Report on Ruapehu (New Zealand) — February 1993
Bulletin of the Global Volcanism Network, vol. 18, no. 2 (February 1993)
Managing Editor: Edward Venzke.
Ruapehu (New Zealand) Little change in crater lake; episodes of tremor
Please cite this report as:
Global Volcanism Program, 1993. Report on Ruapehu (New Zealand) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 18:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199302-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Fieldwork on 17 February revealed little change compared to previous visits. The lake was pale gray-green with a large area of brownish discoloration over the N vent area, where several convection cells were apparent. Blue-green meltwater around the N and W margins increased in area during the day. There was no shoreline evidence of recent surging. Following a decline in January, the lake temperature had increased from 21°C (measured on 12 January) to 24°C. Over the same period the lake outflow decreased from 100 l/s to 90 l/s. The Mg/Cl ratio in the lake water has remained essentially constant since December (table 3), after reaching a low of 0.042 in September.
|Date||Outlet (°C)||Logger Point (°C)||Discharge (l/s)||Mg (ppm)||Cl (ppm)||Mg/Cl|
|21 Dec 1992||--||--||--||311||6731||0.046|
|11 Jan 1993||--||--||--||273||5947||0.046|
|17 Feb 1993||--||--||--||299||6674||0.045|
|03 Jun 1993||14.0||16.5||less than 1||282||6403||0.044|
|18 Jun 1993||10.9||13.0||30||273||6194||0.044|
|03 Jul 1993||11.2||12.5||--||277||6267||0.044|
|06 Aug 1993||19.6||--||85||277||6404||0.043|
|21 Sep 1993||35.5||--||25||283||6753||0.042|
|29 Sep 1993||38.2||--||0||282||6883||0.041|
|09 Oct 1993||35.5||--||0||294||7016||0.042|
|04 Nov 1993||37.2||39.0||0||296||7233||0.041|
|10 Dec 1993||26.0||--||0||293||7175||0.041|
EDM measurements revealed no long-term movements of volcanic significance. A key benchmark, buried by snow since July, was exhumed. Its burial prevented detection of any short-term inflationary changes.
The crater was revisited 28 February-2 March to install telemetry equipment. The lake initially appeared unchanged from 17 February, but by 2 March it was a uniform gray. Outflow was estimated to be 140 l/s. Lake temperatures of 28-29°C measured near the telemetry equipment were similar to the 29.2°C telemetered via ARGOS on 4 March.
Seismicity was generally low with some volcanic earthquakes recorded. Strong tremor was recorded 9-10 January and moderate tremor on 25-28 February.
Geologic Background. 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 dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 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 (Te Wai a-moe), 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.
Information Contacts: P. Otway, IGNS Wairakei.