What Did Yellowstone Look Like Before the Yellowstone Hot Spot?

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We know Yellowstone today as the site of a huge caldera that erupted 631,000 years ago and covered the region in a thick layer of ash. But what was present before the caldera?

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Natali Kragh, graduate student in the Department of Earth Sciences at Montana State University.

What is the difference between a million and a billion? To put these two numbers in perspective, we cycle through one million seconds about every 11.5 days whereas we only cycle through one billion seconds every 31.7 years! So, what does this have to do with Yellowstone geology? Although the modern-day landscape of Yellowstone is covered with volcanic rocks sourced from the hotspot underneath, Yellowstone doesn’t just have rocks from volcanic eruptions. Before the hot spot reached northwestern Wyoming 2.1 million years ago, dozens of different rocks existed in the area that record a history of over 3.5 billion years. These rocks can still be seen today along the northern, eastern, and southern boundaries of the park and can be divided into three formational time periods: the Precambrian (>540 million), the Paleozoic/Mesozoic (540-65 million), and the early Eocene (53-38 million).

Crinoid fossil from northern Yellowstone National Park

Crinoid fossil from a Paleozoic rock unit found in northern Yellowstone National Park.

(Credit: Natali Kragh, Montana State University. Public domain.)

Some of the oldest rocks on Earth reside along the northern boundary of Yellowstone National Park. These rocks formed during Precambrian time, basically anywhere between 570 million to 4 billion years ago. The Precambrian is defined by vast expanses of intrusive rocks (like granite) building on each other, laying the foundation for the continents as we know them today. After they formed, these rocks were continuously buried by younger rocks, and they were eventually heated and pressurized to form metamorphic rocks—the rarest rock type found in Yellowstone.

Following this time, the western United States was fairly flat, and for millions of years after the Precambrian, vast oceans developed and began producing sedimentary rocks, formed through the deposition of sand, mud, shells, and other materials in layer upon layer. As the oceans advanced and retreated over 500 million years (the Paleozoic and Mesozoic eras), limestones, mudstones, sandstones, and everything in between were laid down all across the American West. Yellowstone today has a near complete record of these rocks along the northern and southern boundaries of the park. Plenty of these rocks contain both animal and plant fossils from the time of their formation, exhibiting the diversity of life that has always existed in Yellowstone.

Absaroka Mountains in northeastern Yellowstone National Park

A view into the Absaroka Mountains in northeastern Yellowstone National Park. While the Absarokas of today look a little different due to glacial activity, these are the same mountains that existed over the last 53 million years, well before the arrival of the Yellowstone hot spot.

(Credit: Natali Kragh, Montana State University. Public domain.)

This relatively tranquil period of time came to a screeching halt around 80 million years ago with the onset of the “Laramide orogeny”. This period was a time of massive compression and faulting due to subduction of an oceanic plate (called the Farallon plate) along the western boundary of the North American continent. This event forced the metamorphic rocks back up to the surface, through all the sedimentary rock, and formed the majority of the mountain ranges in Wyoming and southwestern Montana that we see today.

The subduction of the Farallon oceanic plate is also the cause of the final rock-forming period predating the Yellowstone hotspot—an episode that began approximately 53 million years ago. Subduction of the Farallon plate resulted in not only mountain building, but also the formation of magma that rose to the surface in the Yellowstone region. This formed a chain of volcanoes in western Wyoming extending into Montana, stretching over 150 miles in length. To imagine this, think of today’s Cascade Mountains, which include iconic volcanoes such as Mount St. Helens, Mount Rainier, and Mount Hood. This volcanic arc occupied in the exact area of today’s Yellowstone National Park, forming the Absaroka Mountains. The Absarokas are entirely made up of a series of volcanic rocks over 6,000 ft. thick and covering 9,000 sq. miles that currently bound the eastern side of the park. Debris flows off of the Absaroka peaks are responsible for the destruction and then the preservation of the petrified wood found in Yellowstone today.

So, what did Yellowstone look like before the hot spot arrived? Huge volcanic mountains could be found along the eastern and northern borders, with sedimentary rocks filling in between and stretching into southeastern Idaho. But while the animals of the time were roaming the land and glaciers began to advance, the hot spot track was progressing along the Snake River plain

The rest, as they say, is history.  Geologic history!

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