Ruapehu Causes Jitters

The growing threat of a lahar from Mt. Ruapehu’s crater lake has caused minor disruption to travellers on the Desert Road section of State Highway 1 today.

Two days ago, the Department of Conservation announced that the tephra dam, which is preventing the contents of the crater lake from cascading down the mountainside, had started leaking. This afternoon, the lahar warning system was triggered by scientific maintenance work on the volcano, and emergency services swung into action, temporarily closing State Highway 1 to traffic.

At 2797 metres, Mt. Ruapehu is the tallest mountain in New Zealand’s North Island. The mountain is a volcano located at the southern end of the Taupo Volcanic Zone, and last erupted in 1995 and 1996.

A report on the GNS Science website shows that the last two eruption episodes caused significant changes to the crater at Mt Ruapehu. It states: “Portions of the southeastern rim of the crater were eroded and deposition of new tephra has raised the elevation of the rim by 6-7m in the former outlet area. Studies suggest that the potential for collapse of the main crater wall is low but that the weak, poorly compacted and permeable new tephra barrier is strongly prone to erosion and/or collapse once the lake refills. The worst possible scenario in these circumstances is sudden collapse of the new tephra dam causing a lahar as large or bigger than the 1953 Tangiwai lahar.”

It is this tephra barrier or dam that is being closely monitored by GNS Science, the Department of Conservation (DoC), and other agencies. Equipment has been installed on the volcano in an effort to detect a lahar from the crater lake, and emergency services have developed a plan to limit risk to members of the public at nearby ski-fields, on the main trunk railway and on the Desert Road.

Whilst lahars (debris flows) from Ruapehu have flowed in almost all directions from the volcano in historical times, it is thought that the collapse of the weak tephra dam is most likely to cause the next destructive flow. Such a lahar would flow down the Whangaehu River towards Sate Highway 1. The Tangiwai lahar of 1953 followed this route and, because a warning system was not in place, 151 passengers on an express train lost their lives when the train plunged off a bridge on the Whangaehu River that had been destroyed by the lahar minutes earlier.

The lahar warning system that was installed after the 1996 event was triggered by a small eruption within Ruapehu’s crater lake late in the evening of October 4th last year. Since then, the lake temperature and water level has risen – with the spring thaw, ice cliffs have fallen into the lake causing a steady rise in the lake’s level. During the latter part of November, the lake level rose another 60 cm, bringing it half-way to the top of the 7.6 metre-high dam by month’s end, according to a DoC media release.

The November increase brought the lake level to Warning Level 2, and DoC expected that the lake level would reach Warning Level 3 (water level at 5.7 metres above the old outlet) during January or February.

It seems that these projections are accurate, with DoC indicating that the leaking tephra dam could collapse at some stage during this summer.

[Compiled from data supplied by Geonet, GNS Science, Hazardwatch and Department of Conservation media releases and websites.]

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