Report on White Island (New Zealand) — June 1993
Bulletin of the Global Volcanism Network, vol. 18, no. 6 (June 1993)
Managing Editor: Edward Venzke.
White Island (New Zealand) Ash eruptions; additional subsidence
Please cite this report as:
Global Volcanism Program, 1993. Report on White Island (New Zealand). In: Venzke, E. (ed.), Bulletin of the Global Volcanism Network, 18:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199306-241040.
37.52°S, 177.18°E; summit elev. 321 m
All times are local (unless otherwise noted)
Following the 16 March visit by IGNS geologists, ash eruptions were reported on 29 March and 2, 5, and 6 April by fishermen. An E-type earthquake at 1022 on 5 April lasted for 5 minutes, and may have been associated with one of the observed eruptions. Seismicity in April and May was generally low, with only a few A- and B-type events/day, and almost no volcanic tremor. Another E-type event on 20 April was preceded and followed by periods of microearthquake activity.
A visit to the island on 18 May revealed a few millimeters of gray rainwashed ash on the main crater floor. The primary gas emission site was a new fissure vent on the N wall of Royce Crater (figure 18), which was emitting moderate amounts of gray-tinted gas. Voluminous white steam clouds from TV1 Crater and vents high on the N wall of Wade Crater obscured views of those areas. Almost no emissions were coming from Princess Crater. Further wall collapse had occurred in Donald Duck Crater, enlarging it slightly and reducing the width of the dividing wall with the adjacent TV1 Crater. A green pond occupied the floor of Wade Crater.
About 5 mm of new gray ash was seen at peg M (~50 m SE of the Peg XII Sag) on top of the brown ash deposited in February. A total of 40 mm of fine ash has accumulated at this site since May 1992. Also since May 1992, ~100 mm of fine ash has accumulated at a site 20 m from the rim of the 1978/90 Crater on the SW wall of the Peg XII Sag, and 250 mm on the edge of Gibrus Ramp. Fumarole temperatures remained low in all areas, probably due in part to heavy rains in the three days before the visit.
Deformation since the January survey continues to be dominated by strong subsidence centered immediately SW of Donald Duck Crater. The maximum rate of -35 mm in 4 months (-9 mm/month) was registered at peg X, a significant reduction from the previous 5-week period when -20 mm/month was recorded. The subsidence near Donald Duck Crater is interpreted as continuing deflation associated with withdrawal of underlying brine fluids and a reduction of heat flow, possibly accompanied by some subterranean collapse. The subsidence rate has decreased substantially from the unusually high rate measured in 1992. There was also a 7 mm (2 mm/month) rise in the peg D-M area SE of Donald Mound, continuing the trend seen during the previous survey, and resulting in a minor elevated area. This uplift is thought to indicate a possible warming at depth, though no temperature or chemical effects are evident at the surface. Previous episodes of uplift in this area, of approximately half this magnitude, have occurred with no associated surface changes.
A magnetic survey was carried out by Victoria Univ geologists. Diurnal variation during the day was about 10 nT, and the magnitude of the measured changes was comparatively small (<80 nT). A positive trend was distinguished immediately NE of the 1978/90 Crater Complex, probably due to shallow cooling in the active vent area or an apparent increase in hot spring activity noted by the Victoria Univ team since 8 December 1992. This positive trend seems to be superimposed on a more widespread negative trend over most of the crater floor, and is thought to be caused by more deeply centered heating. A possible source of error in the magnetic differences is that the 8 December 1992 survey was done using a GEM GSM-19 rather than the usual Geometrics G8 magnetometer. December 1992 values were adjusted for the different staff lengths (the vertical magnetic gradient was also measured), and it is believed that residual errors would be small in magnitude.
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: I. Nairn, B. Christenson, P. Otway, and C. Wood, IGNS Wairakei; E. Broughton and P. Rickerby, Victoria Univ, Wellington.