The lava lake at Kilauea’s summit crater drained during the 2018 eruption, providing a window into the changing pressure in the underlying magma system. Credit: Kyle Anderson/US Geological Survey

When a magma chamber in Earth’s crust begins to drain, the overlying rock can lose its structural support and collapse, forming a sinkhole called a caldera. The collapsing rock typically drives the formation of fractures and can result in a major eruption. Caldera-forming eruptions are among the planet’s most hazardous natural phenomena: Hawaii’s 2018 Kilauea eruption, for example, destroyed more than 700 homes. But the architecture of subcaldera magma reservoirs and the conditions that trigger collapse are poorly understood because of a lack of robust observations.

Using the extensive data sets gathered during the Kilauea eruption, Kyle Anderson, Matthew Patrick, and their US Geological Survey colleagues have determined how a pressure drop in the volcano’s underlying magma chamber triggered the onset of caldera collapse and drove repeated eruptions at a rift zone far from the collapsing summit. The findings could help geophysicists better use observational data to predict when a caldera-forming eruption will begin and how long it will last.

In 2018 at Kilauea, lava effusion proceeded cyclically for months in residential areas of the lower East Rift Zone, 40 km from the volcano’s central caldera. By combining seismic and infrasound energy measurements with time-lapse, video, and thermal images, the researchers found that the outgassing of lava as it approached the main vent drove brief bursts that alternated between foamy, gas-rich lava and denser, gas-poor lava. Additionally, episodes of summit caldera collapse triggered hours-long, pressure-driven surges in the magma supply that drove the effusions far from the summit. From the measurements, the researchers calculated a magma effusion rate of 500–2000 m3/s during each episode, an important input for models that forecast flow rates and the regions that could be affected by lava outflows. The findings show that caldera-collapse eruptions do not have a steady effusion rate. Rather, the collapse events serve as a piston that drives incremental increases in the lava supply rate to the volcano’s flanks.

To determine the conditions that preceded reservoir failure and the onset of collapse, Anderson, Patrick, and their colleagues used the height of the lava lake that pooled in Kilauea’s caldera as a proxy for the magma reservoir’s changing pressure. Those observations, combined with measurements of ground deformation, indicate that collapse began when less than 4% of the chamber’s magma had been evacuated, which decreased reservoir pressure by about 17 MPa. That event ultimately resulted in the formation of a 0.8 km3 caldera. Surprisingly, Kilauea’s chamber was nearly full at the onset of caldera collapse.

The explanations of how pressures and volumes link caldera-forming summit activity to flank eruptions provide a new tool for forecasting hazardous eruptions. However, the researchers have yet to understand whether the lava-pumping mechanism is common to all caldera collapses. (K. R. Anderson et al., Science 366, eaaz1822, 2019; M. R. Patrick et al., Science 366, eaay9070, 2019; American Geophysical Union Fall Meeting, 12 December 2019.)