Ash from a volcanic eruption is formed when magma blasts to the surface and shatters into fragments of abrasive glass, pumice and minerals. Since the ash is hot, it is less dense than the air and rises. A large eruption that generates lots of heat will carry the ash high into the atmosphere. The Mount Pinatubo eruption in the Philippines in 1991 shot ash up into the air 114,000 to 130,000 feet high. Commercial jets fly at about 30,000 feet.

Only about one-third of what’s ejected when a volcano erupts becomes ash. Another third falls back into the crater and the final third is composed of ground-hugging gas clouds and ignimbrites – a mixture of ash, pumice and fragments of rock – that may only travel 50 to 100 miles from the crater.

By using Yellowstone’s supervolcano as a basis, Mastin and colleague Alexa Van Eaton were able to plug in numbers that would give them a greater understanding of how the heat, mass and particles from a big eruption could generate its own wind by rising up to the edge of the stratosphere, miles above Earth, counteracting the predominant jet stream winds. Such a plume rising up from an eruption creates an umbrella cloud.

The plumes from smaller eruptions tend to look like a fan, spreading downwind from the volcano – like smoke from a smokestack.