Tongariro volcanic eruption and ash fall – experts respond

UPDATED 10.50AM AUG 8: An explosive eruption overnight on Mount Tongariro has spread ash across the Central North Island and led to travel warnings on major highways in the region.

The volcano remains at alert level 2, indicating the onset of eruptive activity and changes to indicators of unrest. GNS Science is closely monitoring the situation.

The Science Media Centre has rounded up reaction to these developments from volcano and hazard experts.

Shane J. Cronin, Professor in Earth Sciences, Volcanic Risk Solutions, Institute of Natural Resources, Massey University, comments:

How significant is the volcanic ash threat?

“Low at the moment – ash is localised mostly to near the volcano – there is drifting fine ash interfering with aviation.”

What is going on with Tongariro in terms of the forces that created the eruption?

“Either, new magma is rising below providing gas and heat to make the shallow hydrothermal system explode, or, there has been a gradual buildup of heat/pressure in the hydrothermal system to the point at which it failed and exploded. Which of the two options is the real one is the subject of our work at the moment in analysing ash samples for evidence of fresh magma.”

 Any comments on the emergency plan underway?

“All measures have been appropriate and well executed, ongoing science response is in full swing from all major nz research institutions relevant to the area (universities, GNS and NIWA and Met)”

 Impacts on the environment, agriculture etc?

“As yet only minor impacts due to limited ashfall, work is underway to characterise the chemistry of the ash and whether it has affected surface waters.”

Assoc Prof Phil Shane, School of Environment, University of Auckland comments:

“At this stage it looks like this may have been a relatively small hydrothermal explosion. It is difficult to know whether it will lead to further activity at this stage. It does show that hot gases or fluids were close to the surface. Some big eruptions at other volcanoes have started with hydrothermal explosions. But it is too soon to forecast what will happen next. Hydrothermal explosions usually are short-lived and only ejecta material over short distances locally.”

Dr Jan Lindsay, Senior Lecturer and volcanologist at University of Auckland comments:

“It will be interesting to see how the eruption develops – whether it continues for months, or is over already. If the former we may see ash reaching Auckland, depending of course on the wind direction. We know from looking at sediment cores from Auckland lakes that ash from Tongariro has reached Auckland many times over the last 80,000 years. Of course it only needs to be in the atmosphere to cause a problem: we are already seeing flights cancelled and the impact on flights in and out of Auckland airport could be significant.”

Dr Thomas Wilson, Lecturer in Hazards and Disaster Management, University of Canterbury comments:

Type of eruption

“There are three main types of eruptions you can get in an explosive event. There’s what we call a phreatic eruption, or steam-driven eruption. This is where water has been superheated by magma under the volcano, but it’s only the water that erupts in an explosive manner. Basically the hydrothermal system underneath the volcano erupts, and this is what seems to have occurred on Tongariro. In a phreatic eruption, it’s existing rock and material underneath the volcano that is fragmented and erupted. So in a purely steam-driven eruption, there’s usually no new magma.

“Another type is a phreatomagmatic eruption, where water has come into contact with magma and caused the magma to fragment in an explosive eruption.

“Then there’s a magmatic eruption, where there’s no water interacting, and it’s just the magma erupting itself. In a magmatic eruption, the ash particles are usually a bit bigger, and it’s very fresh material, so there may be more soluble chemicals which can come off the ash. There are also a lot of magmatic gases which can condense onto the surface of the ash as it cools down. These are usually acidic, which can cause some problems.

“Scientists [from GNS] are out collecting ash samples as we speak, and there’ll be more information in the near future on those specifics, which are relevant for agriculture and human health. The key thing is that it’s a very small volume of ash that’s been produced so far. It’s a very light dusting.”
How quickly could things change?

“Things could change quickly. It’s a bit of an unknown. That’s the ‘volcano problem’, if you like. It’s difficult to know what the volcano is going to do.

“There are three main scenarios:

  • It could stop completely;
  • It could continue with these same-size eruptions, and we might get some more very light ash fall across the North Island, dependent on the wind conditions on the time;
  • It could be the beginning of a bigger eruptive sequence, which would probably mean larger eruptions, with more ash produced, probably leading to more widespread deposition of ash, to a thicker amount.

“I can’t give you any probabilities on the likelihood of these scenarios. We do know that there have been eruptions from this part of the volcano in the past, and these have typically been pretty small on a global scale. But we can’t rule anything out at this stage

“A steam-driven eruption like this could be a sign that there’s magma moving into place under the volcano, and we might see a progression through to a magmatic eruption. Or it could just be that the volcanic hydrothermal system has been unsettled by these earthquakes, and we’re seeing an eruption as a result of this.”

Is it unusual that there wasn’t more warning before this eruption?

“We saw the volcanic threat level raised about two weeks ago, following seismic activity and volcanic gases on the volcano, and that’s the same area where the eruption occurred.

“This is quite normal behaviour for volcanoes. They’ll show vague signs of unrest, like this, and then we get an eruption. The challenging thing is knowing when and where it will occur.

“The key message is that this is a really good example of why we need to be prepared for volcanic eruptions in New Zealand. These are totally normal for these volcanoes, and it’s what we should be expecting. Even if you haven’t been affected around the volcano, this is a good opportunity to think about how to be prepared and read some of the resources online about ash fall and other hazards.”

Shinmoedake volcano in Japan

“In Jan-Feb 2011, there was an eruption in the Shinmoedake volcano in southern Japan, which is very similar to Tongariro, and it erupts a similar composition of magma. It had a magmatic eruption that was much larger than what we’ve seen today. We studied its impacts on critical infrastructure, on towns and communities and agriculture. It did cause some disruption. There was centimetres-worth of ash across vast tracts of agricultural land and some towns. There were some direct impacts to agriculture, but after six months, things were mostly back to normal. There were some big issues with towns and roads, and that created coordination and management issues. There were some disruptions to electricity supplies as well, and surface irrigation water. As far as health concerns, limiting exposure, wearing a mask and avoiding times when ash was blowing around outside was sufficient to mitigate those issues. It’s quite a good example of what might happen if there were a much bigger eruption from Tongariro.”