In the spring and summer of 2018, Kīlauea Volcano on the island of Hawai’i erupted from its lower East Rift Zone. As the eruption progressed, the underground magma chamber beneath the summit caldera evacuated and could no longer support the ground above, leading to collapse at the surface. In total, Kīlauea Caldera sank 500 meters over the course of the roughly 3-month eruption. Notably, the collapse occurred in 62 roughly periodic events of up to 8 meters each. Some of the early events were accompanied by explosions that sent plumes nearly 10 kilometers into the atmosphere.
Understanding the physics at play in these dramatic events is a challenge for scientists, but the Kīlauea eruption provided researchers with the best look yet at how caldera collapses occur thanks to an array of GPS sensors, tiltmeters, and satellite- and drone-based sensors. Segall et al. used ground deformation measurements from these various sensors to create a simplified model of caldera collapse that they believe can explain several surprising features observed in the Kīlauea eruption.
One thing that initially puzzled researchers about the eruption was GPS and tilt data from the volcano’s surface that showed that the ash-charged eruptions were associated with sudden upward and outward ground movements. Such motion—known as inflationary deformation—usually occurs with an increase of magma pressure below the surface. Sudden inflations have been observed in other caldera collapses but are not well understood. Inflation associated with explosions at Kīlauea was surprising as eruptions normally cause magma pressure to decrease. Between collapse events, however, the volcano’s summit showed behavior more in agreement with expectations, subsiding and deflating.
To better understand what might have been happening inside the volcano during eruptions, the researchers created a mathematical model of the system in which faults intersect the magma chamber. When the model was run under the assumption that the faults in the volcano system were vertical or inward dipping (normal), it replicated several key features of the collapses, including both the inflationary and deflationary deformation modes. However, in the case of outward dipping (reverse) faults, the model did not match observations, leading the researchers to conclude that the Kīlauea collapse resulted from inward dipping or vertical faults.
Collapse events in caldera volcanoes are ultimately what keep eruptions going: As the caldera floor sinks, it applies pressure to the magma chamber, pushing more magma toward the surface. Piecing together what happens belowground in these systems should help scientists predict eruptive behavior during future eruptions. (Geophysical Research Letters, https://doi.org/10.1029/2019GL084689, 2019)
—David Shultz, Freelance Writer