A long-dormant supervolcano in California still holds over 1000 cubic kilometres of semi-molten magma. The find suggests the supervolcano is not entirely extinct, but a major eruption remains extremely unlikely.
Long Valley Caldera in east California is 32 kilometres across and almost a kilometre deep. It marks the spot where, 760,000 years ago, a supervolcano spewed out over 1400 cubic kilometres of material in just six days. Clouds of super-hot ash and rocks blanketed the surrounding 50 kilometres, and ash landed as far afield as Nebraska.
The volcano has not erupted on a similar scale since, although smaller eruptions continued until about 100,000 years ago. Since then it has been dormant. But since the late 1970s, the dome in the centre of the caldera has been slowly rising. So geologists have closely studied the area to find out whether it might ever erupt again.
“There have been more than 20 studies in the last 40 years,” says Ashton Flinders of the US Geological Survey in Menlo Park, California. But he says they either imaged “small features down to shallow depths of just a few kilometres” or “much deeper but only really large features”. “This has left a bit of a shadow zone in the mid crust, where the shallow studies can’t see and deeper studies tend to blur anything they do see.”
To fill in the gaps, Flinders and his colleagues pulled together data from all the seismic stations within 150 kilometres of the caldera within the last 26 years, and converted it into a three-dimensional image of the rocks. This enabled them to estimate how much magma was still molten and how much had solidified.
They identified a zone in the middle of the crust that appears to contain over 1000 cubic kilometres of magma. That is a lot of magma, but Flinders emphasises that just because it is there doesn’t mean it will erupt.
That’s because the magma will only erupt if a large fraction of it is molten. “Magmas with less than 50 per cent melt typically don’t erupt,” he says. His team estimates that 27 per cent of the Long Valley magma is molten – making a large-scale eruption unlikely.
However, it is possible that there are smaller pockets where the melt fraction is significantly higher than 27 per cent. The study could not pick out such fine details. If one region achieves a melt fraction of 50 per cent or more, it could lead to a localised, normal-sized eruption – but not a supervolcano blast.
A dying volcano
“I imagine there could be smaller eruptions and indeed they are the most likely,” says Robert White of the University of Cambridge, UK. “But there hasn’t been anything for a very long time.”
“Despite the large volume of magma beneath the caldera, that magma is extremely deep, and we haven’t seen definitive evidence of surface signals that would suggest it’s started to ascend,” adds Flinders.
A 2017 study by his USGS colleague Wes Hildreth concluded that the uplift in the centre of the caldera was caused by watery liquid escaping from the magma as it solidifies – so the uplift was not a sign of an imminent eruption. Flinders agrees, except he thinks the magma is still rather more molten than Hildreth.
It seems what we are seeing at Long Valley Caldera is a supervolcano dying slowly, its vast magma reservoir gradually crystallising into solid rock. If Flinders is right, it will take a long time before it is fully dead, and it might have a few ordinary-sized eruptions left in it – but the time when it might have unleashed a supereruption is long past.
This is a stark contrast to the Yellowstone supervolcano, which remains a live threat – albeit one that may not erupt in a big way for thousands of years. A 2015 study found 10,000 cubic kilometres of magma under Yellowstone.
Journal reference: Geology, DOI: 10.1130/G45094.1