Report on Ruapehu (New Zealand) — April 1989
Scientific Event Alert Network Bulletin, vol. 14, no. 4 (April 1989)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Heat flow declines
Please cite this report as:
Global Volcanism Program, 1989. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 14:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198904-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Since February, no discrete eruptions have been reported although steam passively rising from Crater Lake has occasionally been witnessed. When geologists visited the volcano 21-22 March, slight upwelling in the N vent area formed broken sulfur slicks. Crater Lake's temperature had fallen to 32°C (a 10.5° drop over 23 days) representing a decline in heat flow to ~10% of its previous rate. Lake level had decreased to 100-150 mm below overflow. Lake chemistry was stable, showing little change in Mg/Cl since 11 January. Minor inflation was measured across the N crater rim. On 5 April, geologists observed slightly increased upwelling in the N vent area. The lake temperature was 31.3°C. N-rim inflation had largely disappeared. NZGS geologists noted that some previous pulses of inflation/deflation have been followed by renewed lake heating (or strong seismicity). Few tremor episodes and volcanic earthquakes were recorded on seismic records through . . . 5 April.
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: P. Otway, NZGS Wairakei.