Heightened volcanic unrest at Mount Ruapehu, New Zealand


The temperature of the Crater Lake (Te Wai ā-moe) at Ruapehu volcano has decreased from 43 to 41 °C (109 – 106 °F) during the past week. The level of volcanic tremor continues to be moderate to strong and a small number of shallow volcanic earthquakes have been recorded. While the crater lake may have stopped heating, volcanic activity at Ruapehu remains elevated — the Volcanic Alert Level remains at Level 2 and the Aviation Colour Code at Yellow since December 21, 2020.

"Since our last update [on December 21, 2020], Ruapehu’s Crater Lake (Te Wai ā-moe) temperature has decreased slightly. The temperature reached a maximum of 43 °C (109 °F) on December 21 that subsequently declined to about 41 °C (106 °F)," GeoNet's Duty Volcanologist Agnes Mazot said on December 28.

A lake temperature peak of about 40 – 46 °C (104 – 115 °F) is common during these heating-cooling cycles; on at least 7 occasions since 2007, the lake temperature exceeded 40 °C.

Using this lake temperature decrease, it is estimated that the energy input into the lake has decreased from ~400 to ~200 MW (megawatt).

Elevated volcanic tremor is still ongoing along with a few volcanic earthquakes, Mazot added.

The largest of these earthquakes was M2.2 on December 26, located under the volcano. Volcanic earthquakes of this size are uncommon and the combination with elevated tremor and still high lake temperature indicate moderate to heightened volcanic unrest. Because of this, the Volcanic Alert Level remains at Level 2.

"We continue to closely monitor Ruapehu, and this week we will attempt a gas flight to measure volcanic gas emissions. We will also attempt to sample the water at the crater lake for further chemical analyses. Together, these will help us identify processes occurring at depth beneath the volcano."

Mount Ruapehu on December 19, 2020. Credit: Copernicus EU/Sentinel-2, TW

The Volcanic Alert Level is still at Level 2 which reflects the current level of volcanic activity. Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of volcanic unrest. The Aviation Colour Code remains at Yellow.

The Volcanic Alert Level should not be used to forecast future activity, however, at Volcanic Alert Level 2, eruptions are more likely.

Volcanic Alert Level 2 indicates the primary hazards are those expected during volcanic unrest: steam discharge, volcanic gas, earthquakes, landslides and hydrothermal activity. While Volcanic Alert Level 2 is mostly associated with environmental hazards, eruptions can still occur with little or no warning.

The Department of Conservation (DOC) has closed the summit area of Ruapehu, including a 2 km (1.2 miles) radius from the center of Te Wai ā-moe/Crater Lake.

"Our priority is to protect public safety, so we ask visitors to respect the exclusion zone under Volcano Alert Level 2 [VAL2] for their own safety," Tongariro Operations Manager Connie Norgate said.

Mount Tongariro and the Tongariro Alpine Crossing are not affected by increased volcanic unrest at Mount Ruapehu and remain open.

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. (GVP)

Featured image: Mount Tongariro Crater Lake on December 19, 2020. Credit: Copernicus EU/Sentinel-2, TW


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