The amount of magma in the Long Valley Caldera is so large it could support an eruption equivalent to the massive one which occurred 767,000 years ago, which released 140 cubic miles of material into the atmosphere.
By comparison, the 1980 Mount St Helens eruption resulted in the release of 0.29 cubic miles.
While the Long Valley Caldera is unlikely to blow anytime soon, the report, written by scientists from the United States Geological Survey (USGS), the University of California, and the University of Rhode Island, said: “We can conclude the mid-crustal reservoir is still melt-rich.
“We estimate the reservoir currently contains enough melt to support another super eruption comparable in size to the caldera-forming eruption at 767 ka.”
The report, published by the Geological Society of America, stresses however that there is no need to start panicking, adding: “This volume and a relatively high melt fraction in no way ensures that the magma is eruptible.”
The team used cutting-edge techniques to inspect the volcano in great detail, enabling them to reach their stunning conclusions.
The Long Valley Caldera is one of the Earth’s largest calderas, measuring about 20 miles long, 11 miles wide and up to 3,000 feet (910 m) deep.
After four strong earthquakes shook the Long Valley area in 1980, USGS scientists also detected evidence of renewed volcanic unrest in the region.
They subsequently found that the central part of caldera was slowly rising.
A fact sheet issued by the USGS states: “Because such ground deformation and earthquakes are common precursors of volcanic eruptions, the USGS has continued to closely monitor the unrest in this region.
It is natural to wonder when and where the next volcanic eruption might occur in the Long Valley area. Geologic processes generally proceed at a slow pace, and when viewed on the scale of a human lifetime, volcanic eruptions and destructive earthquakes happen rarely.
“Nevertheless, the long history of volcanic activity in the Long Valley area indicates that future eruptions will occur.”
Geologists have found that after its creation in the massive eruption 760,000 years ago, clusters of smaller volcanic eruptions have occurred in the caldera at roughly 200,000-year intervals.
About 100,000 years ago, the most recent of these eruptions formed the Mammoth Knolls, low hills just north of the Town of Mammoth Lakes.
Mammoth Mountain, a young volcano on the rim of Long Valley Caldera, was created by a series of eruptions which occurred between 220,000 and 50,000 years ago.
Volcanoes in the Mono-Inyo Craters volcanic chain, which extends from just south of Mammoth Mountain to the north shore of Mono Lake in Mono County, California, have erupted frequently over the course of the last 40,000 years.
During the last 5,000 years, an eruption has occurred somewhere along this chain every 250 to 700 years.
The nearby Inyo Craters and associated lava domes were the result of a series of small to moderate eruptions 550 to 600 years ago, while the most recent eruptions along the volcanic chain took place about 250 years ago at Paoha Island in Mono Lake.
The fact sheet added: “When an eruption does break out in the Long Valley area, its impact will depend on the location, size, and type of eruption, as well as the wind direction.
“Also, an eruption during the winter months could melt heavy snow packs, generating mudflows and locally destructive flooding.”
David Rothery, Professor of Planetary Geosciences at the Open University, told Express.co.uk: “A major eruption in the next 100 years is extremely unlikely, but a there is a greater than 50 percent chance of major eruption in the next few hundred thousand years.”
The report, authored by Ashton F Flinders, David R Shelly, Philip B Dawson, David P Hill, Barbara Tripoli, and Yang Shen, states: “Despite 40 years of diverse investigations, the presence of large volumes of melt in Long Valley’s magma reservoir remain unresolved.
“Here we show, through full waveform seismic tomography, a mid-crustal zone of low shear-wave velocity.”