Elevated volcanic unrest at Mt. Ruapehu, CO2 output now 2nd highest ever recorded, New Zealand

Elevated levels of volcanic unrest continue at New Zealand’s Ruapehu volcano. This is primarily manifested as strong volcanic tremor, slowly rising Crater Lake temperatures, and high gas outputs. These changes indicate magma may now be driving the unrest, increasing the chances of further activity. The Volcanic Alert Level remains at Level 2.

Over the past seven days, the elevated volcanic unrest has continued, increasing to heightened levels.

The tremor levels remain elevated but have declined from the peak reached on April 6 and 7. An airborne gas flight completed yesterday has confirmed an increase in gas output through the Crater Lake, in particular of CO₂, which is now at the 2^nd highest ever recorded value.¹

The Crater Lake (Te Wai ā-moe) temperature is now 38 °C (100 °F) and modeling suggests that about ~280 MW of heat is required to sustain this lake temperature.

“Temperature and heat input remain within the typical range for a heating cycle. The crater lake remains a battleship grey color with upwellings at the northern vents area and a small overflow. Sulfur slicks are present on the lake surface,” said GNS Duty Volcanologist Brad Scott.

The sustained nature of the volcanic unrest now differs from those typically seen during the start of a heating phase. The volcanic tremor signals and elevated gas output are now more indicative of processes being driven by magma interacting with the geothermal system at depth in the volcano.

“The most likely outcome of this unrest episode is no eruptive activity occurs, as no eruptions have followed unrest in the past 15 years,” Scott said, adding that there is also a possibility of a single or multiple eruptions that could impact the summit area and generate lahars into some catchments draining off the volcano, especially the Whangaehu Valley.

“The size of these eruptions, if they did occur, would probably be like the September 2007 event.”

“The chances of a prolonged and larger eruption, such as occurred in 1995-96 with wider ashfall impacts, is possible but remains very unlikely. Such an eruption would most likely only follow a sequence of smaller eruptions,” Scott concluded.

Geological summary

Ruapehu, one of New Zealand’s most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200 000 years ago.

The 110 km³ (26.4 mi³) dominantly andesitic volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km³ (24 mi³) ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank.

A series of subplinian eruptions took place between about 22 600 and 10 000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake, is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene.

Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3 000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.²

References:

¹ Mt Ruapehu elevated volcanic unrest continues. Volcanic Alert Level remains at Level 2 – Volcanic Activity Bulletin – RUA – 2022/05 – GeoNet – April 12, 2022

² Ruapehu – Geological summary – GVP

Featured image credit: James Shook