After someone learns about the massive, devastating eruptions that have been unleashed from the Yellowstone Caldera, their usual response is two-fold: Will that happen again? And how much warning would we get? In addition to those incredible events, however, Yellowstone and other calderas like it see smaller eruptions of lava much more frequently. These "small" eruptions are still about the size of the largest eruptions the world has seen in the last century—like Mount Pinatubo. So how much warning can we expect for them?
These eruptions spit up rhyolite lavas that are cooler but much more viscous—and therefore violent—than the familiar, chemically distinct, and comparatively tame Hawaiian volcanoes. Magmas vary in chemistry and evolve over time as minerals with lower melting points separate from others that are still solid. For stagnant magmas hovering around those melting points, a fresh shot of hot melt can sometimes stir the pot and cause an eruption. For many of the lava eruptions at Yellowstone, some of which have followed long periods of calm, that kind of rejuvenation is responsible.
There’s a lot we don’t know about that process, though, like how quickly it can happen. To answer that question, Arizona State’s Christy Till, USGS researcher Jorge Vazquez, and UCLA’s Jeremy Boyce had to go small. They put individual crystals of a flavor of feldspar from a Yellowstone lava that erupted around 260,000 years ago under a serious microscope. These crystals clearly have an outer rim younger than the interior. That outer rim represents the rejuvenation episode before the eruption, like extra snow added onto an existing snowball.
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