Buoyant material from a hot spot rises through the upper mantle, bringing heat from the Earth's interior closer to the surface.

The Yellowstone hot spot impinges on the base of the North American plate, one of several rigid plates that make up the Earth's crust. These plates move a few inches per year with respect to the stationary hot spots and each other, sometimes causing great earthquakes as the plates collide, grind past one another, or split apart.

The Yellowstone hot spot has interacted with the North American plate for perhaps as long as 17 million years, causing widespread outpourings of basalt that bury about 200,000 square miles in Washington, Oregon, California, Nevada, and Idaho under stacks of lava flows half a mile or more thick.

Some of the basaltic melt, or magma, produced by the hot spot accumulates near the base of the plate, where its heat melts rocks from the Earth's lower crust.

These melts, in turn, rise closer to the surface to form large reservoirs of potentially explosive rhyolite magma.

Catastrophic eruptions have partly emptied some of these reservoirs, causing their roofs to collapse.

The resulting craters, some of which are more than 30 miles (50 kilometers) across, are known as volcanic calderas.

Because the plate was moving an inch or so per year southwestward over the hot spot for millions of years as the calderas formed, groups of calderas are strung out like beads on a string across parts of Idaho and Wyoming.

From: Dzurisin, Christiansen, and Pierce, 1995, Yellowstone: Restless Volcanic Giant: VOLCANO HAZARDS FACT SHEET: USGS Open-File Report 95-59

Yellowstone lies at the intersection of the Basin and Range tectonic province, dominated by E-W extension, and the eastern Snake River Plain, a linear downwarp or graben that has been a locus for basaltic volcanism since middle Miocene time.

According to one popular model, the rhyolitic Yellowstone Plateau marks the current location of a "hotspot" or melting anomaly in the upper mantle, and the basaltic Snake River Plain records the hotspot's northeastward track across the mobile North American Plate.

From: Newhall and Dzurisin, 1988, Historical Unrest at Large Calderas in the World: USGS Bulletin 1855

A few hotspots are thought to exist below the North American Plate.

Perhaps the best known is the hotspot presumed to exist under the continental crust in the region of Yellowstone National Park in northwestern Wyoming.

Here are several calderas (large craters formed by the ground collapse accompanying explosive volcanism) that were produced by three gigantic eruptions during the past two million years, the most recent of which occurred about 600,000 years ago.

Ash deposits from these powerful eruptions have been mapped as far away as Iowa, Missouri, Texas, and even northern Mexico.

The thermal energy of the presumed Yellowstone hotspot fuels more than 10,000 hot pools and springs, geysers (like Old Faithful), and bubbling mudpots (pools of boiling mud).

A large body of magma, capped by a hydrothermal system (a zone of pressurized steam and hot water), still exists beneath the caldera.

Recent surveys demonstrate that parts of the Yellowstone region rise and fall by as much as 1 cm each year, indicating the area is still geologically restless.

However, these measurable ground movements, which most likely reflect hydrothermal pressure changes, do not necessarily signal renewed volcanic activity in the area.

From: Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate Tectonics: USGS Special Interest Publication