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Yellowstone National Park, Part 1

© Geoff Habiger

In 1872, President Grant signed into law the Yellowstone Park Act, creating the first National Park in the United States. For over half a century, white explorers, trappers and traders had passed through Yellowstone. They had recounted seemingly fanciful tales of a land where the ground boiled and erupted, and smelled of sulfurous fumes. Jim Bridger, the legendary trapper and known for his many tall-tales, described the landscape as “the place where Hell bubbled up.” It wasn’t until the expeditions of Ferdinand Hayden, in 1871, that the facts of Yellowstone came to light to the public. It was his expeditions and staunch lobbying of Congress that helped get the Yellowstone Park Act passed.

Yellowstone is a region that is defined by its geology. The land has been raised, destroyed, repaved, and sculpted by geological forces. Today, it is one of the most geologically active places on the planet, with erupting geysers, steaming hydrothermal vents, and thousands of tiny earthquakes every year. But what is the engine driving this activity? How did the Yellowstone we see today become this way?

The area we now call Yellowstone National Park sits atop a hot spot. A hot spot (or mantle plume) is a point where hot, molten magma has risen from the mantle into the crust. This causes the surrounding landscape to rise and creates volcanoes as the magma flows to the surface and erupts. Hot spots tend to be relatively stable, while the lithosphere above them is in constant motion through the action of plate tectonics. Most hot spots occur under oceanic crust, such as the Hawaiian hot spot, which created the Hawaiian Islands, and the Iceland hot spot. The Yellowstone hotspot is unique in being located under a continent. Current research suggests that the magma that feeds the Yellowstone hot spot rises from only a depth of about 125 miles beneath the surface. This plume of magma rises to a depth of around 50 miles and pools beneath Yellowstone. Here, large blobs of magma rise from the top of the plume and move up through the mantle and into the crust. Here the magma heats the surrounding crust, creating magma chambers. As the magma collects, the pressure builds, and in time a volcanic eruption occurs. Unlike other hotspots, which erupt basaltic lava (which is fluid-like and usually flows out as lava), the magma under Yellowstone is rich in silica, and erupts as rhyolite. The rhyolite, being thick and viscous, creates explosive eruptions. Yellowstone’s history and all the activity we see at Yellowstone, from the geysers and hot springs to the earthquakes are the result of this hotspot.

Porcelain Basin

Lower Falls of the Yellowstone River

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