“An eruption at the scale of what we saw yesterday can occur suddenly, as it did yesterday, with virtually no immediate precursor signals,” GNS Science senior volcanologist Graham Leonard told reporters Tuesday. The government-owned company is responsible for monitoring the Pacific Rim nation’s many geological hazards, including earthquakes, tsunamis, and landslides. It also issues periodic hazard bulletins for a dozen volcanoes in the region, of which White Island is considered the most active.
An 80-minute boat ride across the Bay of Plenty from New Zealand’s mainland, White Island rises like a barnacle. Every year more than 13,000 tourists visit to snap selfies in front of the blurping mud pots and sulfurous vents of its horseshoe-shaped crater, the tip of a massive undersea volcano. Its magma reservoirs sit very close to the surface, feeding heat and gas into a shallow aquifer above it, superheating any water trapped in the rock and pressurizing the surrounding sediment. Any external shock to the system—an earthquake, a surge of gas from deeper in the magma, or changes to the aquifer water level—can release the built-up pressure, often to violent effect. At those temperatures, water expands into steam at supersonic speeds, with enough energy to shatter solid rock. The resulting explosion is called a hydrothermal or phreatic eruption.