What's Under Long Valley? Water, Heat, and Molten Rock!

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A picture is worth a thousand words—USGS Post Doctoral Fellow Jared Peacock's new 3D geophysical model for the Long Valley Caldera and Mammoth Mountain reveals a subsurface marked by active hydrothermal reservoirs (hot water and fluids), bodies of partial melt (molten rock), and rock that has been pervasively altered to clay by fluids from a now extinct hydrothermal reservoir.

Using magnetotelluric (MT) data, Jared located a robust hydrothermal reservoir with a source located about 4 km (2 and a half miles) under Deer Mountain. Groundwater heated in this reservoir flows upward and eastward towards the caldera's Resurgent Dome. A separate hydrothermal reservoir was identified under Mammoth Mountain at a depth of about 1 km (0.6 mi). Both reservoirs are fueled by heat emanating from small bodies of partially molten rock located more than 8 km (5 mi) below the surface. The new model verifies and improves upon the results of earlier geophysical investigations.

Model of reservoirs underneath Long Valley Caldera...

Main hydrothermal features of the new (2016) 3-D model of the subsurface at Long Valley Caldera. This schematic is based on a survey of the electrical properties of the earth (magnetotellurics) below. Arrows show subsurface water flow, with colors keyed to changing water temperature, from blue (cold) to red (hot). Purple arrows show an extinct hot water pathway. See 2016 article by Jared Peacock and others (doi:10.1002/2016GL069263).

(Public domain.)