It's been just over a week since White Island erupted, leaving 16 people dead and more injured. This morning, scientists gave a 25 to 40 per cent probability of another blow within the next 24 hours. All the while, they've been building a picture of what happened at 2.11pm on December 9 - and what might yet unfold at New Zealand's angriest volcano. Science reporter Jamie Morton put some of the key questions to GNS Science volcanologist Dr Craig Miller.

What do we now understand about the eruption, its type, and what caused it?

With the information we currently have, we believe it was a phreatic eruption.

The eruption likely would have been caused by an intrusion of magma at around 1km depth under the volcano some time in the months prior to the eruption. The magma heated water under the ground, which turned into steam.

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Pressure in the volcano's hydrothermal system reached a critical point and erupted, sending a pyroclastic density current across the crater floor out to sea. The current contained super-hot ash, steam, and gas.

We will need to test ash samples to determine whether magma was directly involved in the initial phase. We believe magma is now close to the surface - only a few tens of metres deep.

It's like a pot of water sitting on the stove. It doesn't start boiling until you turn the element on. In this case the "element" was an intrusion of magma.

We aren't yet sure when the "element" was switched on.

How does this eruption compare with events in the past?

Smaller-scale eruptions occurred between 2012 and 2013 and in 2016. Luckily, all these were at night.

Eruptions at Whakaari/White Island have happened in a number of different ways.

The 1970 to 2000 eruptive period contained more magmatic eruptions, where hot lava bombs landed on the crater floor and some were thrown out to the ocean.

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Ash eruptions were also common during this period. Since 2011, we have seen more phreatic-style eruptions.

However, in 2012 a lava dome was produced from degassed magma that slowly made its way to the surface of an active vent in the crater lake area.

What's happened over the week since in terms of general activity?

Following Monday's eruption, we had a two-day period of very high volcanic tremor, between December 11 and 13.

This was followed by a sudden large, decrease in tremor on December 13 and has remained at a low level since then.

Since the tremor dropped, we've established that there is an open vent at the back of the crater basin. Steam and gas can now pass through this vent unrestricted, which probably resulted in the drop in tremor.

Since the eruption, there have been high gas emissions. As the system is now freely degassing, gas emissions remain high.

We've confirmed this with gas detection flights and through visual observations of the vent area from helicopter.

Scientists have given White Island a 25 to 40 per cent - or unlikely - chance of erupting in the next 24 hours. Photo / White Island Flights
Scientists have given White Island a 25 to 40 per cent - or unlikely - chance of erupting in the next 24 hours. Photo / White Island Flights

How has GNS Science been monitoring it?

We've been receiving continuous data on volcanic tremor and earthquakes via two seismometers on the island. Both of these stations continue to function.

We also use three volcano cameras for visual observations of activity. Only one of these cameras is reliably functioning at the moment.

Our volcano geochemists have conducted two gas flights and one visual observation, thermal imaging flight. We receive daily satellite observations of sulphur dioxide gas.

Continuous GPS measurements from two stations on the island measure any ground swelling in the crater floor, or deformation. We are also working with MetService to detect ash in the atmosphere.

What do we know about the gas concentrations and general environment on the island right now. What are the most dangerous elements?

Measurements in the air don't accurately reflect levels on the ground, and we don't have any direct measurements of gas concentrations on the island.

However, we can say conditions are likely to be very unpleasant.

Are GNS Science staff looking at returning to the island any time soon?

We have no plans to send staff to the island in the near term. We have permissions to allow gas and visual overflights to occur, but we do not plan to send staff physically onto the volcano.

We can take gas and visual flights a safe distance from the volcano.

The 24-hour eruption probabilities have been falling somewhat over the week. Can we read much into that?

The situation on Whakaari/White Island is still volatile, and our 24-hour scenarios still contain a degree of uncertainty.

As time goes on, our understanding of what has occurred increases. As we collect more data we are able to fine-tune our scenarios of likely future activity and gradually adjust our probabilities/expert judgement to reflect that.

Is there any chance of another eruption producing effects that could reach the mainland?

It is possible that people might smell gas or that very fine, trace amounts of ash may make its way to the mainland. These are not life threatening to those on the mainland.

The volcano has been in a state of unrest since 2011. Is this activity likely to extend that period for a long time to come?

It's too early to tell. If there's new magma involved, this period may continue for a while. It all depends on the magma supply to the volcano.

An ash-coloured White Island after its eruption on December 9. Photo / Auckland Rescue Helicopter Service
An ash-coloured White Island after its eruption on December 9. Photo / Auckland Rescue Helicopter Service

If there is a magmatic eruption, that may signal a longer period of unrest with likelihood of future eruptions.

Why shouldn't people be concerned about White Island's activity having any correlated effect elsewhere?

Although all the active volcanoes in New Zealand are linked broadly via the plate boundary, they all stand alone. So that means activity or volcanic unrest at one volcano does not influence any of the others.

The eruption at Whakaari/White Island is not going to cause other volcanoes or geothermal systems in New Zealand to change.

The best way to think about it is to liken Aotearoa's volcanoes to a street full of houses.

The Taupo Volcano Zone is the street, and the volcanoes are different houses on that street.

Each house has its own plumbing system, so someone flushing a toilet in one house doesn't make the sink overflow in another house. This is the same for volcanoes.