Long Valley Caldera is a volcanic depression near Mammoth Mountain in California, which has experienced extensive hydrothermal activity over two periods of time since it formed about 767,000 years ago. The caldera is home to an active hydrothermal system, which has been extensively studied close to the surface but less well characterized deeper underground. Hot fluids flow underground and escape to the surface through hot springs, but the heat source and reservoir for the labyrinthine hydrothermal system have not been thoroughly studied.
Now, Peacock et al. have used magnetotellurics to create a three-dimensional (3-D) model of the ground down to 10 kilometers underneath the Long Valley volcanic system. To create this 3-D model, the team measured the Earth’s electrical response to natural magnetic fields at 61 stations across Long Valley Caldera.
From those measurements, the researchers produced a model of the ground’s electrical resistivity. Because geothermal systems like the one in Long Valley Caldera consist mainly of electrically conductive fluids and clays, the researchers used the contrast between those hydrothermal characteristics and the surrounding resistive host rocks to study five conductive zones under the caldera.
In studying the conductive zones, the team found several separate zones of fluids and areas of partial melt. One of the zones of fluids—located 4 kilometers below Deer Mountain—is interpreted as the reservoir for the hydrothermal system. Moreover, the heat source could be an area of partial melt at 8 kilometers in depth, although the authors explain that other plausible heat sources could exist.
The researchers also discovered two additional bodies of aqueous fluids, one beneath a valley within the caldera that’s bound by faults (graben) and one under Mammoth Mountain. Both aqueous fluid bodies could originate from zones of partial melt 8 kilometers below the surface.
These conclusions agree with past studies of the shallow part of the system, and future work will combine the new study’s findings with fluid flow modeling to confirm that researchers’ interpretations of Long Valley Caldera’s hydrothermal system are valid. (Geophysical Research Letters, doi:10.1002/2016GL069263, 2016)
—Leah Crane, Freelance Writer