Q:   How large is the magma chamber under Yellowstone? And is it growing?
— Kit W.
A:  

The magma chamber is believed to be about 40 by 80 kilometers (25 by 50 miles) across, similar in size to the overlying Yellowstone caldera. The top of the chamber is about eight kilometers (5 miles) deep and the bottom is around 16 kilometers (10 miles) deep. However, the chamber is not completely filled with fluid magma. It contains a partial melt, meaning that only a portion of the rock is molten (about 10 to 30 percent); the rest of the material is solid but, of course, remains hot.

The method that scientists use to discern this information is similar to medical CT scans that bounce X-rays through the human body to make three-dimensional pictures of internal tissue. In an analogous manner, a method called seismic tomography uses hundreds of seismograms to measure the speed of seismic waves from earthquakes and small, intentional dynamite explosions — data that allow geophysicists to make 3-D pictures of structures within the Earth. Scientists compare these seismic velocities, and infer the composition from deviations of these from average, thermally undisturbed values. (From the Yellowstone Volcano Observatory Web site.)

Regarding the part of the question asking whether the magma chamber is growing, based on heat loss or energy output of the system, the magma chamber appears to be crystallizing on the order of about 0.1 km3/year.
— Lisa Morgan, Research Geologist

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Q:   Is the same amount of detective work and vigilance directed toward the Yellowstone caldera also being applied to the Long Valley, Calif., caldera?
— Tien L., Seattle
A:   Yes, the U.S. Geological Survey established the Long Valley Volcano Observatory prior to the Yellowstone Volcano Observatory. Please see U.S. Geological Survey Hazards Program Web site for more information.
— Lisa Morgan, Research Geologist

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Q:   What would be the initial signs of an eruption at Yellowstone? And how much warning could we expect to have before a major eruption?
— Chris H.
A:  

Precursors to volcanic eruptions include strong earthquake swarms and rapid ground deformation, and typically take place days to weeks before an actual eruption. Scientists at the Yellowstone Volcano Observatory (YVO) closely monitor the Yellowstone region for such precursors. They expect that the buildup to larger eruptions would include intense precursory activity (far exceeding background levels) at multiple spots within the Yellowstone volcano. As at many caldera systems around the world, small earthquakes, ground uplift and subsidence, and gas releases at Yellowstone are commonplace events and do not reflect impending eruptions.

In addition to increased earthquake activity, active monitoring would detect significant changes in gas chemistry, thermal features, and uplift and/or deformation. Given the currently deployed system of multiple seisimic monitors, gas surveys and deformation analyses in place in Yellowstone, we would know well in advance of a pending large eruption.
— Lisa Morgan, Research Geologist

Lisa Morgan is a research geologist for the U.S. Geological Survey. With co-author Ken Pierce, Morgan developed major concepts and a model for development of the Yellowstone hot spot, documenting the northeastward spatial and temporal progression of volcanism, faulting and uplift along the 16-Ma Snake River Plain-Yellowstone Plateau volcanic province. Morgan has worked in this province over for 20 years. She has a Ph.D. in geology and geophysics from the University of Hawaii where she worked with George P. L. Walker concentrating on physical volcanology and the evolution of large caldera systems.

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