Increased seismicity and temperature at Te Wai ā-moe, Mount Ruapehu, New Zealand

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GNS Science volcanologists are reporting another phase of high lake temperatures at Te Wai ā-moe (Crater Lake), Mount Ruapehu along with moderate levels of volcanic tremor. The Volcanic Alert Level remains at 1.

GeoNet is continuously recording the temperature of Te Wai ā-moe since 2009. Over this time, the temperature has often cycled between ~ 15 and 45 °C (59 – 113 °F) over a period of about 12 months.

However, in September 2018 they recorded a departure from this, and for the following 6 months the lake temperature remained elevated (at ~ 30 °C / 86 °F).

Over the last two weeks, the lake temperature has risen further, at a rate of around 0.5 °C (0.9 °F) per day, to 42 °C (107.6 °F) on April 9. To put this in context, only 1% of post-2009 temperatures have exceeded 42 °C and a similar temperature last occurred in May 2016.

The level of volcanic tremor typically also increases when the lake temperature rises, and this has been the case in the last 2 weeks. The current tremor intensity is moderate. In previous heating cycles, this increased tremor lasted for a few days to several weeks.

In the past, eruptions at Ruapehu have occurred more often when the lake exceeded 45 °C (113 °F). However, a temperature of 46 °C (114.8 °F) was recorded in 2016 with no eruption.

While Volcanic Alert Level remains at one, which is a minor unrest, 'it is a useful reminder that eruptions can occur with little or no warning,' Duty Volcanologist Agnes Mazot said. GNS Science continues to closely monitor Mt Ruapehu and the country's other active volcanoes.

The last eruption of this volcano took place on September 25, 2007 – Volcanic Explosivity Index of 3. Its last major eruptive phase started on June 16, 1996, and lasted until September 1 of the same year (VEI 3).

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 credit: D. Townsend

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