Why White Island volcanic eruption came as a surprise? Experts say it was caused by steam, not magma, which is harder to track
At 2.11 pm local time on Monday, a devastating volcanic eruption rocked Whakaari/White Island, off the east coast of New Zealand, claiming five lives.
The eruption created a steam and ash column to 12,000 feet (3-4 km) above the island. The island in New Zealand’s Bay of Plenty is a tourist destination, and 47 people were on it when it erupted on Monday afternoon.
Volcanologists at GeoNet, which operates a geological hazard monitoring system, described it as an “impulsive, short-lived eruption, of 1-2 minute duration", which impacted the main crater floor area. Ash and debris cover the crater floor.
According to an update on Tuesday at 11.45 am local time, since Monday’s eruption, no further eruptions have occurred. However, scientists continue to observe localized steam and mud jetting from the active vent area. The volcanic alert level stays at 3, which means minor volcanic eruption.
"Since Tuesday morning, the seismic activity has remained weak, yet we continue to see periodic steam and gas driven jetting from the active vent area," said GeoNet in its third update on December 10.
Was there any warning
Three weeks before the eruption, GeoNet raised the volcanic alert level from 1 to 2 on November 18 to reflect heightened activity at the island. Volcanic alert level to 2 implies moderate to heightened volcanic unrest.
Is it unusual to go from level 2 to an eruption? According to GeoNet, an eruption can occur at any volcanic alert level (level 1 or 2). There are also times when the volcano is raised to level 2 with no subsequent eruption.
“About three weeks ago, we raised the alert level to 2. That reflected a couple of things: there was more sulfur dioxide, which is a gas that comes out of magma at relatively shallow depth. But there was also a lot of steam and mud bursting events, and that is a sign there is something that is changing at the volcano. For us, it is difficult to tell when the eruption would be. We were raising the level of concern because there is a lot of uncertainty in understanding these kinds of events,” volcanologist Geoff Kilgour from GNS Science, a New Zealand-based consultancy group, told the Breakfast show.
He said: “This kind of bursting event, which is essentially like a balloon bursting, is almost impossible to predict. So, our raising of the alert level was to reflect that kind of uncertainty.”
Kilgour also told the National Geographic: “Monitoring efforts and reports from tourist companies also picked up some geyser-like convulsions at the time, along with an uptick in gas emissions and seismic rumblings. So, authorities did raise the volcano’s alert level. Alert level rises don’t mean that an eruption is inevitable, and in many cases, no eruption is forthcoming. Unfortunately, this time was a deadly exception.”
Why the eruption took everyone by surprise
In an era of global satellite monitoring with proliferating networks of instruments on the ground, why can we still not accurately predict volcanic eruptions?
When erupting, all volcanoes pose a degree of risk. However, the risks are not equivalent from one volcano to another because of differences in eruptive style and geographic location. According to scientists, an eruption like the one at White Island, though not very common, is also not exceptional. They have happened at volcanoes elsewhere in the world and will continue to occur with little or no warning.
The National Geographic article says: “But for this volcano, and for the type of eruption style involved, it was nothing out of the ordinary. Similar eruptions, though not everyday occurrences, have happened at many volcanoes all over the world, and they will continue to appear without much warning.”
Sitting 48 km offshore, White Island is currently New Zealand’s most active cone volcano, which has been built up by continuous volcanic activity over the past 1,50,000 years. About 70% of the volcano is under the sea, making this massive volcanic structure the largest in New Zealand.
However, experts say the eruption at White Island was caused by steam and not by magma, which is harder to track. Such an eruption can happen suddenly and with little or no warning.
“Whakaari/White Island is a wet volcano with a crater lake and typically has explosive eruptions. These occur with a little to no warning and often impact the main crater floor area. These explosions eject hot rocks (cannonball-like projectiles), ash clouds, and surges (pyroclastic density currents),” says GeoNet.
Monitoring, prior warnings for such eruptions are difficult
Shane Cronin, Professor of Earth Sciences, University of Auckland, describes such a steam-driven eruption as a hydrothermal or a phreatic eruption. He said scientists do not usually see these eruptions coming.
According to Cronin, “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 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,” Cronin wrote in The Conversation.
According to scientists, multiple types of eruptions can occur at each of New Zealand’s volcanoes — the eruption type can vary minute to minute.
Experts say the style of eruption depends on many factors, including the magma chemistry, temperature, viscosity (how runny the magma is), volume, and how much water and gas is in it, the presence of groundwater, and the plumbing of the volcano.
A hydrothermal eruption is driven by the heat in a hydrothermal system. Hydrothermal eruptions pulverize surrounding rocks and can produce ash, but do not include magma. These are typically very small eruptions.
A phreatic eruption is driven by the heat from magma interacting with water. The water can be from groundwater, hydrothermal systems, surface runoff, a lake or the sea. Phreatic eruptions pulverize surrounding rocks and can produce ash, but do not include new magma.
“A phreatic eruption is driven by superheated steam and gas. Often this steam and gas build-up behind a rock and mineral seal, and when the strength of that seal is exceeded by the gas pressure, an explosive eruption can occur. The gas driving the eruption likely comes from a deeper source of magma, but the magma itself is not directly involved. Phreatic eruptions can also occur when hot magma comes into contact with cold groundwater, but we don’t believe this is the situation at Whakaari/White Island,” said GeoNet expert.
What happens now
GeoNet’s monitoring equipment continues to operate, and they are closely monitoring the situation on the island and providing near real time data.
While experts have seen a steady decline in activity, there remains significant uncertainty about any future activity. GeoNet experts say over the next 24 hours they estimate an equal likelihood of either no eruption or a smaller/similar-sized eruption that would impact the main crater floor.
"There is a high level of uncertainty associated with this estimate and we are working to reduce that uncertainty. We also estimate the least likely scenario is a larger eruption. There is an extremely low likelihood of any ash impact to the mainland, but people may smell gas, depending on the prevailing wind direction," says GeoNet.
GNS Science also continues to closely monitor Whakaari/White Island for further signs of activity.
According to Cronin, monitoring and warning for hydrothermal eruptions is a considerable challenge. Many systems, he explains, are already “primed” for such events, but the triggers are poorly understood. “Our only hope for anticipating these events is to track potential vapor and liquid pressure in hydrothermal systems and to learn from their long-term behavior when they are at a super critical state. Unfortunately, there are no simple rules that can be followed, and each hydrothermal system is different,” he says.