Volcanic Lightning: Where does it come from?

Article: Experimental generation of volcanic lightning

Authors: C. Cimarelli, M.A. Alatorre-Ibargüengoitia, U. Kueppers, B. Scheu, and D.B. Dingwell

Article URL: http://geology.gsapubs.org/content/42/1/79.full.pdf+html

Figure 1: Volcanic lightning from an eruption of Puyehue volcano, Chile. Image source

All types of lightning, especially volcanic lightning (Figure 1), are poorly understood by scientists. Because of our inability to observe lightning from within the thundercloud or ash plume, we do not know the specifics for their formation. The high concentration of ash in the eruption column prohibits direct, visual observation of the electric potential near the volcanic vent, where lightning is first observed. Once we know how lightning generation relates to volcanic eruptions, we can determine first-hand information on eruption location, eruption column structure, and the amount of ash erupted.

A group out of Ludwig Maximillian University, Germany successfully generated volcanic lightning under controlled laboratory conditions. The scientists wanted to figure out the dominant mechanism that controls particle electrification during the onset of explosive volcanism. Lightning was generated in rapid decompression experiments where concentrated ash was accelerated into a large tank of air, analogous to an active volcanic eruption. Throughout the experiment, high-speed cameras were used to visually capture lightning as it was generated within the ash cloud (Figure 2). Finally, the electric potential at the vent (where the ash was ejected into a tank of air) was measured with a pressure transducer and copper antennas.

Figure 2: A still-frame capture of lightning during the experiment. The concentrated ash at the bottom of the eruption column spreads out with increasing height. This spreading out, or decompression, of fine ash particles creates an electrical charge. This is similar to the generation of static electricity. Width of lower jet is about 6 cm. Image is from the original article. 

The experiments revealed that the number of electrical discharges directly relates to the amount of fine particles ejected. Thus, an increase in the number of fine particles ejected during an eruption results in an increase in the number of lightning strikes in the ash column. Additionally, they found that the clustering of fine particles provide a more efficient way for charge generation and lightning discharge in the eruption column. Another interesting find is that when there are two dominant particles sizes, the larger ash particles focused near the center of the jet while the finer particles were accelerated to the edge. This is an effective mechanism for the separation of positive and negative charges and for subsequent lightning generation. In an eruption, the concentrated ash particles near the vent rapidly spread apart with height and generate different types of lightning, where short lightning is generated near the vent and longer, more luminous lightning is generated higher up in the eruption column. This matches with observations of Sakurajima volcano in Japan, where an impulsive explosive eruption on February 8^th, 2010 generated short-lived, frequent lightning near the crater and longer, luminous lightning hundreds of meters high.

These experiments have opened a new way to investigate the generation of lightning within volcanic eruptions. Combining high-speed camera observations with other tools like Doppler radar, which can detect lightning hundreds of miles away in real-time, can provide early information about lightning generation at an active volcano. Measuring the lightning frequency at an erupting volcano can help us determine some eruption characteristics in real-time, such as the amount of fine ash being ejected. These ash estimates can then be input into ash forecast models for an early determination of ash transport, providing more time for hazard assessments and potential evacuations.

Blowing stuff up for fun and education

Our dispatches crew had a fantastic time recently doing a 3 day pop-in with a local middle school classroom learning about volcanoes. True to form, we made a wonderful mess of it (in the best possible way).

On Day 1, students explored the impact of viscosity on bubbles by using straws to blow bubbles in milk, ketchup, and peanut butter. It takes some serious lungs to get a bubble going in JIF! For the lower viscosity fluids, bubbles were able to rise up and even form a foam. In low viscosity magmas, the bubbles will be able to rise independently of the melt, which creates opportunities for passive degassing. In a high viscosity fluid (chunky JIF moves a bit like molten rhyolite), bubbles that form are much more likely to be trapped in place; as the magma depressurizes, the bubbles will grow next to each other like bubbles in rising bread dough. If the bubbles grow too quickly for the magma to stretch around it, the bubble walls will rupture like popped balloons -- but with much greater force.

Day 2 witnessed much larger bubbles than any of us could manage with a straw: it was time for trashcano eruptions! In a trashcano, the eruptive energy comes from the expansion of liquid nitrogen as it transitions to the gas phase. The liquid nitrogen, in a sealed pop bottle held underwater in the trash can by an anchoring brick, expands until it creates too much pressure inside of the bottle and then explodes. All of that expanding gas has to go somewhere as it escapes the now-useless bottle, so it pushes the water from the trash can up and out of the way in a glorious eruption. Our middle schoolers were real troopers, despite the freezing conditions! We set off a couple of eruptions, then worked together to map the maximum trajectories of the ejecta (packing peanuts) that were thrown out along with the water during the eruption. The highlight for the students? Professor Mike M sacrificing himself and one of the (unwitting) graduate students to the freezing spray in the name of showmanship!

