Volcanology

Rockfall-induced eruption

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Credit: USGS / M. POLAND

As a tourist attraction, Kīlauea Volcano in Hawai'i beats all other volcanoes on Earth. It is also high up on the list of the most active volcanoes: the Halema'uma'u Crater at Kīlauea's summit has been spewing out poisonous gases continuously since 2008, leading to increased levels of volcanic smog and concerns about air quality.

The gas emissions started in March 2008 with the opening of the Overlook Vent, and have been punctuated by explosive eruptions caused by the sudden degassing of volatile elements from magma. On 12 October 2008, a small explosive eruption occurred at this new vent. Rebecca Carey and colleagues (J. Geophys. Res. http://doi.org/jhn; 2012) show that the 12 October eruption was triggered by rocks falling from the steep, overhanging vent walls into the lava lake below, rather than by uprising gas from the deeper magma plumbing system, as happens usually during basaltic explosive eruptions. They argue that rocks hitting the lava surface created a pressure wave — a train of high and low pressure areas — whose decompression component triggered degassing of the magma.

Micro-textural analysis of pyroclastic rocks ejected on 12 October 2008 reveal numerous tiny bubbles. These small-scale features formed when rock impacts at the lava lake surface caused a decrease in magma pressure, analogous to the formation of bubbles in a newly opened bottle of fizzy water.

The tiny bubbles record a sudden change in pressure, but their formation was not responsible for the 12 October event. The micro-textural analyses also revealed numerous larger bubbles that form more slowly, and thus must have existed in the magma before the eruption. Calculations show that in the wake of the rockfalls, decompression could cause pre-existing, large bubbles in shallow parts of the lava lake to expand. Growth and explosive expansion of these pre-existing bubbles probably triggered the eruption.

In the light of the findings, it would seem wise to let a sleeping lava lake lie undisturbed.

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Whitchurch, A. Rockfall-induced eruption. Nature Geosci 5, 767 (2012). https://doi.org/10.1038/ngeo1628

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