Report on Ruapehu (New Zealand) — August 1993
Bulletin of the Global Volcanism Network, vol. 18, no. 8 (August 1993)
Managing Editor: Edward Venzke.
Ruapehu (New Zealand) Temperature and outflow from crater lake increase; activity remains low
Please cite this report as:
Global Volcanism Program, 1993. Report on Ruapehu (New Zealand). In: Venzke, E. (ed.), Bulletin of the Global Volcanism Network, 18:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199308-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
Heatflow in the crater lake continued at low levels in June and July but increased significantly in August. Little change was observed in the pale gray color of the lake during fieldwork on 3 June, 18 June, and 3 July. Snow was present along the lake's edge and there was no shoreline evidence of surging on any of the field dates. Moderate upwelling was observed with associated yellow slicks over two sites in the N vent area on 3 and 18 June. Some weak convection and a faint short-lived dark slick were visible above the central vent on 18 June. Yellow sulfur globules (hollow spheres with long tails) were also seen floating and stranded on the shore near the outlet on 18 June. Weak upwelling from the N vents on 3 July produced a sulfur slick extending nearly 150 m E of the outlet on the S shore to Logger Point, but no slicks were seen from the central vent. Convection had recommenced from the central vent by 6 August and produced khaki-yellow slicks on the lake surface. The calculated August heatflow was ~3x that of July. Temperature and discharge from the outlet (at the S end of the lake) had increased since June, while the temperature at Logger Point declined (see table 3).
EDM distances measured on 3 June showed a minimal change of -7 mm overall since the last leveling survey on 20 April, which is unlikely to have any volcanic significance. No distance measurements were made in July. Changes since 3 June were generally below detection limits (<5 mm ) with the exception of the shortening of two lines to a station SW of the crater lake (station E), continuing the trend since February. Two other stations moved ~18 mm closer to station E. None of these movements is thought to have any significance in terms of future eruptive activity. Snow was still an obstacle during the 6 August survey with deep drifts around stations on the W side of the crater lake.
Detailed seismic monitoring in June was hindered by the failure of the Dome seismograph, which was struck by lightning in early April. Very little tremor was recorded by the Chateau seismograph. The signal from Dome was restored on 18 June and some very small A-type events were recorded on many days. During a 6-minute period on 3 July, a sequence of five events with a dominant frequency of 2-3 Hz was recorded, indicating low-frequency volcanic earthquakes or short duration tremor. Some very small local high-frequency events were also recorded in early July and were attributed to probable ice-induced quakes. No records were obtained from Dome during 12-20 July, and when recording recommenced a 2-Hz volcanic tremor of variable amplitude was occurring. The tremor was continuous up to the time of the 6 August report, with the dominant frequency rising from 2 to 3 Hz on 27-28 July. Small high-frequency events continued to occur in early August.
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, B. Scott, and A. Hurst, IGNS Wairakei.