What causes supervolcanoes to erupt?

Supervolcano eruptions are the most devastating natural disasters on the planet, unleashing destruction that can level entire continents and kick off new ice ages. We've long struggled to understand what causes these unimaginable eruptions... but now, there might be an answer.

There are only a handful of known supervolcanos, with the most famous probably being the one beneath Yellowstone National Park. The eruptions are at least a thousand times worse than any volcanic eruption in recorded history, with the most recent occurring about 26,500 years ago. Because we have so little direct knowledge of these eruptions, geologists have found it difficult to determine the precise causes of these eruptions.

Now researchers at Oregon State University have developed a model that could explain what triggers this super-eruptions. Surprisingly, eruptions appear to have a lot less to do with what's going on beneath the volcano's magma chamber as it does with what's going on above. Basically, a ring of rock builds up around the magma chamber, and the magma chamber slowly expands into this rocky halo over tens of thousands of years, slowly but surely building up pressure. Faults build up in the surface above the caldera of the supervolcano, and when these ultimately collapse, the super-eruption is triggered.

Researcher Patricia Gregg explains the process:

"You can compare it to cracks forming on the top of baking bread as it expands. As the magma chamber pressurizes at depth, cracks form at the surface to accommodate the doming and expansion. Eventually, the cracks grow in size and propagate downward toward the magma chamber. In the case of very large volcanoes, when the cracks penetrate deep enough, they can rupture the magma chamber wall and trigger roof collapse and eruption."

This model departs from previous attempts to explain supervolcano eruptions, all of which focused on changes within and below the magma chamber. If something was going on beneath the supervolcano to cause the eruption, then we should be able to find evidence of smaller precursor eruptions in the areas around ancient super-eruptions. But we haven't been able to locate any such precursor eruptions, which is why the Oregon State team decided to turn their attention to what was going on above the supervolcano.

Fellow researcher Shanaka de Silva explains:

"Instead of taking the evidence in these eruptions at face value, most models have simply taken small historic eruptions and tried to scale the process up to super-volcanic proportions. Those of us who actually study these phenomena have known for a long time that these eruptions are not simply scaled-up Mt. Mazamas or Krakataus — the scaling is non-linear. The evidence is clear."

Gregg says that it takes an exceedingly unlikely combination of circumstances to create a super-eruption. The supervolcano's chamber needs huge influxes of magma to heat up the surround rock and make it malleable enough for the chamber to start growing into the surrounding rock. If the temperature isn't hot enough, that process can't start, and the massive pressure buildup required for an eventual supervolcano eruption can't take place. Instead, the chamber is likely to simply expel the excess magma in a series of normal, relatively harmless eruptions over the course of tens of thousands of years.

This is good news for anyone who is planning on visiting Yellowstone in the next several hundred centuries, as Gregg says the Yellowstone caldera is still a long ways off from building up the sort of pressure needed to replicate its last super-eruption, the Huckleberry Ridge eruption about two million years ago. She explains:

"The uplift of the surface at Yellowstone right now is on the order of millimeters. When the Huckleberry Ridge eruption took place, the uplift of the whole Yellowstone region would have been hundreds of meters high, and perhaps as much as a kilometer."

If that's the case, then we've got a very good chance of seeing super-volcanoes coming several centuries before they're due to erupt. Of course, whether we can actually do something about them is another question entirely.

Via the Geological Society of America. Image by Krumma on Flickr.