A team of volcanologists, from the Hiroshima University in Japan, proposed how earthquakes can act as triggers for distant volcano eruptions. The trigger mechanism in question is the sloshing of the bubbly magma. The scientists have simulated the effect of earthquake shockwaves on a laboratory equivalent of a magma chamber. The study has for the first time brought proven engineering concepts about sloshing, the movement of liquid's surface, from outside of geology to aid in understanding the mechanism.
The scientists have long had a reason to suspect the connections between the occurrence of earthquakes and distant volcano eruptions. Historically well-known earthquakes have often been followed by volcano eruptions in the distant radius, but the idea had often seemed too far-fetched. In 1835, even Charles Darwin guessed about the possible connection between the great Concepcion earthquake in Chile and the eruption of the Osorno volcano that occurred a month later.
A huge eruption of Mt. Pinatubo in the Philippines was also preceded by an M7.7 earthquake about 100 km (62 miles) from the epicenter that occurred a year before. The scientists from the University of Oxford, UK, led by the volcanologist David Pyle, discovered the eruption rates of Chile volcanoes, generally speaking, significantly increase in a 12 month period following the earthquakes of or above magnitude 8.
Over time, volcanologists have reached the conclusion that the connection between earthquakes and volcano eruptions is determined by the state of the volcano and the presence of bubbly magma. The sloshing of the bubbly magma is the newly proposed trigger mechanism, which holds a great potential for explaining the connection.
Sloshing is a well-known mechanism, investigated in engineering. For example, trucks carrying liquids such as petroleum need to have specially designed tanks to carry the fluids. Static petroleum storage tanks can suffer fractures and roof collapse after the earthquake motion affects the motion of the fluid inside. Volcanologists from Japan have wondered whether earthquakes can influence the volcanic magma in a similar way.
Researchers have used a rectangular tank attached to a shaking table to study the effect. Instead of magma, they utilized a thick glucose syrup containing irregularly-shaped bits of plastic to replace suspended rock crystals, precipitating out of magma. The sloshing process can occur in only partially filled containers, like, for example, molten rock in an open volcanic conduit leading to the surface. In full magma chambers, sloshing can take place between liquids of different densities. Liquid layering is thought to be frequent in magma chambers because the foamlike bubbly magma overlies denser magma.
The scientists have tested the sloshing of three different magmas - an open, single-liquid layer, single-foam layer, and a closed two layer system with foam overlying the liquid. They captured each tank shaking in a ten-second interval in a film. The shaking has been conducted with different frequencies and amplitudes, with artificial magmas of variable viscosities, volumes, crystal contents, and bubble fractions.
The team found out that a large increase in sloshing resulted from the tank shaking near the liquid's resonant frequency. The bubbles in the foam layers suffered deformation and became interconnected. This process caused the foam to collapse. In a real-world volcano, the escape of hot gasses from the collapsed foamy magma in a closed reservoir could cause the increase in magma pressure, thus triggering an eruption.
In the double layered experiment, the foam layer collapsed, and the remnant foam has mixed with the underlying liquid layer, which would, in the real world, also slowly increase the magma pressure, and cause increased activity or a delayed eruption. This discovery may answer the question, as to why the volcanic eruptions can happen months after the earthquake.
Following their laboratory experiments, experts decided to investigate the earthquake conditions which could cause real magma to undergo a foam collapse. They discovered that volcanic vents wider than 0.5 m (1.6 feet) and low-frequency seismic waves would be necessary, and this can explain why only large quakes could trigger volcanic eruptions. A typical magma in a 3-m (9.8 feet) wide volcanic tube would require an M7.5 earthquake to cause the sloshing-induced foam collapse from a distance of about 100 km (62 miles).
According to experts, the large spherical magma chambers at intermediate depths under the volcanoes should resonate with seismic waves, as well, as long as the denser magma layer fills up to a sufficient level in the reservoir.
The intriguing discovery will require further testing, but, according to experts, it does hold a great potential to solve the complex problem of the connection between earthquakes and volcano eruptions.
- "Sloshing of a bubbly magma reservoir as a mechanism of triggered eruptions" - Atsuko Namikia, Eleonora Rivaltab, Heiko Woithb, Thomas R. Walterb - Journal of Volcanology and Thermal Research (2016) - doi:10.1016/j.jvolgeores.2016.03.010
"How earthquakes might trigger faraway volcanoes" - Article published in ScienceMag (accessed April 26, 2016)
Featured image: Mt Pinatubo, Philippines, May 1, 2009. Image credit: Marc Reil Gepaya (Flickr-CC)
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