Death from Below: Supervolcanoes and What Makes Them Tick

A couple weeks ago we learned about how rocks from space can destroy cabins, cities, and even civilizations with little to no warning. Very few things in nature hold as much destructive potential as a wayward hunk of solar system leftovers on an unlucky path, but there is one other event that comes close and you don’t need to look far to find it. Approximately 30 km (18 miles) beneath you right now is a hot, churning mass of semi-liquid rock we call the Earth’s mantle and in a few select places around the planet, it has found a way to say hello in the most terrifying of ways.

Mantle plumes are columns of magma that rise up from deep within the Earth and form reservoirs of molten rock relatively close to the surface. The reservoirs contain the full range of materials that make up the inner-Earth, including solid rock and dissolved gases. The trouble with these reservoirs is that as more material flows into them, pressure builds. Sometimes, it builds to the point where the Earth’s crust cannot contain it and it explodes upward with startling force. This process is similar to what happens with the Earth’s many volcanoes, except it tends to be much, much bigger, and for that reason, we call these reservoirs supervolanoes.

The name is a little misleading because the processes behind (or more accurately, beneath) supervolcanoes occur on such a scale that they only vaguely resemble their smaller cousins. When these babies go off, there isn’t much you can do except head for your doomsday bunker. The generally accepted lower-bound size limit for a supervolcano is a reservoir with the potential to erupt 1000 km² of material. By comparison, the 1991 eruption of the regular volcano Mount Pinatubo  released 5 km² of material; just enough to circle the Earth a couple times and reduce average temperatures in the Northern Hemisphere by half a degree C for a year or two afterwards.

Supervolcanoes erupt fairly frequently in geologic time and when they do, the effect goes a little beyond needing a sweater for a few extra days a year. Supervolcanoes release enough ash to block out the sun and usher in the ice ages. The most recent eruption from one of these beasts was 26,000 years ago in New Zealand. Another event at Lake Toba in Sumatra occurred 74,000 years ago and nearly wiped out the human race – geneticists have pointed at the Toba eruption as an explanation for the lack of diversity in the human genome. Apparently, our species was reduced to a few thousand people in the wake of the blast and the subsequent volcanic winter. The biggest eruption we know of took place 28 million years ago in Colorado and left behind over 5,000 km² of deposits, roughly the size of the island of Trinidad.

So where will the next world-shaking eruption happen? Basically, we have no idea. Despite being enormous and built into the planet we live on, supervolcanoes are hard to study. Actually, they are pretty hard to even find. The problem is that the destruction occurs on such an unimaginable scale that we tend to overlook it. The most telltale sign of a sleeping supervolcano is often a gigantic lake (flooded crater) or an absence of mountains where you would expect some to be. The latter is what allowed scientists to identify the caldera (aka magma reservoir) below Yellowstone National Park in the American west. Yellowstone’s last eruption blew up 50 km of mountains and left a caldera 50 by 70 km (30 by 50 miles) in size.

If you really want to figure out the odds of a supervolcano erupting, Yellowstone is the example to look at. On average, the hotspot beneath the park has produced an eruption once every 730,000 years. That puts the odds at around 0.00014% for any given year. The last eruption at Yellowstone was around 640,000 years ago, so you’ve probably got at least a few more years to go see Old Faithful and herds of bison. That could change though; scientists continually monitor Yellowstone for disturbances. The park experiences between 1,000 and 3,000 earthquakes per year as the caldera churns beneath it, so an increase in activity could mean an increased risk of eruption … or, it could mean pressure is being released and everything is safe.

Much like with death from the sky, supervolcanoes are unnerving in their ability to surprise.