Report on Ruapehu (New Zealand) — March 1991
Bulletin of the Global Volcanism Network, vol. 16, no. 3 (March 1991)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Lake temperatures decrease, then stabilize
Please cite this report as:
Global Volcanism Program, 1991. Report on Ruapehu (New Zealand) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199103-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Crater Lake temperature continued to decrease ... to 24°C at the end of December, then fluctuated between 24 and 27°C through 21 March (figure 12). Several dips to 20°C were believed related to heavy rainfall. Upwelling and occasional yellow sulfur slicks were observed above the N vents, although no activity was detected above the central vents. The Mg/Cl ratio decreased from 0.053 on 9 October to 0.046 on 27 December, then ranged between 0.45 and 0.50 during 18 January-21 March.
|Figure 12. Crater Lake temperatures at Ruapehu, January 1990-24 March 1992, from a continuously-recording instrument (curve) and individual measurements by geologists (circled crosses). An arrow marks the January 1990 phreatic explosions.|
Seismicity has remained low since August. Low levels of 2-Hz tremor were recorded until early February, when low to moderate levels of 3-Hz tremor began. Until early 1988, 3-Hz tremor usually occurred during periods of high lake temperatures, but this pattern is no longer evident. Deformation measurements showed that the crater width was stable from 9 October with minor shortening measured 8 February-21 March.
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 110 km3 dominantly andesitic 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, 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 3000 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: B. Scott, DSIR Rotorua.