Using Seismic Waves to Image the Yellowstone Magma Storage Region
How do we know what's beneath Yellowstone, and how can we image the shallow magma? Seismologists at the University of Utah (UU, a YVO member agency) have worked with several other institutions to create an image of the Yellowstone magma reservoir using a technique called seismic tomography. By looking closely at data from thousands of earthquakes, they discovered that there are two magma reservoirs—one shallow and one deep—and that they are much larger than originally believed.
To create an "image" of the magma storage region (reservoir) beneath Yellowstone, the research teams reviewed data from thousands of earthquakes. Seismic waves travel slower through hot, partially molten rock and faster in cold, solid rock. Researchers make a map of the locations where seismic waves travel slower, which provides a sub-surface image of the hot or partially molten bodies in the crust beneath Yellowstone.
The deeper magma storage region extends from 20 to 50 km depth, contains about 2% melt, and is about 4.5 times larger than the shallow magma body. Seismologists at the University of Utah, the University of New Mexico, and the California Institute of Technology published the 2015 study in the journal Science, which examines the magmatic connection between the deep Yellowstone mantle plume and its shallow crustal magma reservoir.
To obtain the deeper view of the Yellowstone volcano plumbing system, the research team used earthquake recordings from the Yellowstone Seismic Network together with seismic data from the EarthScope Transportable Array (a network of seismometers that span the U.S.). The Yellowstone seismic network has closely spaced seismometers that are better at making images of the shallower crust beneath Yellowstone, while data from EarthScope's seismometers are better at making images of deeper structures.
The shallower magma storage region is about 90-km long, extends from 5 to 17 km depth, and is 2.5 times larger than a prior, less accurate, study indicated. In 2014, UU and the Swiss Federal Institute of Technology seismologists published this research in the Journal of Geophysical Research Letters. The magma reservoir contains between about 5 and 15% molten rock (melt) that occupies pore spaces between solid (crystalline) material. Although this is the crustal magma storage region that has fueled Yellowstone's past volcanic activity, magma typically does not erupt unless it has greater than 50% melt.
It is important to note that this shallow low-velocity body extends about 15 km NE of the caldera at depths shallower than 5 km. This shallow region to the northeast is most likely due to the presence of hot water, other fluids, and gasses coming off of the main body of the magma reservoir. This northeasterly progression of the magma system is consistent with the southwestern movement of the North American Plate at ~2.35 cm/yr over a stationary plume of hotter mantle material (the Yellowstone hot spot) that is located beneath about 60-90km.
The magma reservoir at 20-50 km depth is a key connection of a continuous magma conduit between the mantle plume (at depths up to ~60 km) and the shallow crustal magma reservoir at 5-15 km. Importantly, it gives a much more complete picture of the "volcanic plumbing system" of the Yellowstone hotspot and how magma and heat are transferred from the mantle to the surface.
These findings do not increase the assessment of volcanic hazard for Yellowstone—the inferred magma storage region is no larger, the research simply makes a better image of the magmatic system. Simply, we have more key information about how the Yellowstone volcano works.