THE GEOLOGY OF YELLOWSTONE NATIONAL PARK

Posted by Kelly Wilson on April 14th, 2021

By Dr. David W. Barber

The northwest corner of the state of Wyoming, in the middle of the Rocky Mountain chain that forms the backbone of the North American continental plate, contains a large, high plateau surrounded by mountain peaks of the Rockies. A large lake on the south side of this plateau, Yellowstone Lake, feeds the Yellowstone River near its headwaters just south of the lake. This river exits the plateau to the north and then turns northeast to join the Missouri River on its way southeast toward the Mississippi River which, in turn, flows south to the Gulf of Mexico. This high plateau provides a home for many of the largest mammals found in America including buffalo, elk, bear, moose and deer. Even more extraordinary, numerous geothermal features, such as geysers, fumaroles, mud pots and hot springs, dot the area. The beauty and amazing visual displays of the region were recognized as a natural treasure as the US expanded westward, and its protection was strongly advocated. So, in 1872, just after the close of the Civil War, President Ulysses S. Grant signed into law the creation of Yellowstone National Park, the first national park in the world.  

The spectacular scenery of Yellowstone originates from a geologic phenomenon underneath its crust called a hot spot. These are areas in the upper mantle where hotter mantle material has plumed up or invaded cooler mantle rocks normally above them. These upwelling masses of hot rocks can push up the crustal rocks on the surface causing them to stretch apart and become thinner by fracturing and faulting. These fractures and faults allow water to penetrate the crust and become heated from the hotter rocks below. Turned into steam, the superheated water escapes back to the surface explosively to form the geysers, or more gradually to form the other geothermal features of the park.

Accompanying these spectacular, scenic features are earthquakes, some 1,500 to 2,500 per year, generated along the faults produced by the upwelling magma dome. Most are too small to be felt but some can trigger landslides such as occurred during the Hebgen Lake earthquake in 1959, a massive 7.2 magnitude quake that killed 28 people. The small quakes generally occur in swarms, multiple quakes occurring over a small interval of time. This crustal unrest adds to the excitement of visiting Yellowstone, reminding us of the incredible power contained in the earth beneath our feet.  

The seismic activity of Yellowstone demonstrates ongoing tectonic deformation, that includes two areas of active domal uplift, the Mallard Lake and Sour Creek domes about 35 km apart. This movement has given rise to speculation about the possibility of a super-volcanic eruption from Yellowstone, a cataclysmic event that could expel 2,500 times more material than Mount St. Helens’ flank eruption in 1980. Although such an event is unlikely, the Yellowstone Super-volcano has erupted in the past, leaving the Yellowstone Caldera as its footprint.  

A caldera is formed when the crust overlying the upwelling magma dome becomes too thin to contain it and allows it to escape either violently, blowing a huge amount of crustal rocks and magma into the atmosphere, or more slowly, causing the magma to flow out on the surface over a large area. Generally, the outer rim of the caldera preserves the edge of the crustal rock that was pushed up by the magma. The ejected material can form various kinds of rock formations. Extrusive lava flows turn solid, forming rhyolites (silica rich) or basalts (silica poor). Obsidian forms when felsic lava cools quickly to produce glass. Pyroclastic rocks, composed primarily of ash but other solid rock fragments welded together, are called tuffs if the particles are smaller than 2.5 inches or breccias if the particles are larger. These rocks provide a geologic record of the history and extent of the volcanic activity in Yellowstone.

Detailed studies of the volcanic rocks in and around the park have helped geologists decipher the history of the activity of the area. The Yellowstone Super-volcano has erupted several times in the past, creating a large complex of overlapping calderas extending from Lake Yellowstone westward out of the park into Idaho. The timing of these events stretches from 2.1 million years, to 1.3 million years, to 640 thousand years, to the latest and smallest, 174 thousand years ago.  Geologic studies of the Snake River Plain, which stretches to the southwest of the park through Idaho into northern Nevada, have demonstrated the hot spot under Yellowstone has created a chain of calderas stretching back to 16 million years ago. This pattern demonstrates that the North American plate has been moving over a relatively stable hot spot from northeast to southwest over the last 16 million years. Some helpful tools like drill core logging enable Geologists to precisely record and measure as much of this information as possible. 

Complex seismic tomographic studies of earthquake waves that pass through the upper mantle underneath Yellowstone have revealed that two magma bodies are present there. The shallower one composed of rhyolite lies about 3-10 miles deep, is 25 by 55 miles in width and contains 5-15% melt. The deeper one composed of basalt lies 12-30 miles deep, is 4.5 times bigger, but contains only 2% melt. Both rhyolite and basalt flows are present on the surface indicating that both sources have been involved in the eruption history of Yellowstone, but the exact nature of their interaction awaits further studies. 

Analysis of the risk of another super-volcanic eruption is complicated by the lack of knowledge about the strength of the crustal covering. Although some of that knowledge could be obtained from wellbore instruments which measure the physical properties of rocks penetrated by a drill bit, drilling in the park is prohibited to protect the natural beauty of Yellowstone. In this case, aesthetic considerations would seem to trump ethical issues, but sudden increases in the apparent instability of the area in the future might overturn that evaluation. For now, we can continue to enjoy the scenic splendor of this national treasure while maintaining a vigilant monitoring of its tectonic activity.