EXPERT REACTION: Whakaari/White Island volcano eruption

Volcanoes are categorized into extinct, dormant and active. White Island belongs to the latter category, as part of the Pacific Ring of Fire - a network of volcanoes along the boundaries of the plates that make up Earth's outer shell.

Active volcanoes have a so-called plumping system; a network of subsurface molten rock and extremely hot fluids. During cooling, this magma network forms crystals and through this process gets "thicker" (higher viscosity) and builds up vapor, like in a pressure cooker. When new, fresh hot melt is replenished from the deep Earth's interior, this reservoir overpressures and evacuates melt and gas towards the surface. Close to the surface, the pressure is so high that rock (formed from lava) bursts into tiny particles - ash. A cloud of hot ash, mixed with toxic, acidic gases (CO2, SO2 and Cl-F) then erupts. The mix is often fatal, mainly because of the extreme temperatures of the gas (> 800 degrees C) and an initial jet velocity of > 600 meters per second.

The trigger of this eruption is not predictable and can, in theory, reoccur any time. Geologic process in volcanoes often occur on timescales that are longer than human perceptions of events classify as "non-active". So it seems, and rightly so, that active volcanoes are harmless. Until, like now, they suddenly erupt. The eruption was probably not foreseeable or clearly not predictable. Subsurface processes are very complex, and as geologists, we only have limited ways to "look into the deep".

White Island has been a disaster waiting to happen for many years. Having visited it twice, I have always felt that it was too dangerous to allow the daily tour groups that visit the uninhabited island volcano by boat and helicopter. White Island lies almost 50 kms offshore from Whakatane, and experiences significant explosive eruptions every 3 to 5 years. It has a very active geothermal system with many steaming gas vents and varying numbers of hot water filled crater lakes in the floor of an amphitheatre shaped large crater. The volcano is a marine stratovolcano. In the late 1800's a sulphur mine was established on the island, with a small settlement for the workers. This was destroyed in 1914 when a rock avalanche of hydrothermally weakened rock collapsed from the crater walls and inundated the mining buildings, most of which were destroyed with significant loss of life.

Many of the explosions are hydrothermal, resulting from superheating of the geothermal waters by molten rock or magma within the volcano. Most of the material that is explosively ejected is hydrothermally altered country rock for around the geothermal vents, but sometimes the ejected materail includes fragments of fresh magma. In addition to the hot rock fragments and fragments of magma, large volumes of volcanic gas and superheated steam are released which produce a hot plume of gas and rock that rises above the vent, sometimes to heights of thousands of metres. The temperature of the erupting mass can the several hundred  degrees Celsius. Hazards include rock projectiles, noxious gases and burns from the hot gas cloud.

In the lead up to major eruptions there can be elevated levels of steam release, small explosions and increased seismicity, as occurred on this occasion in the last two weeks.”

White Island is an active volcano off the east coast of north island NZ. It is part of the Taupo volcanic chain that crosses the north island. It is known as a stratovolcano - similar to Mt St Helens and many other classic come-shaped volcanoes. It is related to subduction of oceanic crust into the mantle along a plate boundary east of NZ - the Kermadec-Tonga arc.

It erupts sporadically and often unpredictably from time to time. It is a gas-charged andesitic volcano - similar to Stromboli and other Mediterranean. It has erupted many times historically and has claimed lives before. The latest eruption appears to have been a phreatic or gas eruption which produced a column of ash and debris

The number of tourists visiting active volcanos is increasing globally as part of an increase in both geological tourism and adventure tourism. Part of the attraction is to visit an unpredictable natural environment and for most tourists they assume that they will be able to visit such dangerous sites in relative safety.

However, despite the increased science behind predicting volcanic eruptions, the science is not infallible and ‘active’ volcanos may erupt at any time. Therefore volcanic hazard management guidelines need to be easily understood by the public so that visitors can weigh up their their risks in relation to visiting such sites.

One such strategy is to simply allow visitors to view active volcanoes from a distance and not allow them on to any volcano deemed still in its active phase. In this way risks to the public will be reduced whilst still allowing adventurous visitors a chance to see and experience those elements of the landscape which are active and dramatic.

White Island is the emergent peak of a large submarine volcano located about 50 km offshore of the North Island of New Zealand in the Bay of Plenty. It is one of a chain of volcanoes stretching from Ruapehu southward in the North Island, northwards through numerous (about 35) submarine volcanoes to the Kermadec group (Curtis, Macauley and Raoul) and onwards to the Tonga chain. The volcano is one of many in the 'Ring of Fire'.

White Island has been showing signs of unrest for the past few weeks according to the monitoring group of the Institute of Geological and Nuclear Sciences in New Zealand. The eruption this afternoon at 1411 hours NZ local time, was a short-lived explosive event. In addition to gases, fragmented rock particles (ash) rose ~4000m above the vent; ash covered the floor of the crater. Camera footage shows that tourist helicopters have been severely damaged.

Tourists were exploring the floor of the crater, and were reported to have been affected, some severely, by the explosive activity.