https://phys.org/news/2018-06-magma-beneath-yellowstone-supervolcano.html

Some 640,000 years have passed since the volcano's last major eruption. But it can be "super," having produced one of the largest known blasts on Earth and spewing more than 2,000 times as much ash as Mount St. Helens did in 1980.

A major element in the volcano's power is the explosive, silica-rich rhyolite that break's through the Earth's crust during an eruption. Larson and his colleagues focused on the plume of basalt magma heating the rhyolite from below.