A catastrophic burst of steam
Whakaari/White Island is in a state of perpetual reconstruction, rebuilding itself in a new image after every eruption over its 150,000 year life.
What we can see of White Island – a ragged, steep-sloped amphitheatre, pock-marked with ashen craters and vents, all in the post-apocalyptic colour scheme of grey, white, and brown – is actually the summit of a vast marine mountain, around 1900m tall and similar in size to neighbouring Tongariro.
It has been called New Zealand’s most active volcano, at least in recent times. There have been dozens of recorded eruption events, six of which have happened in the past two decades. They happen quickly and sharply and always without warning.
The operating force on White Island, like a lot of New Zealand’s most dramatic landscapes, is tectonic. The volcano is part of the Taupo¯ Volcanic Zone, which is on the boundary of the Pacific and Australian plates, clashing and grinding together endlessly.
At the northern part of the boundary, the Pacific plate is subducting beneath the Australian plate (the reverse is happening at the southern end). This produces hot, gas-filled material called magma. The magma rises and pushes its way upwards within volcanic systems like White Island, where it settles in a chamber within the volcano.
The heat and gases from this magma rise, escaping from the chamber through cracks and vents. At White Island, it creates an active hydrothermal system around the volcano, warming up the groundwater and rainfall and producing steam.
The steam is mostly water, but also contains sulphur and carbon dioxide, and escapes through springs and small vents on the crater called fumaroles. This is the cause of the unusual white clouds often billowing around the island, and the bubbling crater lakes that periodically appear and disappear, which together serve as a visual demonstration of the volcano’s volatility. While this is happening, minerals are crystallising within the volcano, causing blockages in the vents that allow pressure to build.
It is not yet certain what caused Monday’s eruption, but experts say it was likely a phreatic eruption, caused when steam pressure can no longer be contained. These eruptions happen when water combines with something extremely hot, like magma or molten rock. The water becomes steam, quickly and massively expanding in volume, which erupts with such power it can obliterate rock.
Based on photographs from witnesses, the eruption appears to have come from the western side of the crater, potentially from within the crater lake itself. Much like previous eruptions at White Island, there seems to have been little evidence of magma or lava within the debris. It appears to be primarily steam, ash and rock.
Experts say the eruption may have been a consequence of new magma entering the system, evidenced by increased seismic activity and volcanic gas emissions in recent weeks. If this new, blazing hot magma combined with water, it would have burst through the volcano’s crust almost instantaneously.
It is not possible to accurately predict such eruptions, although there are signs that they are more likely to happen in a given period. It is likely the force of the eruption and the cascading debris has formed new craters, opened new vents and collapsed existing landforms, as the volcano once again reiterates itself.