On the third day, we used 'Eject!' software to figure out our vent velocities during the eruption based on  ejecta distances and ambient conditions. The program, written by Larry Mastin of the USGS, lets students model trajectories for various conditions using a simple GUI. The students worked in teams of 3-4 to see who could come up with the best fit conditions to describe the eruption from the day before. One group, determined to test the limits of the program, nearly sent their packing peanut into orbit!

It was a fantastic opportunity to share our science with a local school, get dirty, and practice our outreach skills. The students were absolutely fantastic! Thank you so much to Mrs. Casselman and Grace Lutheran School for inviting us over to play. We hope to see some of the students in our 101 classes in a few years!

Pouring Lava on Ice: for Science!

Eruption plume above Eyjafjallajokull volcano, Iceland, April 17, 2010.
 In the foreground is the meltwater floodpath from eruption-induced ice melting.
Photo credit: Eyjoful Magnusson, University of Iceland. Source

In 1973, a new volcano erupted on the island of Heimaey, part of the Westman Islands off the southern coast of Iceland, and threatened the very existence of the inhabitants and their way of life as they knew it. The main town of the island, Vestmannaeyjar, became partially inundated with lava but the inhabitants knew they would be able to rebuild their town. However, the lava was also advancing toward the only harbor on the island, threatening to seal off the harbor and destroy this fishing community’s livelihood as well as one of Iceland’s main fishing port. The Icelanders came up with a novel plan to desperately attempt to save the town. The idea? Throw enough cold water on the lava in hopes of creating a solidified lava wall to divert the rest of the lava away from the harbor. It took two months of water pumping and the entire arsenal of pumping ships from the area (high-capacity pumps were even brought in from the US) but the harbor was eventually saved. This event is one rare case of a “successful” fight against an advancing lava flow; however, half of the town was still destroyed and a similar event could occur at any time.

Lava flows are not the only volcanic threat to life and infrastructure. Lava flowing over ice or snow can induce rapid melting, leading to an unexpected debris filled flood event. Volcanic eruptions are dangerous and need to be quantitatively measured in order to better understand the hazards associated with them. Specifically, understanding the interactions between lava and ice is critical in developing hazard assessments. We care about the interactions between lava and ice because of the rapid melting involved and the increased risk for flash flooding and debris flow events, such as lahars and jokulhlaups. However, because volcanoes are often in difficult to reach places and an eruption involving lava and ice is inherently dangerous, it is difficult to observe the lava/ice interactions closely. Moreover, a lava flow traveling over ice may melt sufficiently through it and begin flowing underneath and/or through the ice, where it is no longer observable from the surface.

In an attempt to address these observational and measurement difficulties, a group of scientists out of Dickinson College, Pennsylvania, in collaboration with Syracuse University, New York, have created a way to observe and measure the interactions of lava with ice and snow (figure 1). In short, their experiment is to pour basaltic lava onto layers of ice or snow in a controlled setting to observe and measure the interaction. Multiple experiments were performed using blocks of ice, shaved ice, and sand (to simulate an ash covered ice surface).

Images of the experimental lava flows advancing over ice and snow. The top image is lava flowing over an ice bed 12 cm thick (the width of the flow is 15 cm). The second image is lava flowing over snow 12 cm thick (the width of the flow is 20 cm). Images are from the original article. 

Experiments reproduced two end-member melting behaviors consistent with field observations: (1) flows that melted quickly through ice and (2) flows with slow initial melting. In experiments without a sand layer, lava advance was inhibited due to it quickly melting through and sinking into the ice. Eventually the lava melted through the ice to the container bottom and again flowed downslope along the container-ice boundary. These experiments show the ability of lava to exploit these crevasses to flow downward and through ice. By controlling the starting geometries for ice-lava boundaries we can test different hypotheses for ice confinement of lava flows.

One of the more interesting observations is that during some experiments the lava would appear to skate across the ice; this is thought to occur from the trapping of a vapor phase between the ice and lava (the ice transitions to steam, is trapped, and acts as a lubricant between the ice and lava). The existence of this trapped gas may also act as a buffer for heat flow between the lava and ice, which would melt the ice slower and allow the lava to advance further before sinking into the ice.

Formation of bubbly Limu o Pele surface in experimental lava flow. Liquid water and stream from the melting of the ice blocks below the lava is incorporated into the lava. The steam expands within the lava and forms large bubbles that then solidify, known as Limu o Pele. Image is from the main article.

Rapid and extensive formation of large bubbles (Limu o Pele, see figure 2) occurred within the lava during the no-sand experiments. This may be the first published demonstration of large-scale Limu production with water originating underneath lava. The abundance of bubbles that formed as lava flowed across the ice indicates that there may be a strong relationship between external water incorporation and the formation of Limu.

Additional reading: 

http://www.earthmagazine.org/article/do-it-yourself-lava-flow

http://lava-dev.syr.edu/research.html

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