White Island volcano eruption: what happened?
On 9 December 2019 the volcano on White Island, about 50km away from the northeast coast of New Zealand’s North Island, erupted explosively. Of the 47 people on the island at the time, 18 were killed and many more severely injured. White Island has been classified as New Zealand’s most active volcano since 1976, when it began a period of 24 years of almost continuous eruption. A second period of eruption started in 2011 and continues today. Volcanologist Prof Bill McGuire takes us through what happened.
HOW MUCH OF A SURPRISE WAS THE WHITE ISLAND ERUPTION?
The eruption came as no surprise to volcanologists. Like Stromboli, which lies in the Tyrrhenian Sea off the western coast of Italy, Mount Etna on the eastern coast of Sicily, and Kilauea in Hawaii, White Island is a volcano that is almost continuously showing signs of activity. A feature that makes it very attractive to tourists.
The volcano has an open vent that is typically steaming or sending out jets of gas. Periodically, activity escalates, producing small to medium eruptions of ash, blocks and volcanic bombs – pieces of molten rock with diameters greater than 64mm. Before this most recent blast, the volcano had erupted five times in the last 20 years.
WHAT EXACTLY HAPPENED? DOES THE WHITE ISLAND VOLCANO HAVE ANY UNUSUAL FEATURES?
White Island is actually two overlapping volcanoes poking out of the Bay of Plenty, 50 kilometres northeast of New Zealand’s North Island. A major collapse of the crater wall in 1914 wiped out a sulphur mine on the volcano and killed those working there.
The most recent eruption was caused by rising magma coming into contact with groundwater, resulting in a violent but short-lived – just 12 minutes – hydro-magmatic blast [a violent combination of water and magma] that sent a cloud of ash up to four kilometres into the atmosphere. At the same time, ground-hugging clouds of steam, hot ash and rock poured rapidly across the terrain.
WERE THERE ANY WARNING SIGNS?
Yes. There were clear signs that an eruption of some sort might have been on the way.
“There were clear signs that an eruption of some sort might have been on the way”
White Island had shown signs of becoming increasingly restless since at least October, with a rise in sulphur dioxide gas emissions and elevated levels of ground tremors. The alert level was raised from 1 to 2 on 15 November and by early December the volcano was producing violent jets of gas, as well as ejecting mud and debris up to 30 metres into the air. [Volcano alert levels go from 0 (no volcanic unrest) to 5 (major volcanic eruption).]
WHAT MAKES VOLCANO ERUPTIONS SO DIFFICULT TO PREDICT?
Provided volcanoes are being monitored, their eruptions are not difficult to predict. To reach the surface, magma must break rock, which generates swarms of small earthquakes detectable by seismometers. Magma also causes the ground to swell, which can be measured using GPS and similar techniques.
Often, the rates of increase of earthquakes and ground swelling can be used to estimate the start of an eruption. But predicting the exact timing of small hydro-magmatic blasts, like the one at White Island, is notoriously difficult.
IS IT SAFE TO VISIT ANY VOLCANO AS A TOURIST?
No volcano is completely safe, either for the volcanologists working on it or for tourists who are just visiting. Most tourists want to see some sign of activity, and this is the problem. The volcanoes that tourists visit most – White Island, for example – are more active. This means that the risk of an eruption causing injury or death is higher.