Geochemistry, Mineralogy, Volcanology News

Sparks May Reveal the Nature of Ash Plumes

In lab experiments and models, researchers uncover how ash can affect the standing shock waves of erupting volcanoes. Their findings may lead to new predictions of volcanic ash hazards.

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Shake up a bottle of soda, open it, and watch a plume of explosive gas, sugar syrup, and flavoring go spewing into the air. The culprit? Gases and ejecta suddenly allowed to escape high pressures inside the bottle.

Scale up this experiment, add some molten rock, and you have a pressure release that shoots gases, ash, and lava into the sky. Ash plumes can drift through the atmosphere, spreading volcanic shards far from their source.

Although volcanic eruptions can’t be predicted, a group of scientists is trying to forecast the characteristics of ash released in the dark plume of debris. At the American Physical Society’s annual meeting in October, researchers presented a way to relate radio frequency measurements of volcanic near-vent discharges to the ash that’s ejected.

Their work can be used in volcano monitoring, especially when it comes to volcanic hazards.

Atmospheric Ash

The Eyjafjallajökull volcanic eruption disrupted air travel for 6 days, grounding flights from Europe to North America. The Icelandic volcano is far from the only plume producer: There are about 1,500 active volcanoes around the world.

“Ash can damage turbines of planes and also affect visibility,” said Jens von der Linden, a physicist at Lawrence Livermore National Laboratory and lead author of the new research. “It’s very hazardous for commercial aviation to fly through areas that may have ash.”

He explained that because of the danger there’s interest in understanding how much ash is ejected and where weather patterns might carry it.

Von der Linden and his colleagues want to make better, early predictions of ash content after an eruption. To do this, they decided to use a scaled-down lab-created version of a volcanic eruption: shock tube experiments ejecting ash into a large expansion chamber where discharges occur.

“[Researchers] have a high-pressure tube that they fill with gas, a high-pressure gas, and they can also put ash, particles, or glass beads in it,” explained von der Linden. During a simulated eruption, the gas and particles burst from the tube into a lower-pressure expansion chamber.

The setup mimics what happens in a volcanic eruption. “It’s like a rock conduit; initially, there was a rock blocking the high-pressure gases that were building up in a volcano,” he said. Suddenly, that rock bursts, and “the gas and ash of the volcano are moving up this rock conduit, where it expands into atmosphere.”

This sudden change in pressure from high to low creates a shock wave above the volcanic vent. And although shock waves often propagate outward and move, the waves move in such a way that a standing shock wave, or Mach disk, forms.

Lightning and Ash

Corrado Cimarelli, one of the group’s collaborators, had previously noticed in laboratory experiments that discharges occur below a flat surface formed by a standing shock wave called a Mach disk. The team modeled how ash might affect the Mach disk and compared the results to the shock tube experiments.

They found the amount and type of ash had different effects on the Mach disk. “You still get a shock surface, but it’s thinner and a lower height,” said von der Linden.

“The concept of a standing shock wave isn’t new, but doing modeling to see how ash affects the shock is new,” said Sonja Behnke, a remote sensing scientist at Los Alamos National Laboratory who was not involved in the study. “Jens is the only one I know of doing this work.”

But it wasn’t just the different shape of Mach disks that caught the researchers’ eyes. They observed sparks near the volcanic vent, outlining the shock wave surface. Although the sparks look like bolts we see in the sky, they might not be the type of lightning we’re familiar with.

Model, simulation, and experiment images of volcanic sparks
Researchers created a model, simulation, and experiment of volcanic sparks and shock waves during an eruption. (a) The model showing the standing shock wave surface, or Mach disk, is indicated in red. The sparks occur within the low-pressure region inside the Mach disk boundary. (b) In this simulation of the gas outflow speeds, speeds drop at the shock surface, piling up gas and increasing pressure. (c) In the experiment, sparks form below the Mach disk surface while upper sparks mark out the shock surface. Credit: Model and simulation images by Jens von der Linden et al.; experiment image courtesy of Clare Kimblin and Ian McKenna, Special Technology Laboratory, Mission Support and Test Services

“We’ve never actually seen the near-vent discharge [in nature], this nonlightning discharge, because it’s probably not bright enough and the plume is too dark to look through,” said von der Linden. “But we know from the radio frequency signatures that it exists and it’s very different from lightning because the [sparks] only have high-frequency components.”

“Vent discharges are very small sparks/electrical discharges that occur right at the vent of a volcano concurrent with an explosive eruption,” said Behnke. She added that “they aren’t very lightning-like because they are very small—maybe a few meters, but we don’t know for sure.”

Behnke said that “lightning observations may be able to quantify the height, width, and lifetime of a standing shock wave,” all of which are useful for volcano monitoring.

Measuring Mach Disks

“The near-vent region of a volcano is very hard to diagnose,” said von der Linden. “It’s very hot, rocks are flying through there—it’s very hard to access it.” He noted that pairing his team’s modeling work with antenna field monitoring could triangulate where the spark signals are coming from.

Behnke agreed that much more work will need to be done to tease out the details of volcanic ash in shock waves. “It isn’t clear what the difference might be between the laboratory sparks and what we see in an eruption,” she said.

“It will be critical to compare the modeling work to field observations of volcanic lightning,” Behnke noted. “Laboratories are very different than nature.”

Understanding the characteristics of ash is important in aviation, said von der Linden. He stressed that their method “isn’t predicting ruptures, but it’s predicting right when eruption occurs.”

“This work is showing that in the future, we could potentially use lightning observations as a proxy for the duration of ash venting, or perhaps the density or size distribution of ash particles,” said Behnke.

She added the approach could be “used by volcano observatories to model the plume from an eruption and better forecast ash hazards.”

—Sarah Derouin (@Sarah_Derouin), Science Writer

Citation: Derouin, S. (2019), Sparks may reveal the nature of ash plumes, Eos, 100, https://doi.org/10.1029/2019EO136482. Published on 15 November 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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