3D imaging enables scientists to make more accurate forecasts of volcanic unrest

New, highly precise and high-resolution 3D imaging technique has been employed on the magma chambers and volcanic "plumbing" below the Mono Craters volcanic basin, Eastern California, US. Results have provided valuable information on the size of the chambers, their shape and the possible timing of the next eruption, US Geological Survey reported on October 29, 2015.

Last known eruptions at Mono Craters area happened about 600 years ago at Panum Crater and about 350 years ago on Paoha Island in the middle of Mono Lake. However, the Mono Craters volcanic area is still placed among the most dangerous, high threat volcanoes of the nation.

Mono Lake and Mono Craters landscape overview, California, US, November 23, 2013. Image credit: Jared Peacock, USGS

USGS scientists used magnetotellurics to obtain the new 3D images. The movement of ions in the Earth's magnetic field creates an electrical current which is then measured by this technique. Electrical current depends on the rock's compositional properties. For example, older, more denser rocks have higher resistance and therefore the current flow is decreased while the opposite situation happens with fractured rocks filled with ground water fluids, or hydrothermal fluids related to volcanic activity.

Magma chambers, for example, are filled with liquid, melted rock or partially crystallized "mush" and their resistivity to electrical current is very low, accordingly.

The obtained resistivity images have uncovered at least two vertical magma columns 10 km (6 miles) deep. One of them is located below the Panum Crater, and its giraffe's neck shape suggest the existence of hydrothermal fluids near the surface. Another one, underneath South Coulee, shows no signs of recent near-surface activity. the 3D imaging technique also allowed the scientists to distinguish an older granitic rock, a pluto, between the columns of younger magma.

Three–dimensional resistivity model (view looking northwest) showing anomalies below Mono Craters. The warm red and orange colors indicate material with low electrical resistively, and the cool blue colors indicate a high resistivity to an electrical current. Note the giraffe’s neck” -shaped anomaly below Panum Crater. Black dots are earthquake hypocenters. Image credit: USGS

Results of the study were published in the Journal of Geophysical Research, on October 19, 2015.

“These new 3D images add detail to our existing knowledge of the volcanic system east of the Sierra Nevada, and enable us to make more accurate forecasts of volcanic unrest,” said Margaret Mangan, Scientist-in-Charge of the USGS California Volcano Observatory

The new electrical resistivity model in combination with previously acquired data allowed the researchers to shed light on an old question of how the volcanic systems are related to nearby faults. A conductive anomaly situated west of Mono Craters has been long known as a site of long-period earthquakes, and it turns out this correlation could be caused by volcanic fluids from the magma below or from the release of fluids from nearby hot, pressurized metamorphic rocks.

In order to establish if the long-period earthquakes are caused by magmatic or metamorphic processes more research needs to be done.

Reference:

• "Imaging the magmatic system of Mono Basin, California, with magnetotellurics in three-dimensions" – J. R. Peacock, M. T. Mangan, D. McPhee, D. A. Ponce – Journal of Geophysical Research (2015) – doi: 10.1002/2015JB012071

​​Featured image: Mono Lake and Mono Craters landscape overview, California, US, November 23, 2013. Image credit: Jared Peacock, USGS