Eruption debris may extend snow seasons (10/6/2007)

Debris-covered snow and ice on Mt Ruapehu, in New Zealand

Skiers and snowboarders may have the recent eruption to thank for an extended ski season, says Massey University glaciologist Dr Martin Brook.

Dr Brook is a lecturer in physical geography who has specialised in the study of glaciers. On hearing of the eruption last Tuesday, he and a team headed up Mt Ruapehu to install monitoring equipment to assess the glaciological response.

"The eruption dumped a lot of volcanic material on the upper snowfields at Ruapehu, which act as source accumulation areas collecting snow," he says. "This is in turn turns into firm snow and then glacier ice for the Whakapapa Glacier in particular. As we are now moving into the spring and summer melting season, where the sun is at a higher angle, and the days are longer, snow and glacier ice on Ruapehu usually melts rapidly until the following autumn. However, this year, there is now debris cover on the ice of varying thickness, so this will protect the snow and ice from melting in the accumulation area, keeping a base of snow and ice in place for a longer than usual. That also gives us the tantalising prospect of enhanced snow at the beginning of the autumn ski season in 2008."

Were the layer of debris thinner - or thicker - it would have a different impact.

"Melting is enhanced under debris up to about 8mm thick, due to absorption of shortwave radiation from the sun. The debris re-emits this as long-wave radiation into the adjacent snow and ice. This is because dark colours have a low reflectivity, and do not reflect sunlight like lighter colours do. However, with a debris cover thicker than about eight to 10mm, this actually acts to insulate the ice and snow below, as the debris is too thick for any radiation received at the surface to be transmitted downwards to the snow below."

Dr Brooks says New Zealand is unusual in glaciological terms. "It's doubly intriguing; New Zealand's glaciers are not your average glaciers. Those on the west coast of the South Island (Fox, Franz Josef) respond to snowfall, which appears to overprint the effect of temperature.

"Hence, we have a situation in New Zealand, with global warming heating the oceans, evaporating more sea water into the atmosphere, leading to enhanced precipitation on the West Coast of the South Island, and a short seven-minute volcanic eruption perhaps leading to insulation of parts of the Ruapehu glaciers and snowfields on the North Island."

Dr Brooks and his team have a permit application with the Department of Conservation to install an automatic weather station and an array of ablation stakes in the summit snowfield, and the top of the Whakapapa Glacier. They hope to return to install the equipment with the assistance of Mt Ruapehu ski field staff this week.

Note: This story has been adapted from a news release issued by Massey University

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