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Measuring the temperature of lava erupting on the Earth’s surface is possible with the right equipment. But how do you figure out the temperature of something miles below the surface? This week, we explore how scientists have tried to take the temperature of the Long Valley Caldera. 

A drill rig mounted on a box truck suspends a cable over a square hole in the ground lined with wooden slats. One scientist in a green shirt and ball cap is halfway in the hole, while another in a safety helmet and gloves uses a long-handled tool to make an adjustment to equipment on the ground. In the background, rocky peaks dotted with snow rise above sagebrush-covered hills.
USGS scientists measuring temperature in a deep borehole in Long Valley Caldera with the Sierra Nevada mountains in the background.  USGS photo by Chris Farrar.

Long Valley Caldera in eastern California was formed 767,000 years ago following the eruption of magma that deposited the Bishop Tuff. No volcanic eruption has occurred in the caldera in the past 100,000 years, which might suggest that any remaining magma has solidified and cooled. However, the uplift of a resurgent dome in the center of the caldera is thought to be driven by a deep and hot magma intrusion. The uplift is significant - almost one meter (almost 40 inches) since 1980! Additionally, imaging the caldera’s subsurface with seismic waves (a method called seismic tomography) has suggested that partially crystalized magma may exist at depths of about 5 kilometers to more than 20 km (3 to > 12 miles). Another imaging method called magnetotellurics (measuring the electrical resistivity of the subsurface) found conductive layers that may be partially crystallized magma, at depths below 8 kilometers (5 miles).

These methods suggest that there might be molten magma below Long Valley, but other observations lead to different conclusions about what’s found in the upper few miles below the caldera. Studies have shown that hot water supplying the Casa Diablo geothermal power plant and springs at Hot Creek comes not from deep within the caldera but rather from Sierra Nevada snowmelt that seeps underground. This surface water is then heated to temperatures as high as 220°C (428°F) as it comes near partially molten rock under the western part of the caldera and then flows eastward toward Casa Diablo and Hot Creek. Temperature measurements in deep boreholes show much lower temperatures than would be expected if the water was heated by magma under the caldera; for example, the temperature at a depth of 3 kilometers (2 miles) in the Long Valley Exploratory Well in the center of the resurgent dome is only 100 °C (212°F). An alternative to the intrusion model to explain uplift of the resurgent dome that is consistent with the low measured temperatures in the boreholes suggests that episodic release of water-rich fluids from a nearly crystalized magma generate large pressures capped by a low-permeability lid.

So we’re still stuck with the question, how hot is Long Valley Caldera’s deep subsurface? Since it’s very hard to make direct observations at depth, the answer will require reconciling diverse proxy datasets that can be collected from the surface – things like new and more detailed seismological and geophysical surveys, more precise measurements of the resurgent dome’s motion, and careful monitoring of the hydrothermal system in the caldera. 

For a more detailed discussion of Long Valley Caldera’s deep and shallow hydrothermal systems, check out https://pubs.usgs.gov/publication/fs20183009

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