Hot ash surged fast across White Island crater floor

Tuesday, 10 May 2016

The White Island webcam showed ash from the explosion, which has settled on the crater floof.

The hot ash that erupted from White Island without warning a fortnight ago raced across the crater floor at speed, rather than being ejected skyward then falling back to earth.

'Those sort of steam explosions have a vertical component but also a very large horizontal component,' GNS Science volcanologist Brad Scott said on Tuesday.

An aerial view showing some of the green ash that covered about 80 per cent of the White Island crater.

'The horizontal steam blast was basically just sand and gravel; very hot and travelling very fast.'

It was not known how fast but it would have been quicker than a person could run. 'It has a texture like apple crumble and was very dry, this indicates it was very hot when deposited,' Scott said.

'We are really happy the eruption happened at night. The blast would have been travelling fast and was hot hence survivability during the eruption would have been low in many areas of the crater,' he said.

'You see the rocks, and you will see the side facing the crater is all plastered, and the side facing away isn't. That's pretty indicative of something travelling horizontally … going fast enough it leaves a shadow on the down wind side.'

The eruption was driven by steam and gas, which would have essentially made up the 4km high plume picked up on MetService rain radar at 10pm on April 27.

The green-tinged ash contained no new juvenile material, just old lake sediments and crater wall material. Had there been new material it could have indicated the start of a whole new phase of activity for White Island. The green colour was due to the unique combination of minerals on White Island.

The 'very energetic blast' happened during a period of volcanic unrest and covered much of the Main Crater floor and the north eastern part of the volcano with ash, Scott said. A new crater was created, landslides were generated and some of the Crater Lake floor was excavated.

Aerial photographs and thermal images were used to identify the site of a new vent that erupted on April 27. There was also significant collapse in that area.

It was still considered too risky to allow people into the crater but photographs suggested the ash could be up to 300mm deep.

Around 500 metres from the eruption site the ash was about 5mm thick, with the ash extending off the island to the north east.

The volcanic alert level for White Island was lowered to 1 at noon on Monday, indicating minor volcanic unrest, with both volcanic gas output and seismic activity decreasing.

The level had been lifted to 3 after the eruption, indicating a minor volcanic eruption, then lowered to 2 by the evening of April 28.

'Our alert level system isn't predictive. It's more about the current status of the volcano,' Scott said. '

A low alert level doesn't mean the risk has gone away.' GNS kept a constant watch on White Island.

There was no link between activity at White Island and Mt Ruapehu except 'they are both active volcanoes'.

A volcanic earthquake swarm was recorded beneath the Crater Lake of Mt Ruapehu in late April and early May but has stopped.

The lake remains hot with a temperature reading of 45.2 degrees Celsius taken during a flight over the crater on Tuesday morning.

That would be the highest temperature recorded since the lake was established after 2000. 'The Crater Lake is still certainly very hot … In the last couple of days the temperature has only changed by about 1 degree, that's not indicating it's getting hotter or colder,' Scott said.

'We've had eruptions from cold lakes, eruptions from hot lakes, lots and lots of hot lakes and nothing happened.'