I hadn’t come to Chile to study violent volcanic processes, but then again I hadn’t anticipated a spectacular eruption from the volcano in my backyard. The March 3 Villarica paroxysm, which culminated in a lava fountain nearly a mile high and spawned raging torrents of mud and rock, was serendipitous for me. My instrumentation, deployed only a month prior, sampled the passage of these currents and led to improved understanding of their destructive force.
My initial research goals were admittedly more esoteric than the study of deadly deluges. My proposed investigation had been to focus on the peculiarities of low frequency volcano sounds, or infrasounds, produced by a volcano’s vent and recorded by specialized microphones that are capable of detecting the inaudible. I wanted to understand how the atmosphere affects infrasound propagation. Toward this end, my microphones had been deployed to the north, south, east and west of the volcano, near glaciers, on mountain ridges and in dense araucaria forests. Sensors were located as distant as 25 km, and on the summit itself. Johnson’s funding from a National Science Foundation CAREER grant and from the Fulbright Scholar’s Program permitted him to spend six months in Chile from January through June 2015. His two children attended public school in the town of Pucon and enjoyed seven days of cancelled school due to the volcanic activity. Through much of January and February, my team of Boise State University students and I hiked and drove to all approaches of the 9,341-foot high volcano, which looms over the clear waters of Villarrica and the town of Pucon, perched on its shores.
The highlight of our fieldwork was at the summit. We deployed on January 18, a brilliant (Austral) summer’s day, as seasonal snow was fast melting. Despite the mushy glacier, we hiked quickly up the 4,000-foot vertical rise, following convenient tracks made by earlier climbers. At the summit, the acid gas was potent, but no worse that usual, and with full face respirators we didn’t even notice the sting.
My Boise State tech and I enjoyed ourselves as we strung out cables, hooked up batteries and started logging data. The crater morphology, rock features, boulders and permanent ice appeared unchanged from my previous field trips in 2002, 2004, 2010 and 2011. We installed our sensors just inside of the crater rim and at the edge of an intimidating 300-foot precipice at the bottom of which an unseen lava lake roiled and churned producing sounds like distant surf.
As we wrapped up our work, I hadn’t the slightest premonition that my instrumentation would soon be lost.
What is a lahar?
Lahars, or volcanic debris flows, are arguably the most insidious of volcanic hazards. They travel downslope faster than lava, extend further than pyroclastic flows and manifest more acute devastation than ash fall. Of the four most commonly-occurring volcanic hazards, lahars are the only menace produced both coincidently with eruption, or spontaneously and potentially long after an eruption has ceased. Along with pyroclastic flows, the scourge of Pompeii, lahars are the primary volcanic killers.
The most infamous of historic lahars occurred in 1985 when Nevado del Ruiz, in Colombia, erupted with modest size, yet melted enough glacier to induce slurries that accumulated mass as they progressed down six river valleys on all sides of the volcano. The city of Armero, situated on flat alluvium 40 km from the volcano, was obliterated more than two hours after the eruption. Twenty thousand died in Armero in a quagmire of mud, which set to a consistency of concrete shortly after the flow stopped.
On March 3, the summit hardware was blown skyward by a sudden eruption of unanticipated violence. Shortly after 3 AM local time, the low-level lava lake activity transitioned to a sustained lava spray, which graduated to a spectacular fire fountain reaching its apex 5,000 feet above the vent.
The paroxysmal eruption concluded in less than 30 minutes, barely long enough for my family to stumble bleary-eyed from our beds in the town of Pucon. We watched awestruck from my backyard patio 10 miles away.
During the week following the eruption I drove and hiked to my sites to retrieve the data collected by my digital recorders. Though the summit stations were irretrievably lost, and one station at 3 miles distance was impaled by falling scoria, the data collected at stations around the volcano’s periphery was of unprecedented quality. Each time I connected a laptop to a data acquisition box was like Christmas morning… I had no idea what goodies I might uncover. The infrasound data documented beautifully the story of the March 3 eruption and revealed intricacies of its eruptive sequence.
When I downloaded station H, however, located in a bucolic cattle pasture six miles from the volcano summit, I received the best prize. The eruptive activity was clearly evident, but it was another signal, immediately following the eruption, that caught my eye.
Villarica Locator Map
Unlike data from other stations, array H was above a river valley and revealed a three-hour-long sub-audible rumble, building rapidly in intensity and then tapering to background. Station H consisted of three infrasonic microphones spaced 50 feet apart, which were designed to detect the direction of incident sound wavefields.
Using the science of array processing, this is what the data told me:
At 3:25 AM on March 3 a powerful source of sound originated not from the summit, but from the upper slopes of Villarrica. This sound, propagating in tones mostly inaudible to humans, reached Station H 30 seconds later.
Between 3:25 AM and 3:31 AM the source of sound migrated rapidly downslope, following the precipitous path of the Pedregoso River. The moving source descended more than eight miles in six minutes indicating an incredible advance of 80 miles per hour.
The sound source remained detectable for hours and at distances further than 10 miles from the summit. The estimated sound energy radiated by the flow was 10 Giga-Joules, more than the explosive power yielded by a ton of TNT.
Our study was the first to directly observe a lahar front using acoustics and last month my Chilean partners and I submitted our discoveries for consideration in the scientific journal Geophysical Research Letters. In this submission we argue that acoustic monitoring should play an important role in the timely detection of potentially destructive lahar activity. Even with the rapid velocity of the advancing flow, an infrasound station and telemetered system could give more than 20 minutes notice to the exposed town of Pucon and its vulnerable bridge over the Turbio River.
Images of Villarica:
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In the wake of our computer-driven data analyses we also spent days reconnoitering more than 10 miles of flow path and climbing to the source zone. We climbed over lava flows stripped smooth by mud and boulders and scrambled over VW-bug sized erratics transported miles from their origins high on the volcano. Only a stream of water flowed in the drainage during our reconnaissance, but the flow path scar, 10 miles long, 50 to 200 feet wide and denuded of all vegetation, was testament to the lahar’s power.
In the scheme of potential lahar intensity, the event of March 3 was junior sized. It destroyed two small bridges on the Pedregoso/Turbio drainage and temporarily isolated about 40 homes for a week, but no significant damage resulted and no one was killed. Historic mudflows spawned from activity of 1971 and 1984-1985 were considerably larger by comparison and the geologic record tells us that lahars of the past few centuries lie beneath the homes of several thousand current Pucon residents.
Villarrica continues to be the most frequently explosive volcano in Chile and there is little doubt that future activity will transpire in our lifetimes. Next time Villarrica erupts violently perhaps infrasound will be used to mitigate its impact.
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