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Glacial erratic

This huge boulder was dropped by a retreating glacier on the north rim of the Grand Canyon of the Yellowstone in Yellowstone National Park. It is evidence of Yellowstone's icy past.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mike Poland, geophysicist with the U.S. Geological Survey and scientist-in-charge of the Yellowstone Volcano Observatory.

The volcanic history of Yellowstone is well known, and rightly so, given the size of some past eruptions. But did you know that Yellowstone was also home to the largest glaciated area in the Rocky Mountains during the last two ice ages?

Ironically, it is Yellowstone's volcanic characteristics that make it such a good place for ice to accumulate during glacial periods.

Because of the hotspot that lies beneath Yellowstone (and that supplies the heat needed to generate magma), the Yellowstone Plateau sits at a higher elevation than its surroundings. In essence, the region is buoyed upward by the heat beneath the surface. The higher elevations mean that there is more precipitation in the Yellowstone area than elsewhere in the Rocky Mountains.

The track of the Yellowstone hotspot also helps to increase precipitation in the region. Motion of the North American plate westward over the hotspot has resulted in a series of volcanic centers that trend from the oldest, along the Nevada-Oregon border, to the youngest, where Yellowstone is today. The older volcanic areas destroyed preexisting mountain ranges and have created a lowland to the southwest of Yellowstone called the Snake River Plain. This lowland funnels storms right to Yellowstone.

Thanks to these combined effects, precipitation in the Yellowstone area is prodigious. The area sees an average of more than 12 feet (nearly 4 meters) of snow every year, with perhaps twice that much falling at the highest elevations.

During colder climatic periods, Yellowstone has been covered by ice. The two most recent glacial periods are referred to as the Bull Lake and Pinedale glaciations.

The Bull Lake glaciation covered Yellowstone in ice from about 150,000 to about 130,000 years ago, extending south of Jackson, Wyoming, and west of West Yellowstone. The extent of the ice is evident from moraines, or piles of jumbled rock that are left behind when glaciers melt. Bull Lake ice dammed rivers, and deposits from the glaciers are found above and below Yellowstone lava flows, meaning that the ice age occurred before and after times of Yellowstone eruptions.

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Yellowstone's glacier

This map shows ice cover in the Yellowstone region. Light shaded areas bounded by black and red lines indicate areas covered during the Pinedale (about 20,000-15,000 years ago) and Bull Lake (about 150,000 years ago) glaciations, respectively. Blue lines are contours in thousands of feet on the maximum reconstructed Pinedale glacier surface. The circled numbers schematically depict the southwest migration of the center of mass of the greater Yellowstone glacial system through time (in thousands of years).

The more recent period of thick ice in Yellowstone is the Pinedale glaciation, which corresponds to Earth's last "ice age" and covered the Yellowstone area in an ice cap from about 22,000 to 13,000 years ago. Pinedale ice obliterated much of the evidence of the Bull Lake glaciers in the central Yellowstone region. Pinedale glaciers also blocked the Yellowstone River, leading to the formation of a lake in Hayden Valley. The current characteristics of the valley — gentle hills and valleys with large meadows — are because of the former presence of that glacial lake. Pinedale glaciers also created the fjord-like southern arms of Yellowstone Lake.

One might wonder if all of the heat from Yellowstone's geysers and hot springs could have melted some of the glacial ice. Calculations based on the current heat, however, suggest that the thermal output from the geyser basins along the Firehole River (like Upper Geyser Basin, where Old Faithful is located) would only melt about 1 foot of ice per year from the base of the ice cap. This is trivial, given the amount of snowfall and the fact that the ice sheet was thicker than 4,000 feet in places.

After melting of the ice, there was some uplift due to "rebound" — removal of the ice load caused the ground surface to buoy upward. This is actually still occurring in places like Greenland and Scandinavia, which were covered by epic ice sheets during the last major glaciation, but it stopped in Yellowstone thousands of years ago.

Interestingly, there were no eruptions of magma after the Pinedale glaciation. Studies of glacier melting in Iceland suggest that unloading due to ice removal can cause melt formation and magma ascent, resulting in an increased number of volcanic eruptions. The last time magma reached the surface in Yellowstone was 70,000 years ago, so clearly Pinedale glacial unloading didn't have any impact on melting, perhaps because the Yellowstone magma body is mostly solid. The unloading might, however, have caused some hot water to flash to steam, resulting in a few hydrothermal explosions on the north side of Yellowstone Lake.

Yellowstone is certainly a land of fire. But as you appreciate the landscape of Yellowstone National Park, don't forget that ice has also played a major role in shaping the valleys, mountains, and geologic deposits throughout the region.

For more information on the glacial history of Yellowstone, check out a review article written by University of New Hampshire professor Joe Licciardi and retired USGS geologist Ken Pierce, published in Quaternary Science Reviews last year.

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