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The magma reservoir beneath the Yellowstone Caldera contains almost twice as much melted rock as previously thought. The increased estimate, however, doesn’t meet the threshold that suggests an eruption at Yellowstone is more likely.

The Yellowstone Caldera in north-western Wyoming is one of the largest volcanoes in the world. In the past 2.1 million years, it has seen three catastrophic eruptions that blanketed North America in ash and a number of smaller eruptions where lava flowed within the caldera, most recently 70,000 years ago. Researchers closely monitor Yellowstone for any changes that could signal an eruption, such as ground deformation or earthquakes.

Eruptions are supplied with magma from two huge reservoirs beneath the caldera, one near the mantle and one a few kilometres below the surface. Once thought of merely as “big tanks” of magma, the reservoirs contain a complex “crystal mush” of melted rock and crystals, says at the University of Illinois Urbana-Champaign. The composition of this mush partly determines the volcano’s likelihood of erupting: a higher proportion of melted rock to solid crystals makes the magma more likely to mobilise.

Because seismic waves move more slowly through areas with more melted rock, Maguire and his colleagues were able to analyse seismic data recorded around Yellowstone over the past 20 years to estimate the proportion of melted rock in the shallower magma reservoir.

Where past analyses used a simple model that treated the waves as linear rays, their analysis used supercomputers to model the waves in three dimensions to get a more complete view of the reservoir.

They found the reservoir is composed of 16 to 20 per cent melted rock on average – compared with a previous estimate of about 9 per cent – depending on assumptions made about the shape of spaces between solid crystals. That suggests the reservoir contains about 1600 cubic kilometres of melted rock, or almost twice as much as the previous estimate of roughly .

Even at the high end of their estimate, Maguire says the proportion of melted rock is still well below the 35 to 50 per cent threshold needed for an eruption. “Yellowstone can spend large amounts of its life cycle with some melt without eruption,” he says.

at the University of California, Davis, who wrote a , says, depending on how the melted rock is distributed, there could be enough for a small eruption, but this work shows there’s definitely not enough there for a catastrophic eruption.

“It’s a big improvement in our ability to understand what’s beneath Yellowstone,” says Cooper.