Out of more than 1,000 known volcanoes in the world, only around 20 of them are classed as so-called supervolcanoes. The latter is thousands of times more powerful than its smaller sister eruption and is driven by buoyant magma.
Classed as VEI 8 on the eruption scale, supervolcanoes deposit material greater than 1,000 cubic kilometres, or 240 cubic miles, when they erupt.
Their locations are wide and varied, but some of the world’s most dangerous sit in the US, like California’s Long Valley Caldera.
One of the Earth’s largest calderas, Long Valley sits next to Mammoth Mountain and has struck fear into volcanologists for decades for, if it erupted, there is little anyone could do.
The valley measures around 20 miles long and 11 miles wide and is up to a staggering 3,000 feet deep.
It was formed 760,000 years ago when a huge eruption in the region released hot ash that eventually settled to form Bishop Tuff, a large outcrop that resembles a mountain but is made entirely of volcanic flows.
The caldera hasn’t erupted with such ferocity ever since, but this has only raised concerns among expert circles.
More recently, creeks and hot springs around the area that Long Valley covers appeared to be coming to life, belching a bubbling with hot water — ordinarily signs that something was ready to give beneath the surface.
Although nothing came of the resurgent activity, researchers during the Science Channel’s documentary, Secrets of the Underground, explored the potential for the caldera to blow its lid again.
Rob Nelson, a scientist, noted that around the park there are constant “alarming signs of possible volcanic activity” given the geothermal energy beneath the caldera.
“And there are clues pointing towards an imminent eruption scattered throughout this valley — the site of the second largest explosive volcanic eruption in North America,” he said.
An event in the 21st century wouldn’t come close to that witnessed 760,000 years ago. The devastation would, however, still swallow up the entire region, and pose grave knock-on effects for the rest of the world. Mr Nelson noted that an eruption posed an “existential threat” the the millions who live around Long Valley.
In the 1970s, a swarm of earthquakes emanated from the caldera which sent scientists on high alert. Then, in May 1980, an earthquake that measured a magnitude of six on the Richter scale struck the southern margin of the caldera, coinciding with an uplift in the floor which signalled a growth in magma.
Things have since calmed down, but scientists now see the events as part of a longer theme of resurgent activity that comes up to the present day.
When the Science Channel set out across the Caldera, their investigation found several instances of smoke rising from beneath the ground.
Geophysicist Jared Peacock also identified a worrying feature of the caldera. Using InSAR remote sensing technology — which creates a detailed map of a region — he found a “resurgent dome” near Mammoth Lakes, a town in the Sierra Nevada mountains.
More tests confirmed that liquid was accumulating beneath the dome’s surface, usually a sign of volcanic activity. However, the activity wasn’t centralised but rather scattered, meaning that for now at least, it isn’t a cause for concern.
“We can say conclusively that there is no giant magma chamber below. But there are smaller satellite ones around the area,” noted Mr Peacock.
In 2018, researchers writing in a paper published in the journal GeoScience World revealed evidence they found of ground deformation at Long Valley.
They discovered an “ongoing uplift [which] suggests new magma may have intruded into the reservoir” since at least 1978.
The uplift could prove that molten rock has been moving below the ground, or that material deep within the chamber has been crystallising.
The study reads: “Despite 40 years of diverse investigations, the presence of large volumes of melt in Long Valley’s magma reservoir remains unresolved.”
