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Its volcanic activity is currently 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. 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 explosive eruptions, similar to the 1980 Mount St. Helens eruption, 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). As a result of all this activity small earthquakes are common; in 2002 about 2,300 earthquakes were recorded in Yellowstone park. Most of these were too small to be felt by humans.

These 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.

A full-scale eruption of the Yellowstone supervolcano could result in millions of deaths locally and catastrophic climactic effects globally. Fortunately there is little indication that such an eruption is imminent in the near future, 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 per year, 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 is not going to 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.

Apparently Yellowstone is riding on one of the planet's few dozen hot spots, where light hot molten mantle rock rises towards the surface. HawaiiFor the 1959 novel and 1966 movie, see Hawaii (novel). Hawaii ( Hawaiian/ Hawaiian English: Hawai‘i with the ‘okina is the archipelago of the Hawaiian Islands in the Pacific Ocean. Hawaii constitutes the 50th state of the United States, and as of the 2000 and IcelandThe Republic of Iceland is an island nation in the northern Atlantic Ocean, located between Greenland and Scotland, northwest of the Faroe Islands. Lydveldid Island ( In Detail) (Full size) National motto: none Official languageNone. Icelandic de facto''. 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 plate20th century. Plate tectonics (from the Greek word for "one who constructs", τεκτων tekton ) is a theory of geology developed to explain the phenomenon of continental drift. In the theory of plate tectonics the outermost par moves across the stationary hot spot. The calderas' apparent motion to the east-northeast forms the Snake River PlainThe Snake River Plain is a geological feature of (primarily) the American state of Idaho. It stretches 400 miles (650 km) westward from northwest of the state of Wyoming to the Idaho- Oregon border. The plain is a wide flat arcuate depression. It is conca. 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.