Volcanism

While there are no active volcanoes in the Northwest Central US today, past volcanism has left its mark on the area. Igneous activity continues today in and around Yellowstone National Park in northwestern Wyoming, which overlies a hot spot in the Earth’s mantle. During the Cenozoic, as the North American plate traveled over this mantle plume, the crust melted and produced a trail of volcanic rock that crosses southern Idaho, forming the Snake River Plain and ending at Yellowstone National Park. The trail of volcanic eruptions from the hot spot works its way east along this path. For example, the rocks at Craters of the Moon National Monument in southeastern Idaho formed during eight major eruptive periods between 15,000 and 2000 years ago. During this time, lava associated with the Yellowstone hot spot erupted from the Great Rift, a series of deep cracks that start near Craters of the Moon’s visitor center and stretch 84 kilometers (52 miles) to the southeast. Over the course of eruption, the lava field grew to cover 1600 square kilometers (618 square miles).

The recent geological history of volcanism at Yellowstone has led the area to be classified as a supervolcano—a volcano capable of producing more than 1000 cubic kilometers (240 cubic miles) of ejecta. Supervolcanoes can occur when magma rises under the crust from a hot spot, but is unable to break through. Eventually, the crust ruptures when it can no longer contain the built-up pressure. Although the Yellowstone area contains no active volcanoes today, the Yellowstone hot spot was the source of several prehistoric supereruptions (Figure 10.8): the Huckleberry Ridge, 2.1 million years ago, which produced 2450 cubic kilometers (588 cubic miles) of ejecta; the Mesa Falls flow, 1.3 million years ago, which produced 280 cubic kilometers (67 cubic miles) of ejecta; and the Lava Creek flow, 630,000 years ago, which produced 1000 cubic kilometers (240 cubic miles) of ejected material. The Mount St. Helens eruption in 1980, by contrast, produced only 0.19 cubic kilometers (0.046 cubic miles) of ejecta. While there is concern about another supereruption occurring at Yellowstone, the probability of an explosive eruption within the next few thousand years is very low.

Figure 10.8: The extent of the three most recent ashfalls from Yellowstone supervolcano eruptions, as compared to the eruption of Mount St. Helens in 1980.