GPS Used To Predict Height of Volcanic Plumes

 Photo of eruption Grímsvötn, May 2011. (SPR AFP/Getty Images) 

Scientists in Iceland have developed an innovative method to predict volcanic plume height using changes in magma pressure.  Using a series of anchored GPS stations, they were able to detect inflation of the Grímsvötn volcano before its eruption in May 2011 and the sudden co-eruptive subsidence.

These GPS stations, originally installed to monitor the movement of tectonic plates over time, continuously recorded position and elevation during the pre-eruptive and eruptive phases of the explosion.  They found that the volcano expanded radially approximately 20 inches in the hour preceding the eruption and sank 10 inches during the initial eruptive phase, resulting in tilting of one hundredth of a degree.  By tracking the change in volume of the volcano, the scientists were able to track the change in pressure of the magma chamber during the eruption.  This process is similar to a tire deflating – as air (or in the case of a volcano, magma) leaks out the tire pressure decreases. 

Based off this change in pressure, the total change in volume of the magma chamber was .027 km3, about 10x smaller than the total erupted volume. Pumice is extremely porous and can contain over 50% void space; this is a result of bubbles being trapped in the solidifying melt as they expand due to decreased pressure at the volcanic vent relative to the magma chamber. The scientists were then able to determine the rate of the eruption using the overall duration and amount of ejected material.  The mass eruption rate is directly correlated with the height of the plume; the faster the eruption rate, the taller the plume. Using this novel method, they estimated that the plume height peaked at 15 km, which compares favorably to satellite imagery.

Column height is an important parameter to predict where an ash cloud will travel.  The heat produced by jet engines cause the ash particles to melt, gunking up the engine and causing the plane to stall.  Currently, the protocol for avoiding this is to completely close airspace.   The eruption of Grímsvötn, which lasted for seven days, caused airspace closures in northern Europe, grounding approximately 900 flights.  Better predictions of column height would result in more accurate hazard maps and fewer plane delays. This new method to estimate plume height uses real time data that does not rely on satellites.  Aviation experts can then better seed their models to create smaller no fly zones due to more accurate predictions of ash movement.

Original Article: Hreinsdóttir, Sigrún, Freysteinn Sigmundsson, Matthew J. Roberts, Halldór Björnsson, Ronni Grapenthin, Pórdur Arason, Thóra Árnadóttir et al. "Volcanic plume height correlated with magma-pressure change at Grimsvotn Volcano, Iceland." Nature Geoscience (2014).

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.