December 9, 2019

expert reaction to New Zealand volcanic eruption

On December 9th the White Island volcano in New Zealand erupted.

Dr Dave McGarvie, Volcanologist, Lancaster University, said:

“The Whakaari /White Island eruption of 9 December 2019 occurred suddenly and without warning. This is a volcano that has a history of erupting unexpectedly. Because of this, many volcanologists consider Whakaari /White Island to be one of the most dangerous volcanoes on the planet to visit.

“At Whakaari /White Island, the combination of hot magma just beneath the surface, the ready availability of water, and an unstable volcanic architecture beneath the surface, provides a recipe for sudden and unexpected explosions.

“Eruption footage taken from boats near the island shows billowing clouds of ash obscuring the island and rolling across the sea. Photos taken from further away on the mainland show a weak and pale eruption plume that quickly dispersed. Indications from New Zealand volcanologists are that this was a short-lived and moderate explosive eruption, involving only local production and dispersal of ash. Local dispersal of ash means that flights did not have to be diverted.

“Based on the imagery and information coming out on New Zealand, one likely interpretation of this sudden eruption is that an unpredictable and rapid interaction between water and magma stored beneath the volcano, which produced what was largely a steam-dominated explosion.

“Steam-dominated explosive eruptions (also called phreatic, hydrovolcanic, hydrothermal) are amongst the most unpredictable of volcanic phenomena, and can occur at volcanoes that are going through a calm phase and that therefore appear safe to visit. A recent notable example being the 2014 eruption of Mount Ontake, which killed 63 people.

“When water suddenly meets hot magma, it flashes to steam, and in doing so there is a c.1700 times expansion in volume (at atmospheric pressure), and this can be a powerful driver of volcanic explosions.

“In places like Whakaari /White Island where there are water-filled crater lakes sitting within the summits of large active volcanoes, a steady state is often achieved when water percolating downwards meets heat escaping upwards, resulting in a gentle exchange of heat and then the heated water rises upwards and loses its heat to the atmosphere. The water in the crater lake essentially ‘buffers’ the heat trying to escape from depth, and can smooth out some fluctuations.

“However, when this steady state is perturbed, for example when a small change in the structure of the volcano occurs as magma pushes upwards and traps water that would normally escape easily, then this water will suddenly flash to steam and this can trigger yet more rapid magma-water interactions and steam production, and result in a substantial steam-drive eruption.

“One of the reasons why the massive 1883 eruption of Krakatua (Krakatoa) was so violent and devastating, was because of large and powerful explosions driven by magma-water interactions.

“Whilst the Whakaari /White Island volcano is monitored by an array of scientific equipment, in reality this is just sitting on the very top of a c.1.6 km high volcano, and so the data gathered will be much less informative than (for example) the data that can be gathered from a volcano that lies completely above the water – such as Ruapehu on the nearby North Island of New Zealand. This makes the task of forecasting eruptions from Whakaari /White Island virtually impossible. The core problem here is that – because most of the volcano lies beneath the sea – it is simply not possible for scientists to collect the type, quality, and abundance of data needed to adequately understand how this volcano behaves, and in turn to provide reasonable forecasts of potential eruptions.”

Prof Bill McGuire, Emeritus Professor of Geophysical & Climate Hazards at UCL, said:

“White Island is a sporadically active volcanic island very similar to Stromboli volcano in the Mediterranean. It does not have major eruptions, but small blasts of ash, blocks and lava are reasonably common. Because the island is only a couple of kilometres across, however, visitors are close to the vent and under threat even if there is a small explosion. The volcano has been restless for a few weeks, resulting in the raising of the alert level, so that this eruption is not really a surprise.”

Dr Jessica Johnson, Lecturer in Geophysics at the University of East Anglia, said:

“The eruption was unfortunate but not completely unexpected. Levels of activity at White Island/Whakaari have been relatively high since September, and even more elevated over the last couple of weeks, with increased numbers of small earthquakes and more volcanic gas detected than usual. As a consequence, the volcanic alert level was raised. Similar eruptions have happened over the last 100 years or so.

“Even though the alert level was raised, it is still very difficult to forecast exactly what will happen at volcanoes. White Island/Whakaari is an andesitic stratovolcano, which means that it can have lots of different types of eruptions. It also has a water-filled crater lake. When water reacts with hot rock or magma, it can create explosions, and therefore, can make eruptions even more difficult to forecast.

“Depending on the prevailing wind, residents near the coast of the North Island may experience some ashfall and may be able to smell volcanic gases. This can be hazardous for people with breathing difficulties. The ash and volcanic gases could also affect the local environment such as the marine reserve. However, on a global scale, this was quite a small eruption and so it is unlikely to affect the environment on a larger scale.

“It is very difficult to say whether there will be more eruptions like this one, but GeoNet (part of GNS Science) are closely monitoring the situation and will communicate any changes in activity to the authorities.

“White Island/Whakaari is a very beautiful and interesting destination that naturally attracts tourism. It is very difficult to say whether tourism should be allowed there. The volcano has displayed similar unrest in the past with no major eruptions. The most that the scientists can do is continue to monitor the volcano and issue information when it is available.”

Prof David Rothery, Professor of Planetary Geosciences at the Open University, said:

“White Island  (Whakaari) had 5 separate eruptive events between the year 2000 and now, with the 9 Dec eruption being the 6th and biggest.

“Background activity on White Island had been increasing over recent days, as reported by Geonet (geonet.org.nz) which has scientific oversight of volcanic activity  New Zealand). I understand that White Island is privately owned, and that the state has no control over access. In view of this unforecast (and possibly unforecastable), sudden moderately large and fatal eruption, questions will be raised about the duty of White Island Tours in allowing their clients to go ashore – but thousands have done so previously to experience the sights and sounds of an active (but not usually erupting) volcano. I was in New Zealand three years ago, and had an earthquake not prevented my travelling from the South Island I would probably have visited White Island myself.

“My guess is that this explosive  eruption was steam-driven, as a consequence of seawater interacting with magma below the active crater. Hot rock fragments and steam were blasted upwards, and  part of this cloud fell back and surged along the ground. A repeat on the same scale in the next few hours or days is unlikely, but cannot be ruled out.

“White Island volcano is a consequence of the Pacific ocean floor being subducted below the North Island of New Zealand – a process that supplies magma to volcanoes onshore on North Island that allows geothermal power generation.  This is a not a major eruption in the grand scheme of things,  and is extremely unlikely to presage increased volcanic activity elsewhere.”

Declarations of Interest 

None received