California's supervolcano

It is well known that a massive potential supervolcano lurks beneath Yellowstone Park in Wyoming, but it is less generally appreciated that another huge pocket of magma sits under the Long Valley caldera in eastern California.

The caldera is 32 kilometres across, one kilometre deep, and is the site of a supervolcano eruption that happened 760,000 years ago, with smaller eruptions occurring up to 100,000 years ago. The site has been dormant since then, but geologists have noted that, since the 1970s, the central dome in the caldera has been rising, albeit slowly. So is the monster waking up?

The chances of a massive eruption any time soon are extremely remote. That is because work has been done to estimate both the size of the magma chamber beneath the caldera, and the nature of the magma that it contains.

It is estimated that the chamber contains up to 1000 cubic kilometres of magma, but the good news is that only a relatively small percentage of it is molten enough to pose any sort of danger. The general consensus is that magma that is less than 50% molten will not erupt, and the figure for the Long Valley caldera is 27%. A supervolcano eruption, which would cause devastation over a vast area and lead to the deaths of millions of people, is therefore highly unlikely, but a smaller volcanic eruption at some time cannot be ruled out entirely.

However, there is also the possibility that the Longo Valley magma chamber is actually moving in the opposite direction, as an example of a supervolcano that is dying as the magma cools down. Eventually it will all crystallize into a granite batholith and may one day – millions of years into the future – emerge into the open as the overlying rock is eroded away.

The real problem for the wider region is the Yellowstone supervolcano. Not only is its magma chamber ten times larger than that underneath Long Valley, but the magma does not appear to be cooling down. However, it could be thousands of years before anything really dramatic happens.