Super volcanoes have super short fuse

A new study has found volcanic super-eruptions capable of ending entire civilisations may have far shorter fuses than previously thought.

Super eruptions are hundreds of times more powerful than conventional volcanic eruptions such as Iceland's Eyjafjallajokull which grounded airlines across Europe in 2010.

The Indonesian Toba super eruption 73,000 years ago, plunged the planet into a volcanic winter that scientists believe may have decimated the human population down to just a few thousand breeding pairs.

Scientists led by assistant professor Guilherme Gualda from Vanderbilt University in the United States, found existing dating methods overestimate the lifetimes of giant magma chambers fuelling super eruptions.

These huge pancake-shaped molten rock pools can be forty kilometres across and five kilometres deep.

In the beginning, the magma in these pools is largely free from crystals and bubbles. But over time, these crystals and bubbles gradually form, progressively changing the magma's physical and chemical properties in a process that halts when an eruption takes place.

"Instead of lasting for hundreds of thousands of years, our study suggests that when these exceptionally large magma pools form, they can't exist for long before erupting, possibly just a few thousands or even hundreds of years," says Gualda.

Gualda and colleagues studied the remnants of the Bishop Tuff, Long Valley super-eruption in east-central California, 760,000 years ago.

Reporting in the journal PLoS ONE, Gualda found the magma chamber had a life span of between 500 and 3000 years from formation to eruption.

When it blew, it covered half of the North American continent with smouldering ash.
Getting the age right

Gualda found conventional dating methods using zircon crystals, which rely on the radioactive decay of uranium and thorium, overestimate magma chamber life spans.

"Zircon is tough and doesn't want to melt once it crystallises; it's actually telling us about the how long the system's been around and the changes that take place in the crust before the magma chamber forms," says Gualda.

Instead, the researchers turned to quartz will remelts, telling scientists how long the magma body existed.

To determine the age of the quartz, Gualda and colleagues sectioned individual crystals to examine the different layering in a micron-sized version of studying tree rings.

"We found quartz crystallisation indicating far shorter time spans than zircon," says Gualda.

"This brings these events from geologic time scales of hundreds of thousands of years, down to historic time scales of hundreds to thousands of years."

As far as geologists can tell, no giant magma bodies capable of producing super eruptions currently exist.

Gualda and colleagues believe this may be because these magma bodies exist for only relatively short periods of time, rather than persisting for hundreds of thousands of years as previously thought.