By Guest Author 30/06/2016

by Dr Benjamin Moorhouse

Recent heightened volcanic activity from Ruapehu has resulted in GNS scientists raising both the Volcanic Alert Level (VAL) from level 1 to 2 and the Aviation Colour Code from green to yellow. But what does this mean?

ruapehuThe NZ Volcanic Alert Level system ranges from level 0 to 5 with level 2 being defined as moderate to heightened volcanic unrest. This level has been designed by volcanologists in New Zealand to indicate when a volcano is showing stronger signs of life and has a higher likelihood of erupting.

The volcano monitoring team at GeoNet base their VAL decision on changes or symptoms caused by the interaction of magma (melted rock), gas and fluids under the volcano. These symptoms, including changes in heat and chemistry, seismicity (earthquakes), ground deformation and gas release can be measured and monitored.

Pit of noise

Ruapehu, the Māori word for ‘pit of noise’ or ‘exploding pit’ is the largest active volcano in New Zealand and the highest peak in the North Island, reaching heights of 2797 m. Located in the southern part of the Taupo Volcanic Zone, a highly active volcanic V shaped area in the North Island of New Zealand created by the subduction (downward movement of one plate of the Earth’s crust into the mantle beneath another plate) of the Pacific Plate below the Australian Plate.

ruapehu pic
Subduction of the Pacific Plate underneath the Australian Plate creating volcanoes. Image: Benjamin Moorhouse.

Ruapehu first started erupting around 250,000 years ago and geological investigations show that three summit craters have been active over the last 10,000 years, with the currently active vent being beneath a crater lake called South Crater. Large eruptions historically seem to occur around every 50 years with recent eruptions occurring in 1895, 1945 and 1995. Minor eruptions are more frequent with 60 being recorded since 1945 and include events in 1997 and more recently on 25th September 2007 when an explosion lasted 7 minutes but no high eruption plume was generated.

Lahars a major hazard

A major volcanic hazard associated with Ruapehu is the occurrence of lahars – a type of mud and debris flow composed of volcanic material, rocks and water caused by the collapse of the crater lake wall, either during or long after an eruption. Famous examples include the Tangiwai disaster in 1953 and most recently a dam break in 2007.

Crater lakes form after major volcanic eruptions when it becomes cool enough for rain water and/or snow melt to build up and create a lake. These lakes can be cool or like Ruapehu, warm to hot and acidic due to still being connected to underlying volcanic plumbing. The crater lake of Ruapehu offers an excellent way to study and monitor the volcano because the geothermal system associated with the lake records signals of what is occurring at depths within the volcano.

GNS Scientists monitor Ruapehu through the GeoNet Project with 2 web cameras, 10 seismographs, 6 microphones, 9 continuous GPS stations to record ground deformation as well as regular water and gas monitoring visits to the crater lake and airborne gas surveys.

Since late 2015, a data logger near the outlet of the crater lake had been recording a steady rise in temperature but between Mid-April and May this year lake temperatures soared from 25 °C to 46 °C.

ruapehu pic 2
Top of Ruapehu and Crater Lake. GNS.

This rise in temperature correlated with a volcanic earthquake swarm beneath the crater lake beginning on the 25th of April, peaking a day later but declining by the 3rd of May. Earthquakes striking in a relatively short period of time like this has not been a common occurrence at Ruapehu in recent years.

Being an active volcano, Ruapehu produces volcanic tremors continuously and the level varies a great deal, caused simply by volcanic processes such as gas release or magma movement and does not necessarily indicate an upcoming eruption. Although common, recent measurements of volcanic tremor at Ruapehu has recorded a persistently increased level.

Elevated gas levels

Two monitoring visits (airborne and ground-based) to Ruapehu on May 10th to measure gas output and sample the crater lake water indicated an increase in both the output of carbon dioxide (CO2) and sulphur dioxide (SO2).

The confirmed increase in volcanic gas output from Ruapehu’s crater lake, the continuation of moderate levels of volcanic tremor and lake temperatures still reaching 45 °C means volcanologists have enough data to suggest that Ruapehu is at a higher level of volcanic unrest. As a result, GNS revised the Volcanic Alert Level from 1 to 2 (moderate to heightened unrest) and The Aviation Colour Code from Green to Yellow.

Since upping the level, GNS volcanologists have reported that, “while at the top end of the level we could have steam and gas driven events (geysering) within the Crater Lake, we are not there at the moment. In the coming days or weeks, we could see activity increase or decrease”.

A volcano can be at Volcanic Alert Level 2 for months, years or even decades and does not necessarily mean an eruption will occur and GNS volcanologist Geoff Kilgour said the likelihood of Ruapehu erupting is still low.

The GNS team of volcanologists continue to closely monitor Ruapehu through the GeoNet project.

Dr Benjamin Moorhouse is a science communicator who completed his Phd at the Department Of Geology at the University of Otago. His PhD thesis was: “Geology of the North Otago region of New Zealand”.