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Mt Ruapehu Crater Lake Status Report

Crater Lake Status Report
2 February 2006
Mt Ruapehu

Date of report: 2 February 2006 - based on a site visit and measurements taken on 27 January 2006.

* 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 27 February 1997.

* There is unchanged hydrothermal activity in the lake.

* The lake temperature on 27 January 2006 was 24.5oC.

* The lake colour is grey.

* The lake level is 2529.3 metres above sea level.

* The lake is about 0.0 metres below the hard rock rim (2529.3m).

* Distance below top of the tephra dam is 7.6 metres.

* Recent Lake net filling rate - not currently known

* Predicted time for lake to reach 2529.3m against tephra dam (warning level 1b) is late January 2006

* Predicted time for lake to reach 2533m against tephra dam (warning level 2) is early March 06 (possible) or 2006/07 summer or later (similar probabilities)

* With the lake fullness at 0.0 metres below the hard rock rim at 2529.3m asl the collapse lahar hazard is very low.

* The overall lahar hazard is classed as Low -Normal.

* The Crater Lake Warning Level (as per lahar response planning agencies) is now Warning Level 1b.

Changes in lake level

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. While the latter can be calculated the others are difficult to separate out (quantify) when they occur together (which is the usual situation). Recently when melting and precipitation have been very low we were able to conclude that the recent rise was probably due to influx of hydrothermal fluids and some thermal expansion - the latter seems small in comparison to the exchange of hydrothermal fluids.

Level drops are due to evaporation, exchange of lake waters back into the vent system and potentially seepage. Again it is usually very difficult to quantify these or separate them out. With the benefit of hindsight and a recent science publication by Tony Hurst of the Institute of Geological and Nuclear Sciences (GNS) and colleagues we assume that the significant drop in lake level last autumn was partly due to some lake water flowing back into the vent system as well as due to evaporation and contraction.

These processes are difficult to predict apart from normal seasonal changes in snow melt and freeze (also variation in them is difficult to predict). There are a lot of uncertainties which are mainly impossible to predict. Therefore forecasts of lake level change are difficult and have inherent uncertainties. Normally we make them on the basis of historic trends and scenarios - that is they are forecasts based on scenarios not predicted trends in monitored processes. Over recent years seasonal melting and freeze up have often coincided with a near annual hydrothermal cycle in the lake which has obscured their relative contribution but its clear that snow and ice melt is dominant but variable. Its GNS’s role to monitor volcanic activity.


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