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.

Atoms of some trace elements can diffuse through crystals of this mineral while it’s still hot but get frozen in place once the lava solidifies. Since the original magma had less of these elements than the rejuvenating magma, there’s a difference in concentration between the inner and outer part of the snowball. Given a lot of time at high temperature, those atoms will diffuse around and blur the boundary. If there’s still a sharp difference in concentration at that crystal boundary, however, you’ll know there wasn’t much time between rejuvenation and eruption. Apply some chemistry and you can put a number on that.

The researchers made spot measurements—tiny spots less than half a micron apart—of barium, strontium, and magnesium crossing the boundary of the crystal rims. Along those lines of measurements, most of the crystals showed smooth ramps from one concentration to the other. The simplest guess at how it would have looked initially is a sudden jump with nothing in between, and modeling the amount of diffusion it would take to smooth that out yields just 10 months for the magnesium measurements. Strontium and barium can’t diffuse through these crystals nearly as quickly, though, and those estimates come to about 4-88 years, and 302-4,329 years, respectively. Those are three very different answers to how long it took the rejuvenated magma to erupt.

However, the researchers think the simple assumption of that initial concentration profile is no good. Rather than the chemical composition of the stagnant magma changing instantaneously with the arrival of a fresh batch, these concentrations should have changed more slowly as the new material mixed in.

Even though they diffuse at different rates, the profiles of strontium and barium are functionally identical. Like a race between Usain Bolt and a slightly above average high school athlete, it shouldn’t take these two long to separate—less than 40 years, in this case. So instead of representing a boundary blurred by diffusion, it seems these smooth ramps record the gradually changing chemistry of the magma.

In three of the four crystals, there’s actually no change in magnesium concentration across the boundary. Since magnesium is the fastest, it could have smoothed away the difference in about 10 years. The fourth crystal, on the other hand, apparently came out of the oven after just 10 months or so. Since all four crystals exited in the same eruption, it could be that there were two rejuvenating pulses of hot magma before the lid was blown off.

Regardless, all this means that it didn’t take Yellowstone long at all to wake up. Prior to this eruption, Yellowstone had been quiet for 220,000 years. But just a handful of months or years after fresh magma stirred things up, it burped up 2 to 3 cubic kilometers of lava.

Odds are good that this will be the type of eruption that occurs next at Yellowstone rather than the rarer “super-eruptions” that inspire Discovery Channel animations. “Fortunately,” the researchers write, “any significant rejuvenation of the reservoir is likely to be associated with deformation or seismicity and identifiable by geophysical monitoring.” So we should have a useful amount of warning that something angry is brewing down in Yellowstone’s volcanic guts. Just not as much warning as we might like.

Geology, 2015. DOI: 10.1130/G366862.1 (About DOIs).