Rainfall's Influence on K?lauea: Unraveling Volcanic Eruption Triggers
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The most recent eruption of K?lauea volcano on the island of Hawaii began in 1983. For 35 years, most of its magma emerged from a set of fissures in the volcano called the upper east rift zone. But on 3 May 2018, K?lauea’s lower east rift zone opened up, giving way to a massive outpouring of lava that devastated the south-eastern part of the island. An important question is why this change occurred in May 2018, rather than earlier or later in the course of the eruption. Writing in Nature, Farquharson and Amelung propose that record-breaking levels of rainfall in early 2018 increased groundwater pressures which, in turn, made it easier for rock to break and hence magma to rise to the surface at new locations.
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The creation of a pathway that brings magma to Earth’s surface begins with the mechanical failure of rocks. This failure can occur in two ways: new cracks can open, or existing faults can slip. Both processes can be promoted by pressure changes in groundwater. For the former, increases in fluid pressure decrease the amount of stress needed to open new cracks. For the latter, faults can slip when the stresses acting parallel to the fault (shear stresses) overcome those perpendicular to the fault (normal stresses). These normal stresses act to clamp the fault shut. Increasing fluid pressure in rocks lowers normal stresses without changing shear stresses, thus promoting fault failure.
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Heavy rainfall increases water levels underground and thus pressure in groundwater. The volcanic rocks in Hawaii are very permeable, which allows water to infiltrate and pressure changes to propagate to a depth of several kilometres, close to where magma is stored. Fluid-pressure changes take time to propagate from the surface to those depths. Thus, downward migration of rock failure over time, along with a time lag between the accumulation of water at the surface and failure at depth, would be key indicators that rainfall was the cause of rock failure at K?lauea.
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Farquharson and Amelung modelled pressure changes at K?lauea caused by rainfall in the months leading up to the eruption on 3 May 2018. Their model showed an increase in pressure of tens to hundreds of pascals at depths of several kilometres. On the basis of these changes, along with four sets of observations indicating that eruptions at K?lauea are associated with patterns of substantial rainfall, the authors propose that heavy rainfall promoted the rock failure that enabled magma to flow into the lower east rift zone. Is their hypothesis plausible? The pressure changes computed by their models are small — smaller than stresses from tides. However, if rocks are already close to breaking, such changes might be sufficient to initiate failure. The 2018 eruption was accompanied by a magnitude-6.9 earthquake, and examples of earthquakes caused by pressure changes on this scale are abundant…
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... Although it is well established that changes in water pressure promote earthquakes, they are not necessarily a direct cause of magma eruption. To begin moving through Earth’s crust, magma must create large enough stresses in the surrounding rocks to open a pathway. Earthquakes triggered in the crust around that stored magma, however, can actually relieve stress — as such, they might make it more difficult for magma to erupt… Ultimately, whether fault failure from water-pressure changes can occur close to stored magma, remains uncertain.
