USGS Home Contact USGS VHP Home

Science-related Questions

How fast is the hotspot moving under Yellowstone?

Actually, the source of the hotspot is more or less stationary at depth within the Earth, and the North America plate moves southwest across it. The average rate of movement of the plate in the Yellowstone area for the last 16.5 million years has been about 4.6 centimeters annually. However, if shorter time intervals are analyzed, the plate can be inferred to have moved about 6.1 centimeters per year from 16.5 million years ago until about 8 million years ago, then slowed to 3.3 centimeters a year for the past 8 million years.

How large is the magma chamber that is currently under Yellowstone? How much magma is there under Yellowstone?

The term magma chamber is commonly thought to be an area of 100% melted rock. However, scientists have found that nearly all magma bodies contain a mixture of solid and melted rock (geologists call the mixture a partial-melt). One way to think about a magma chamber or magma body is to picture a water-filled sponge. The sponge would represent the solid rock whereas the water would represent the melt. Beneath Yellowstone, on average, the magma is about 90% solid rock (like a hot sponge) containing 10% liquid rock in its pores. Some regions may have higher melt fractions, and some may contain lower melt fractions.

Geophysical studies cause us to conclude that the Yellowstone magma body is shaped like a banana lying on its side with ends pointing up - it has a volume of approximately 15,000 cubic km. This irregular-shaped body extends beneath the caldera and is about 60 km long NE-SW and up to 40 km wide. The magma body is closest to the surface near its ends where the top of the body is about 5-6 km deep and bottom of the body is about 16 km deep. The two volcanic resurgent domes at Yellowstone are near to the areas where the magma body is closest to the surface.

The size, shape, location, and composition of the magma body is determined by seismic studies called seismic tomography. Medical CAT scans, which bounce X-rays through the human body to make three-dimensional pictures of internal tissue, are a similar diagnostic technique. Seismic tomography uses seismograms from thousands of local earthquakes to measure the speed of sound waves through the earth. Geophysicists compare the velocity of the sound waves at Yellowstone to the faster waves moving through normal cold rock. From these comparisons the geophysicists can create 3-D images of structures deep in the Earth, and infer the composition of the structures. A decrease in velocity from the average value can be interpreted to be a hot, melt-rich volume. Given the pressure, temperature, and uncertainties in their models, the scientists estimate that the melt part could be about 10% of the low velocity volume or about 1,500-2,000 cubic km. It's important to understand that this tomographic technique gives a very coarse look at the subsurface. We are averaging data over many kilometer-sized distances so that we cannot look at small (< 1 km) regions of the subsurface.

Why is Yellowstone called Yellowstone?

Contrary to popular belief, Yellowstone was not named for the abundant yellow-colored rhyolite lavas in the Grand Canyon of Yellowstone that have been chemically altered by reactions with steam and hot water to create vivid yellow and pink colors. Instead, the name was attributed as early as 1805 to Native Americans who were referring to yellow sandstones along the banks of the Yellowstone River in eastern Montana, several hundred miles downstream and northeast of the Park.

Are earthquakes at Yellowstone related to volcanism?

Earthquakes, volcanism, and hydrothermal features go hand in hand at Yellowstone. The underground plumbing of hot water and magmas beneath Yellowstone is influenced by the same stresses that cause earthquakes. The largest historic earthquake in the Rocky Mountains occurred in the Yellowstone region. The magnitude 7.5 earthquake northwest of the National Park in 1959 was caused by the extension or stretching of the Earth's crust. The earthquake caused the land to rupture, displacing a 40-km-long fault that rose vertically up to 20 feet. The same stretching of the Earth's crust that caused the 1959 earthquake causes a reduction in the horizontal pressures on Yellowstone's magma systems, allowing them easier ascent through the crust. Thus, when earthquakes occur in or around Yellowstone, they can change the stress on hydrothermal and magma reservoirs, leading to enhanced fluid movement. Conversely, volcanic processes can induce earthquakes.

What is the relationship between volcanism and the geysers and hot springs in Yellowstone?

Refer to Figure 4. Heat and volcanic gases from slowly cooling magma rise and warm the dense salty water that occupies fractured rocks above the Yellowstone magma chamber. That brine, in turn, transfers its heat to overlying fresh groundwater, which is recharged by rainfall and snowmelt from the surface. Water boiling at depth below the surface is hotter than the temperature of boiling at the surface. If it rises quickly, this superheated water can flash to steam, propelling both steam and hot water to the surface as a geyser. More commonly, hot water rises and loses its heat at a steady rate, flowing to the surface as a hot spring.

Do you have a list of references that discuss volcanic activity in Yellowstone National Park?

Christiansen, R. L., 1984, "Yellowstone magmatic evolution: Its bearing on understanding large-volume explosive volcanism." Explosive Volcanism, Its Inception, Evolution and Hazards, Washington, D.C., National Academy of Sciences, p. 84-95.

Christiansen, R. L., 2001, "The Quaternary and Pliocene Yellowstone volcanic field of Wyoming, Idaho and Montana." U.S. Geological Survey Professional Paper 729-G, p. 145.

Fournier, R. O., 1989, "Geochemistry and dynamics of the Yellowstone National Park hydrothermal system."Annual Reviews of Earth and Planetary Sciences, v. 17, p. 13-53.

Fournier, R. O., 1999, "Hydrothermal processes related to movement of fluid from plastic into brittle rock in the magmatic-epithermal environment." Economic Geolology, v. 94, p. 1193-1212.

Smith, R. B., and Christiansen, R. L., 1980, "Yellowstone Park as a window on the Earth's interior." Scientific American. v. 242, p. 104-17.

Smith, R. B., and Braile, L.W., 1994, "The Yellowstone Hotspot." Journal of Volcanology and Geothermal Research, v. 61, p. 121-88.

Smith, R. B., and Siegel, L., 2000, "Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Parks." New York, Oxford University Press, p. 242.

More References