From nothing happening to Tarawera-type destruction - the volcanic alert levels

Thursday, 12 December 2019

Before Whakaari/White Island erupted on Monday afternoon it was at volcanic alert level 2. In the minutes after the volcano erupted the alert was raised to level 4, then within about two hours it was down to level 3, and on Thursday morning it was dropped back to level 2.

What does that all mean? GNS Science has made a video showing examples of New Zealand volcanoes at the different levels.

It starts with a summertime picture of Ngauruhoe with nothing happening in the crater. That's level 0 - nothing to see here.

White Island is shown at alert level 2 in 2013, with volcanic activity contained to the crater. 'There will be volcanic unrest hazards and a potential for an eruption,' a narrator says.

* Why White Island erupted, and why there was no warning

What are Volcanic Alert Levels in New Zealand?

* Whakaari/White Island map shows the volcano's red zone**

Tongariro/Te Maari in 2012 is the example for level 3. It looks dramatic with a dark plume shooting skywards, but just the area around the active volcanic vent is affected.

Ruapehu in 1995 and 1996 is used to illustrate level 4 - the level White Island reached briefly immediately after the eruption on Monday. Level 4 indicates a 'moderate volcanic eruption' where hazards 'extend way beyond the active crater onto the flanks of the volcano and maybe even more distant'.

To illustrate a level 5 'major volcanic eruption' the video uses a Charles Blomfield picture of the 1886 Tarawera eruption. An event that big includes 'significant hazards on the volcano and can be affecting communities way away from the volcano'.

In comparison, the seismogram for Ngauruhoe during the same 24 hours.

A seismogram for White Island for the 24 hours to around 7pm on Thursday.

Ruapehu 1995/96 - volcanic alert level 4

A representation of the activity at Whakaari/White Island based on digital data can be seen in a seismogram on the GeoNet website.

Seismograms display seismic monitoring data and are read from left to right and top to bottom, and cover the latest 24 hours.

GeoNet noted volcanoes such as White Island and Ruapehu frequently had earthquakes almost identical to those produced during an eruption when no eruption was under way.