Report on Ruapehu (New Zealand) — May 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 5 (May 1983)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Decreases seismicity; lake temperature lower
Please cite this report as:
Global Volcanism Program, 1983. Report on Ruapehu (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 8:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198305-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
On 15 May, when NZGS personnel visited the volcano, Crater Lake was uniformly battleship gray. Faint upwelling over the central (main) vent was marked by black sulfur slicks. Weak upwelling was observed over the N vents. At the outlet there was no evidence in the new snow surrounding the lake of recent ash deposits or surging. Lake temperature measured at the outlet was 16.5°C, 3.5° lower than on the last visit, 18 April.
The horizontal deformation survey showed that since 24 April the maximum change in distance between pairs of stations around the crater was an increase of 17 mm. This distance, between stations on opposite sides of the 600-m-wide crater, had decreased 22 mm 10 February-17 March, then stayed virtually unchanged until 24 April. There has been little cumulative movement of the several survey stations during the past year, but three stations S of the lake have moved about 18 mm SE. Only 4.5 µrad of apparent inflationary tilt were recorded on the Dome since 22 February.
Sequences of B-type earthquakes occurred every 2-4 days in late February and March, but have been less frequent since then; only two B-type earthquake sequences were recorded 18 April-15 May. Small shocks, interpreted as tectonic, have occurred since 29 April. The largest was M 1.8 on 5 May. There have also been occasional episodes of weak tremor.
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.