Crater Lake status report - 26 April 2008

Crater Lake taken 24 April 2008

Mt Ruapehu

The tephra dam at Mt Ruapehu's Crater lake collapsed on 18 March 2007 just after 11am and a moderate-large sized lahar passed down the Whangaehu River. The emergency response 'worked like clockwork' and the lahar passed down the river with minimal damage to infrastructure in its path and disruption to the travelling public.

This despite the fact that the lahar had a flow rate of about 1000 cubic metres per second at Tangiwai, which is about twice as large as that of the disastrous 1953 lahar. 

Volcano status

• The lake's volcanic status is Volcanic Alert Level 1 (i.e. volcanic alert level 1 on scale of 1-5) and has been at this level since 9 October 2007.
• Slightly enhanced hydrothermal activity.  
• The lake temperature on 26 April 2008 was 34.4^oC.
• The lake colour was milky emerald.
• The lake water level (meters above sea level) is 2529.3 m, the new stable level.

Dirty and crevassed ice cliff with 
remains of the beach exposed when
the lake dropped last year, 13 April 2008

Recent volcanic activity

• The most recent confirmed eruption occurred at 8:23pm on 25 September 2007, after which GNS Science raised the volcano’s Alert Level to 2. This eruption generated a blast of boulders and mud which extended northwards across the Summit Hazard Zone and small lahars down the Whangaehu (eastern) and upper Whakapapa (northern) sides of the volcano.
• A minor disturbance occurred in the lake on 26 April 2008 discolouring the central zone for a period of an hour or more.
• It is GNS Science’s role to monitor volcanic activity, seismicity, lake chemistry and gas output. Updates can be found in the Alert Bulletins on the GNS website.

Changes in lake level

A warning sign at the edge of the
Crater Lake dam warns of the hazard
now associated with the dam
14 Feb 2007

The lake returned to a new “stable” level controlled by the level of the hard rock rim after the collapse of the tephra dam on 18 March 2007.

That event left a temporary low lying bar of boulders across the outlet which held the lake at slightly above 2529.4m until it was washed away by the eruption on 25 September.

The current lake level is 2529.35m which represents a maximum level the lake can reach in the current volcanic environment because the lake now overflows across the hard rock sill. Decreases in level will occur during eruptions and due to evaporation, but the lake will then rise again until it overflows.

Level rises are caused mainly by snow (and ice) melt, heavy precipitation, but also by input of hydrothermal fluids (fluids ‘injected’ into the lake from the volcanic vent under the lake floor) into the lake, rock and ice fall and thermal expansion.

Compare this photo taken 20 March
2007 with that above

Prior to the lahar staff monitored the active ice cliff on the northern side of the lake. Large crevasses develop during summer and autumn and seracs on the lake side of the crevasses sometimes carve off into the lake causing a wave. Waves generated are generally not very large but the largest detected were about one metre high large enough to be quite a nuisance to anyone caught unawares on the lake shore. These waves would not trigger an ERLAWS alarm and would normally not be detectable down the Whangaehu.

After the lake drop on 18 March a beach was exposed by the release of the lake water and this will slowly be covered by the accumulation of snow and ice over the next year or two.

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