The geology of Yellowstone National Park, and its precursors in Oregon and Idaho
October 23, 2020
Yellowstone has the distinction of being the first national park in the U.S., and probably in the world. President Ulysses S. Grant signed the bill to protect more than one million acres—mostly in Wyoming—in 1872. Yellowstone was set aside because of its hydrothermal displays—the park contains more than 10,000 features, including the world’s greatest concentration of geysers, hot springs, mud pots, and steam vents.
During recent decades, as the “super-volcano” idea has been popularized, visitors have become more aware of the volcanic origin of Yellowstone Park. A volcano is characterized as a super volcano if the Volcano Explosivity Index (VEI) is greater than 8, meaning the volume of the measured eruptive deposits are >1,000 cubic km (240 cubic miles).
Most of Yellowstone’s eruptions have been rhyolite—silica-rich magma often containing gases that make the eruptions highly explosive. The three extraordinarily large explosive eruptions in the past 2.1 million years (illustrated above) each created a giant caldera within or west of Yellowstone National Park (see map below). During these eruptions, enormous volumes of hot ash, pumice, and other rock fragments spread outward as pyroclastic flows over vast areas where they were welded together to form extensive sheets of hard lava-like rock, called ash-flow tuffs or ignimbrites, that in places are more than 400 m (1300 ft) thick!
The largest eruptions, which created the Huckleberry Ridge, Mesa Falls, and Lava Creek Tuffs, account for more than half the material erupted from Yellowstone in the past 2.1 million years. Because such enormous amounts of magma were erupted during each explosive event, the roof of the magma chamber collapsed, and the ground above subsided by many hundreds of meters to form the calderas.
But why have these vast volumes of lava and pyroclastic flows erupted at a location that is in the middle of the North American plate? Most volcanic action occurs at plate boundaries. For example, the volcanoes of the Cascade Range are a result of the Cascadia subduction zone, a convergent type of plate boundary (see post on October 16, 2020). In contrast, Yellowstone is a volcano that has formed over a “hot spot” fed by plumes of hot material rising from deep in the Earth, perhaps from as deep as the outer core. As a tectonic plate moves over a hot spot, a chain of volcanoes is produced, with volcanoes younging in the direction from which the plate is moving.
But what’s the vast grey area toward the northwest of the hot spot track that’s labeled Columbia River Basalts? Surprisingly, these volcanic flows are also a result of the hot spot, but they went flowing off in a direction away from the hot spot! The reasons for this are too complicated to explain in this post, but suffice it to say that the continent changes dramatically at the eastern edge of Oregon. Most of Oregon consists of disparate terranes (crustal pieces with diverse origins) that have been “glued” together over millions of years. So most of Oregon has continental rock that is relatively thin and weak. In contrast, the continental rock to the east is older, thicker, and more consolidated. As the continent was moving to the southwest over the hot spot, the top of the plume was sheared off at the discontinuity. This caused a lot of melting along the boundary between weaker and stronger parts of the continent.
Why did two different types of materials erupt from the same hot spot: silica-rich, caldera-forming, mostly pyroclastic flows and air falls to the northeast, and silica-poor, highly fluid lavas to the northwest? Mainly, this is because the magma under volcanoes to the northeast of the hot spot were traveling upward through continental crust and picking up more silica along the way. In contrast, the magma under fissures to the north of the hot spot were traveling upward through crust that was thinner and also less silica rich.
Finally, what is the likelihood that Yellowstone will erupt again? Despite the sensationalism associated with a “super volcano”, the likelihood of Yellowstone erupting with a vast, caldera-forming explosion is highly unlikely in the context of our human time scale. The most likely eruptions would be a hydrothermal explosion or a lava flow within the pre-existing caldera. The USGS’s Yellowstone Volcanic Observatory has a rigorous monitoring program and will let us know if trouble is brewing!