Report on White Island (New Zealand) — August 1991
Bulletin of the Global Volcanism Network, vol. 16, no. 8 (August 1991)
Managing Editor: Lindsay McClelland.
White Island (New Zealand) Tephra emission; shock waves in crater
Please cite this report as:
Global Volcanism Program, 1991. Report on White Island (New Zealand) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199108-241040.
37.52°S, 177.18°E; summit elev. 294 m
All times are local (unless otherwise noted)
Emission of gas/tephra columns from May 91 vent continued through August. During early-August helicopter overflights, R. Fleming noted flashes and strong low-frequency detonations as a hot, dilute eruption column rose from the vent. Crumbly white lithic blocks and lapilli with rare juvenile scoriae had been deposited nearby. Larger-than-normal plumes were often visible from the North Island coast, roughly 50 km away.
During fieldwork 28-29 August, a convoluting pink-brown column was emitted from May 91 vent. It contained very little ash and no evident incandescent material. Visible shock waves emerged from the vent every few seconds as "flashing arcs," lighting clouds above with a flickering glow like that from a poorly-functioning fluorescent tube. The strongest shock waves were manifested as an instantaneous displacement of the plume at the vent, and could be felt 150 m away. Some could be seen to bounce off the crater walls and travel back through the clouds. The shock waves did not seem to affect the rate of plume emission. The activity was accompanied by dull booming and sloshing noises, and occasional sharp detonations. The sloshing sounds were much like those heard in 1988 at Yasur (Vanuatu), where large gas bubbles were bursting through the surface of an active lava lake. Geologists noted that the activity at May 91 vent was consistent with similar gas-bubble discharge through a liquid magma column.
About 200 mm of coarse and fine ash had been deposited just N of May 91 vent since the previous fieldwork on 27 May. Little new ash was evident elsewhere on the main crater floor, but small (< 0.3 m) lithic blocks and their impact craters were found >200 m SE of the vent and to its W. Scarce, widely scattered scoria bombs, most 0.1-0.2 m across but some reaching 0.3 m, were found on top of the May ash, with only a light ash coating. The bombs seemed most abundant a few hundred meters SE-NE of the vent. They had highly vesiculated interiors of black glass with large pyroxene and plagioclase phenocrysts. Internal vesicles were up to 30 mm across, but decreased rapidly to sub-millimeter size toward the surface.
The pattern of deformation between late May and late August was similar to that of the previous 3 months. Strong subsidence at roughly double the previous rate continued to be centered SE of May 91 vent, while relative inflation persisted ~200 m farther SE. A new zone of inflation was measured E of Noisy Nellie fumarole (NE of May 91 vent). Minor deformation associated with activity at May 91 vent is unlikely to be detected, as the nearest part of the levelling network is 100 m away. Most fumarole temperatures had changed little since May, although values at Noisy Nellie had increased from 240 to 411°C.
The volcano had remained seismically quiet until mid-June, when B-type events became more common, continuing at rates of 2-7/day through the end of the month. Very weak volcanic tremor was sometimes visible on seismic records. A sequence of >45 tectonic earthquakes (to ML 3.7) occurred near White Island 1-2 July. A- and B-type events increased markedly on 7 July, accompanied by a small increase in background volcanic tremor amplitude. E-type eruption earthquakes were recorded on 1, 7, and 11 July. Seismicity had declined by 15 July, but a 3-day swarm of >200 A-type events began on 20 July. Significant volcanic tremor also resumed and continued through mid-August, increasing again 21-28 August. Tremor varied from a nearly pure 1.8 Hz signal to a complex pattern with spectral peaks to 8 Hz. A-type events did not occur daily in August, but often numbered 8-10/day. B-type events were very rare after 24 July. E-type eruption shocks were recorded on 14, 15, 19, 20, 23, 27, 29, and 30 August.
Geologic Background. The uninhabited White Island, also known as Whakaari in the Maori language, is the 2 x 2.4 km 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, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of eruptions since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there 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. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities.
Information Contacts: B. Houghton, I. Nairn, and B. Scott, DSIR Geology & Geophysics, Rotorua.