White Island is one of several volcanoes in New Zealand that can produce sudden explosive eruptions at any time. In this case, magma is shallow, and the heat and gases affect surface and groundwater to form vigorous hydrothermal systems.
In these, water is trapped in pores of rocks in a super-heated state. Any external process, such as an earthquake, gas input from below, or even a change in the lake water level can tip this delicate balance and release the pressure on the hot and trapped water.
The resulting steam-driven eruption, also called a hydrothermal or phreatic eruption, can happen suddenly and with little to no warning. The expansion of water into steam is supersonic in speed and the liquid can expand to 1,700 times its original volume. This produces catastrophic impacts.
The expansion energy is enough to shatter solid rock, excavate craters and eject rock fragments and ash out to hundreds of metres away from the vent. We know of sites in New Zealand where material has been blasted out over 3km from the vent by such eruptions.