It is this heat that fuels the world's largest hydrothermal system of geysers, fumaroles and hot springs. From last week's Caldera Chronicles article we know that the ultimate source of the heat is a hotspot that transports material to the surface from deep within the Earth, but how does that heat get to the surface to drive the features that have made Yellowstone famous?

Geologic studies of past eruptions indicate that magma is stored beneath the surface of the caldera, but those data can only loosely constrain the sizes, shapes, and temperatures of magma reservoirs in the subsurface. For more information on those variables, we must turn to geophysics.

To create a window into Yellowstone's interior, a seismological inversion scheme, called tomography, was done using seismic waves from both local and very distant earthquakes recorded on the Yellowstone seismic network. Tomography is like a CAT Scan used by the medical profession, and it reveals properties of the entire magmatic system from the upper mantle to near the surface.

Based on their tomography studies, University of Utah scientists found that there were several areas of low seismic velocity at various depths beneath the caldera. Such signals often mean higher temperatures, and perhaps molten material. The map of the subsurface that the scientists produced sees the top of the hotspot, which is 70 km (43 mi) beneath the ground. The hotspot provides the heat for a large basaltic magma reservoir at 20–50 km (12–31 mi) depth, but the seismic waves suggest that only 2–5% of this body is actually molten (the rest of the volume is hot and mushy). The basaltic magma reservoir in turn provides heat for a lower-temperature rhyolite magma reservoir at depths of 4–14 km (2.5–8.5 mi), but that is only about 5–15% molten. It is this rhyolite magma that fuels Yellowstone's geothermal system of geysers and hot springs.

The basaltic magma body has a volume of 46,000 cubic kilometers (11,000 cubic miles), which is 4–5 times larger than the shallower rhyolite magma reservoir. Seismic data also indicate that the basaltic melt might extend westward beneath the Snake River Plain. In fact, pieces of this structure are probably frozen in the subsurface, marking the passage of the North American plate over the Yellowstone hotspot from its origin almost 17 million years ago in northern Nevada / southwest Idaho / southeast Oregon (see last week's article for more information on the impact of the hotspot on the western USA).

The schematic model of the Yellowstone system might also provide insights to how other magma systems around the world work. In fact, the geometry of Yellowstone's plumbing system is a good starting model for other volcanic systems that have erupted both basalt and rhyolite. Basalt is a hot but less viscous (less sticky) type of magma that is frequently generated at hotspots (like Hawaii). Rhyolite is cooler and more viscous (stickier) and can be created when continental crust melts (in this case, the basalt supplies the heat to melt the crust and make the rhyolite). Based on the Yellowstone example, it may be possible to model magma plumbing systems at volcanoes with similar geologic histories as Yellowstone – Taupo, New Zealand, might be an example – helping to better understand volcano and earthquake hazards in those areas. Yellowstone is a wonderful natural laboratory for learning more about volcanic systems across the planet!

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