mount Moran (pictured below) that is the same as a layer of sandstone 20,000 feet underneath the valley surface!

Mount Moran in grand teton national park

The Teton Fault

The Teton Fault has had a significant impact on the landscape. This fault is a result of the ongoing tectonic activity in the region and is responsible for numerous earthquakes. The location of the fault runs along the eastern side of the Teton Range, forming a kind of geologic boundary between the mountains and the lakes that we love to use as framing devices for photos.

The Teton Fault is what’s known as a normal fault, which means that one side of the fault moves downward relative to the other side. This type of faulting is caused by tensional forces that pull the rocks apart. The movement along the fault has resulted in the formation of a series of fault scarps, which are steep cliffs that mark the location of the fault.

Normally, when we hear the word “fault”, we think “earthquake” – leading us to ask: does the Grand Teton area experience earthquakes? The answer is yes.

Earthquakes along the Teton Fault have occurred throughout history and continue to this day. These earthquakes can range in magnitude from minor tremors to more significant events. The most notable earthquake along the Teton Fault occurred in 1959 and had a magnitude of 7.3. This earthquake caused significant damage to buildings and infrastructure in the region.

The 1925 Gros Ventre Landslide

One of the most significant events in the geologic history of the Teton Mountains was the 1925 Gros Ventre Landslide. This catastrophic event occurred on June 23, 1925, when a massive section of the mountainside collapsed and slid down into the Gros Ventre River Valley.

The landslide was triggered by heavy rainfall that saturated the soil and weakened the underlying rocks. The combination of gravity and the weakened slope caused a massive failure, resulting in the displacement of an estimated 50 million cubic yards of material.

The impact of the Gros Ventre Landslide was devastating. The landslide dammed the Gros Ventre River, creating a temporary lake that flooded the valley upstream. The floodwaters destroyed several homes and infrastructure, causing significant damage to the local community.

Wrapping up

The Teton Mountains are a testament to the power of geologic forces and the impact they have on shaping our planet. Understanding the formation of these mountains is crucial in order to appreciate their beauty and significance. From plate tectonics to glaciers, erosion to landslides, the Teton Mountains have been shaped by a variety of processes over millions of years. By studying and appreciating the geologic history of the Teton Range, we can gain a deeper understanding of the forces that have shaped our world.

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