Pyroclastic surges—super-heated and fast-flowing clouds of gas and rock fragments—represent the deadliest risk from Auckland’s volcanic field. Previous hazard assessments assumed such surges would travel up to six kilometres, but a new study of the Ubehebe crater in California’s Death Valley shows they extend much further, and future assessments should consider distances of 10–15 kilometres from the vent.

University of Otago volcanologist James White says Ubehebe’s eruption was similar to those of some of Auckland’s volcanoes where magma and water mixed to trigger more violent explosions, known as phreatomagmatic. But the Death Valley’s dry environment preserved the pyroclastic deposits exceptionally well, tracing them to at least nine kilometres. Pyroclastic surges are lubricated by a low-friction air cushion which allows them to move across any terrain. White says the somewhat cooler—albeit still lethally hot—surges from magma-water eruptions can travel even greater distances. “What hazard managers are going to be worried about is not distance but area—extending the diameter even by two or three kilometres adds a lot of area.”