12 June 2012

Volcanic lightning examined through ash grains

Posted by kramsayer

A stratovolcano in Galunggung, Indonesia, erupts in this 1984 image, producing some spectacular lightning. Researchers are looking into what ash properties could help generate lightning. (Credit: USGS)

GeoSpace is in Selfoss, Iceland this week, reporting from AGU’s Chapman Conference on Volcanism and the Atmosphere. Check back for posts on the science presented at the meeting, as well as field trips to nearby volcanoes and geologic features.

Selfoss, Iceland — The awesome sight of explosive volcanic eruptions occasionally includes a light show as well. Lightning can be sparked throughout the eruption – as the ash, rocks and gases start hurtling out of the volcanic vent, as the eruptive column rises, and as the ash plume drifts away from the volcano. But why lightning happens during some eruptions, and not in others, is still a question – one that a couple of curious volcanologists have recently been linking to characteristics of the ash itself.

Volcanic lightning “is a pretty common occurrence, but we still don’t fully understand it,” said Kimberly Genareau of Lehigh University in Bethlehem, Penn. Building on what scientists do know about the phenomenon, Genareau has been packing samples of volcanic ash into beakers, adding water, and measuring their electrical conductivity.

Scientists have previously determined that, in the vent and column of a volcano, the collision and break-up of tephra particles creates positive and negative electrical charges that help generate electric fields and trigger lightning. In addition, as ice crystals form higher up in the column, they increase the electrical conductivity of the plume and spur volcanic lightning.

By testing ash samples that she collected from Soufrière Hills volcano in Montserrat and Mt. Taranaki, New Zealand, and that a colleague passed along a sample from Mt. Redoubt, Alaska, Genareau has been finding that fine-grained ash particles are less conductive than the coarser size fractions of the same sample. And, even between different fine-grained samples, electrical conductivity varies.

Genareau presented initial results Monday evening here at the American Geophysical Union’s Chapman Conference on Volcanism and the Atmosphere.

Ash samples can differ in both size and shape – blocky versus angular, for example. Ash that forms when magma bubbles burst tends to have a high surface-area to volume ratio, which could impact its conductivity, Genareau said. And when she measured samples from an ash-venting eruption, they were much more conductive than previously bubbly fine ash that had once been part of a pyroclastic flow, a torrent of scalding ash, rock fragments, and hot gases that rushes down the flanks of a volcano.

“There are a number of different things that are contributing to [ash conductivity], so it’s a little tricky trying to pin down which property is having a bigger effect,” Genareau said. The next step in her research, she said, is to refine the experiments in order to vary a single trait – whether it’s size, shape, or chemical composition – and conduct more in-depth studies to identify the possible lightning-influencers. Genareau is collaborating on the research with Stephen R. McNutt of the University of Alaska, Fairbanks.

Electrical properties of ash also affect infrastructure, such as electrical systems, that volcanic material settles on and infiltrates, and the ability of that infrastructure to operate effectively, according to the work of research groups in New Zealand. Understanding what influences the conductivity of volcanic ash, Genareau said, could help mitigate those infrastructure-disrupting effects.

Kate Ramsayer, AGU science writer