4 December 2012

Fiery volcanos seed lightning-generating ice

Posted by kramsayer

Ice crystals, formed from magma bubbles, are key to some volcanic lightning (Credit: Oliver Spalt / Wikimedia Commons)

Volcanic eruptions conjure up images of huge fiery explosions, searing hot magma and charred, decimated landscapes. But some eruptions also create something very different: ice crystals.

In a poster presented at the American Geophysical Union’s Fall Meeting Monday, atmospheric physicist Arthur Few of Rice University in Houston tied these ice crystals to volcanic lightning, and figured out how they form. Few first pondered the ice crystal connection while watching lightning strikes over the erupting Icelandic volcano Eyjafjallajökull in April 2010.

“The volcano produced a lot of lightning during its eruption,” said Few. “In order to produce the kind of lightning we saw, there had to be ice in the volcanic cloud.”

The type of lightning Few identified in Iceland, called negative-cloud-to-ground discharge, forms when small and large ice crystals collide. The smaller crystals are kicked higher into the air during the collision, carrying with them a small electrical charge. This charge collects in the upper layers of the ash cloud and eventually cascades to the ground as lightning. While other mechanisms can produce lightning during volcanic eruptions, only ice crystals can explain lightning at such high altitudes above the volcano.

But how does a volcano create the ice crystals needed to spark lightning?

Water needs a tiny particle, like dust or soot, to “seed” its transformation into crystal ice. For ice-forming volcanos like Eyjafjallajökull, Few discovered that the seeds come from bursting lava bubbles. Deep in the magma chamber of a volcano, about 7 percent of the magma’s mass is water. As the magma is pushed to the surface, the water boils and forms magma-covered bubbles. The process is similar to the way a shaken bottle of soda creates fizz bubbles.

The rapidly-expanding steam bubbles are the explosive force behind a volcanic eruption. As the magma continues upward, smaller bubbles combine to form one large, fist-size bubble until – pop! – the bubble bursts, sending its molten rock coating flying over 5 miles into the air. As the airborne lava cools, it forms tiny rock particles smaller than the width of a human hair—the perfect size to seed ice crystals, Few explained.

Even though the volcano is blistering hot, miles above it the Icelandic air is cold enough for water in the air to crystalize around the cooled rock particles. These ice crystals then collide, generating the lightning effect seen above Eyjafjallajökull.

Few discovered the bubbling cause of the lightning-generating ice crystals by looking at baseball-sized rocks, called volcanic bombs, blasted from the Eyjafjallajökull eruption. The bombs are the solidified remains of magma bubbles inside the volcano. These bombs not only show a snapshot of the merging and growing magma bubbles, but the volcanic bombs also shed the tiny, crystal-seeding particles that explained the lightning formation.

“I like to say that you can’t fly through a volcano in a balloon or a plane,” said Few. “But as it turns out, the volcano can deliver a sample to you.”

-Thomas Sumner is a science communication graduate student at UC Santa Cruz