Tongariro eruption: All you need to know

Vic Cassin sweeps ash from one of the vehicles parked on his section near Turangi, which was in the path of the ash cloud that resulted from last nights eruption of Mt Tongariro. Photo / Alan Gibson

Mt Tongariro rumbled to life for the first time in more than a century this morning, spewing a giant ash cloud as far as Napier.

Dr Thomas Wilson, a disaster management lecturer at Canterbury University, explains all you need to know about the eruption, its aftermath and what the future may hold.

What type of eruption was it?

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.

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.

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.

Is it similar to any other recent eruption?

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.