The Yellowstone caldera is one of the most geologically active pieces of real estate on the planet, with a high concentration of geysers, hot springs, mud volcanoes, and similar features. These features not only make Yellowstone a magnet for tourists, but they also attract the attention of geologists, who are interested in Yellowstone because it sits on top of a plume of melted mantle that has triggered titanic eruptions in the distant past. Now, two of these geologists have looked into what we know about the region in an attempt to estimate the risks for future eruptions.

A large fraction of the national park, including most of Yellowstone Lake, sits inside an ancient volcanic caldera, formed when a magma chamber blew its roof off about 640,000 years ago, releasing approximately 1,000 cubic kilometers of material in the process. An even larger eruption, about twice the size, occurred about 2 million years ago.

But volcanic activity isn't limited to these super-eruptions. The new paper reviews evidence that between 250,000 and 500,000 years ago, there were a number of smaller eruptions that would still be very significant from the perspective of anyone near Yellowstone at the time they happened. Four of these involved large flows of lava, and two others saw pyroclastic flows, mixtures of hot gas and ash. Large lava flows have also erupted between 174,000 and 70,000 years ago.

More recently, there have been a number of larger earthquakes and swarms of smaller ones, mostly associated with the refilling of the magma stores deep under the caldera or the movement of hot fluids closer to the surface.The floor of the caldera itself has also been rising, in some cases by as much as 15cm over a two-year period. This rise is layered on top of a general rebound from a kilometer-thick ice sheet that was present during the last glaciation.

All in all, the review of Yellowstone's history makes it clear that it's an incredibly complex place with lots of things that can potentially influence its future behavior. Still, the authors manage to extract a few predictions from the chaos.

In the paper, they note that the eruptions, earthquakes, and other indications of faulting cluster along three main lines: one along the western rim, one down the center of the caldera, and a third towards the east, partly underlying Yellowstone Lake. If eruptions were to resume, the authors suggest that these areas are likely to be the focus. But it's difficult to predict whether the eruptions would involve pyroclastic explosions or large lava flows. The extensive groundwater and lakes in the area could also add to the explosiveness of any eruptions.

The good news is that there's no indication that more eruptions are likely to be on the way. The magma reservoir that starts 10-20km down appears to be only partially melted, at about 5-15 percent molten material. Some areas have more shallow extensions and/or increased melt, but the maximum appears to be about 30 percent, and, the authors note, "Such melt fractions are usually too low for magma to be eruptible." So, unless there's a major influx of heat or molten material, Yellowstone should remain in its current, semi-quiescent state.

But there is bad news. An examination of the shape and composition of crystals formed in previous eruptions indicate that the molten material experienced a rapid ascent to the surface, and didn't end up sitting in pools closer to the surface before being ejected. This suggests that a recharging of the deep magma reservoir could lead to relatively rapid eruptions, and there might not be any clear sign of the magma moving towards the surface from there that would tip us off to the coming eruptions.

All of which suggests that monitoring the deep magma reservoir is probably the best way to understand what Yellowstone might be up to.

GSA Today, 2012. DOI: 10.1130/GSATG143A.1 (About DOIs).