What lies beneath Yellowstone?

Description There is more magma than previously recognized, but it may not be eruptible What controls how and when a volcano will erupt? Despite some notable successes (1, 2), forecasting volcanic eruptions remains a challenge—not least because there is no way to directly see what is happening beneath volcanoes. Instead, indirect methods are used to glimpse conditions below the surface. An obvious but key requirement for an eruption is the presence of magma (molten rock, consisting of variable proportions of liquid, solid crystals, and volatiles). This magma also needs to be distributed so that it can mobilize and erupt as a coherent body. Therefore, a key issue for eruption hazard assessment is to ascertain how much magma is below the surface and where. On page 1001 of this issue, Maguire et al. (3) modeled seismic data to image melt (the liquid part of magma) beneath the Yellowstone Caldera. They conclude that more melt is present than had been recognized, and it is located at shallow depths in the crust.