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In 1922, Dr. Thomas Jaggar, MIT professor and founder of the Hawaiian Volcano Observatory, took a horse-pack trip through Yellowstone. After the journey, Jaggar stated: "Anyone who has spent summers with pack-train in a place like Yellowstone comes to know the land to be leaping. ... The mountains are falling all the time and by millions of tons. Something underground is shoving them up."

How prophetic were Jaggar's interpretations of Yellowstone's dynamic geology and how volcanic hotspots help shape Earth's surface? In fact, Yellowstone has been shaping the western United States for nearly 17 million years.

The Yellowstone hotspot is among the largest of some 30 active hotspots on Earth. Most hotspots are beneath the world's ocean bottoms. Giant plates of Earth's crust make up the sea floor, and these plates slowly drift over erupting hotspots, creating island chains such as Hawaii. Other hotspots lie beneath Iceland, the Galapagos Islands, and the Azores.

The Yellowstone hotspot started about 16.5 million years ago beneath what is now northern Nevada near its border with Oregon and Idaho. As the continent drifted southwest at about one inch per year, the hotspot in effect moved northeast beneath Oregon's southeast corner, then across Idaho to Wyoming along a track known as the Yellowstone-Snake River Plain. The hotspot is now beneath Yellowstone National Park, where it fuels one of the largest active continental silicic volcanic fields in the world.

The passage of the North American Plate over the Yellowstone hotspot was far from peaceful. During the past 16.5 million years, dozens of catastrophic eruptions occurred, each leaving a caldera that measured as much as a few tens of miles wide. Because North America was drifting southwest over the hotspot, the eruptions formed a chain of progressively younger calderas stretching from the Idaho-Oregon-Nevada border 500 miles northeast to Yellowstone. Many of these calderas overlapped.

Giant eruptions happen so infrequently on Earth that none have occurred in recorded history, but they are the largest volcanic eruptions known in the geologic record. As caldera-forming eruptions progressed across Idaho to Yellowstone, volcanic ash was repeatedly dumped on half the United States, and existing mountains, like the Teton Range, either were blown away or sank into the giant craters.

The tremendous heat supplied by the hotspot created a bulge in Earth's crust 300 miles across and 1,500 feet tall. This bulge moved across the continent over time due to motion of the plate over the hotspot, like a rug being dragged over an object. In the wake of the hotspot, the surface sank by as much as 2000 feet and was flooded by a series of basaltic lava flows. As a result, the old calderas -- essentially old versions of present-day Yellowstone, which also used to be located at higher elevations -- are now hidden beneath the Snake River Plain lava flows. What was left was a vast valley -- the eastern Snake River Plain – full of fertile volcanic soils that help to grow Idaho's famous potatoes.

The hotspot has been active in the Yellowstone region for about the past 2 million years, producing three cataclysmic caldera eruptions interspersed between smaller lava eruptions in that time. The first such eruption, 2.08 million years ago, produced the Huckleberry Ridge Tuff and the Island Park caldera. Activity next occurred from a smaller center in eastern Idaho -- the Henry's Fork Caldera and the Mesa Falls Tuff -- 1.2 million years ago. Finally, another eruption 630,000 years ago formed the Yellowstone caldera and Lava Creek Tuff. Subsequent flows of sticky rhyolite lava, the youngest of which erupted 70,000 years ago, filled Yellowstone caldera, obscuring the presence of the crater. The three large eruptions from the Yellowstone system respectively, were 2,500, 280, and 1,000 times larger than the 1980 eruption of Mount St. Helens in Washington state.

The next time you look at a map of the United States, have a gander at the Snake River Plain area – the landscape of which was produced by the Yellowstone hotspot – and think about all the fertile soil that was made possible by ash deposits across the west!

Graphic depicting the path left by the Yellowstone hotspot as the c...
Yellow and orange ovals show volcanic centers where the hotspot produced one or more caldera eruptions- essentially "ancient Yellowstones"- during the time periods indicated. As North America drifted southwest over the hotspot, the volcanism progressed northeast, beginning in northern Nevada and southeast Oregon 16.5 million years ago and reaching Yellowstone National Park 2 million years ago. A bow-wave or parabola-shaped zone of mountains (browns and tans) and earthquakes (red dots) surrounds the low elevations (greens) of the seismically quiet Snake River Plain. The greater Yellowstone "geoecosystem" is outlined in blue. Faults are in black. Used with permission from "Windows into the Earth, The Geologic Story of Yellowstone and Grand Teton National Park", Robert B. Smith and Lee J. Siegel, Oxford University Press, 2000.

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