Report on White Island (New Zealand) — September 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 9 (September 1981)
Managing Editor: Lindsay McClelland.
White Island (New Zealand) Little eruptive activity for six months; B-type events increase
Please cite this report as:
Global Volcanism Program, 1981. Report on White Island (New Zealand). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 6:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198109-241040.
37.52°S, 177.18°E; summit elev. 321 m
All times are local (unless otherwise noted)
On 18 September, NZGS personnel found that little eruptive activity had occurred in the last 6 months. The fine tephra evenly mantling the main crater floor showed rain wash, pitting, and considerable erosion along the main water courses. A small landslide near the midpoint of the main crater's S rim had sent several tongues of muddy, sulfur-rich, hydrothermally altered material onto the crater floor. A distinctive pink ash that had formed the surface on 6 March was at about 100 mm depth on the rim of 1978 Crater, but was only 30 mm below the surface in the center of the main crater 300 m to the E.
The active subcrater was 200 m wide and 150 m deep. A shallow green pond occupied most of its floor. A 20-30-m-wide vent on the NW side of its floor was emitting pink-tinged gas at high velocity. Throughout the visit this gas and other vapor from 1978 Crater formed a white column.
Fumaroles in a 300 m-wide zone across the main crater floor E of 1978 Crater appeared to be slightly less active than in March. Temperatures were 560°C in a large, reactivated vent; 550°C (100° lower than in March) in a smaller one nearby; and 340°C at a vent now so enlarged that the thermocouple could reach no more than halfway into the pit.
Except for inflation near the center of the main crater, the levelling survey showed no tilt change since May, in contrast to general subsidence earlier in the year. Significant inflation (115 µrad), had occurred just E of active fumaroles about 300 m E of 1978 Crater. The levelling survey team noted that this tilt reversal bears some resemblance to that in April 1980, which preceded the formation of three fumaroles in the same area.
Seismic records 1 August-20 September showed a marked increase in the number of low-frequency (B-type) events from fewer than 5 to more than 30 per day, reversing a decline from late May through mid-July. High-frequency (volcano-tectonic) events numbered fewer than 5/day except on 11 and 12 September, when ten were recorded each day. On 26 August, and 8 and 10 September, single distinctive seismic signatures (symmetric with emergent onsets) were recorded. The NZGS interpreted these as medium-frequency volcanic earthquakes, probably intrusive events. Volcanic tremor was recorded on 10 September.
Geologic Background. Uninhabited 2 x 2.4 km White Island, one of New Zealand's most active volcanoes, is the emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes; the summit crater appears to be breached to the SE, because the shoreline corresponds to the level of several notches in the SE crater wall. Volckner Rocks, four sea stacks that are remnants of a lava dome, lie 5 km NNE. Intermittent moderate phreatomagmatic and strombolian eruptions have occurred throughout the short historical period beginning in 1826, but its activity also forms a prominent part of Maori legends. Formation of many new vents during the 19th and 20th centuries has produced rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project.
Information Contacts: B. Houghton and I. Nairn, NZGS, Rotorua.