Yellowstone Caldera

The Yellowstone Caldera, also known as the Yellowstone supervolcano, is a highly geologically active region in Yellowstone National Park. It measures 55 kilometers by 72 kilometers.

Volcanism

Yellowstone sits on top of three overlapping calderas. (USGS)

Apparently, Yellowstone is riding on one of the planet's few dozen hot spots where light, hot, molten, mantle rock rises towards the surface. Hawaii and Iceland are believed to lie over similar hot spots. The Yellowstone hot spot has a long history. Over the past 17 million years or so, successive eruptions have flooded lava over wide stretches of Washington, Oregon, California, Nevada, and Idaho, forming a string of comparatively flat calderas linked like beads, as the North American plate moves across the stationary hot spot. The calderas' apparent motion to the east-northeast forms the Snake River Plain. However, what is actually happening is the result of the west-southwest motion of the North American plate with respect to the stationary hot spot deep underneath.

Currently, volcanic activity is exhibited only via numerous geothermal vents scattered throughout the region, including the famous Old Faithful Geyser, but within the past two million years, it has undergone three extremely large explosive eruptions, up to 2,500 times the size of the 1980 Mount St. Helens eruption. The most recent such eruption occurred 640,000 years ago and spread a layer of volcanic ash over most of the North American continent. Smaller steam explosions occur every 20,000 years or so; an explosion 13,000 years ago left a 5 kilometer diameter crater at Mary Bay on the edge of Yellowstone Lake (located in the center of the caldera). Additionally, non-explosive eruptions of lava flows have occurred in and near the caldera since the last major eruption; the most recent of these was about 70,000 years ago.

The volcanic eruptions, as well as the continuing geothermal activity, are a result of a large chamber of magma located below the caldera's surface. The magma in this chamber contains gases that are kept dissolved only by the immense pressure that the magma is under. If the pressure is released to a sufficient degree by some geological shift, then some of the gases bubble out and cause the magma to expand. This can cause a runaway reaction. If the expansion results in further relief of pressure, for example, by blowing crust material off of the top of the chamber, the result is a very large gas explosion.

Welded tuff at Golden Gate was created by one eruption of the Yellowstone Caldera

Volcanic hazard

A full-scale eruption of the Yellowstone caldera could result in millions of deaths locally and catastrophic climatic effects globally. Fortunately, there is little indication that such an eruption is imminent, although study of Yellowstone is ongoing and the system is not yet completely understood. Geologists are closely monitoring the rise and fall of the Yellowstone Plateau, which averages 1.5 cm yearly, as an indication of changes in magma chamber pressure.

Occasionally proposals are suggested for ways to safely relieve the buildup of dissolved gas in the Yellowstone magma chamber, usually involving drilling holes or using explosives to release small amounts of pressure in a controlled manner. However, none of these ideas are likely to have a noticeable impact. The magma beneath Yellowstone is not very mobile, so release of dissolved gases from any given point will not do much to the chamber as a whole, and in any event, the scale of the problem is far too large for current engineering capabilities to handle.

Earthquakes

In addition to the volcanic activity, several pre-existing, active geologic faults run through and near the caldera. The combination of these tectonic faults with the movement of magma and hot water results in numerous small earthquakes; in 2002 about 2,300 earthquakes were recorded in Yellowstone park. Most of these were too small to be felt by humans. Occasional large earthquakes occur as well, the most recent being the 1959 Hebgen Lake earthquake (magnitude 7.5) and the 1975 Norris earthquake (magnitude 6.1). Earthquakes probably present the most immediate threat to human life in Yellowstone.

External links

• Yellowstone Hot Spot and the Snake River plain. (http://volcano.und.nodak.edu/vwdocs/volc_images/north_america/yellowstone.html)
• Yellowstone Volcano Observatory (http://volcanoes.usgs.gov/yvo/)
• USGS Fact Sheet - Steam Explosions, Earthquakes, and Volcanic Eruptions—What’s in Yellowstone’s Future? (http://pubs.usgs.gov/fs/2005/3024/)de:Yellowstone (Vulkan)