Report on Whakaari/White Island (New Zealand) — February 1991
Bulletin of the Global Volcanism Network, vol. 16, no. 2 (February 1991)
Managing Editor: Lindsay McClelland.
Whakaari/White Island (New Zealand) Block/ash ejection from October vent
Please cite this report as:
Global Volcanism Program, 1991. Report on Whakaari/White Island (New Zealand) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199102-241040.
37.52°S, 177.18°E; summit elev. 294 m
All times are local (unless otherwise noted)
The first significant eruption on White Island since TV1 vent formed on 2 October took place on 8 February at about 2000. The tephra ejection episode was in the NE part of 1978/91 Crater and was associated with a 14-minute seismic signal that began at 2003 (table 8). Helicopter overflights by Robert Fleming had revealed no unusual activity on 4 February, but TV1 crater was emitting ash on the 6th. The 8 February eruption deposited a semi-continuous layer of blocks, generally 0.3-0.5 m across with some to 1 m, on the main crater floor within ~50 m of the 1978/91 Crater rim, and 5-20 mm of fine ash was spread over the E end of the island. Many blocks were almost ash-free, indicating that they had been ejected late in the episode. Blocks were more common in small gullies, where impact craters were rare, suggesting to geologists that they had been emplaced by lateral flow processes. No new scoria bombs were found, but many blocks showed evidence of high-temperature vapor-phase alteration and recrystallization, including cavities lined with specular hematite crystals. The top of the ash layer was an anhydrite-rich crust ~1 mm thick, suggesting that the ash was hot (>56°C) when deposited. The ash also included scattered 1-4 mm accretionary lapilli. Several meters of ash had ponded on the floor of R.F. Crater, and longitudinal dunes were evident on the surface of the ash deposit, again suggesting emplacement by a flow process. New impact craters, probably made by the 8 February activity, were visible high on the S wall of the main crater. Wooden pegs near the crater were burned by blocks and charred by gas and ash on the side facing the vent.
|13 Jan 1991||0208-0258||Medium-high frequency|
|21 Jan 1991||1804-1909||Low-frequency|
|22 Jan 1991||0517-0607||Low-frequency|
|04 Feb 1991||1051-1105||Low-frequency|
|08 Feb 1991||2003-2017||Low-frequency onset, then high-frequency eruption signature|
Fumarole temperatures were little changed since December. Deformation was concentrated NE of 1978/91 crater, where a maximum of 20 mm deflation had occurred since November and 50 mm since August.
Seismicity was characterized by 0-5 A-type and 0-3 B-type events/day after data collection resumed on 27 December. On 1 January, 31 A-type shocks (to ML 3.1) were recorded and up to ten B-type shocks/day occurred 26-28 January. Five episodes of E-type seismicity were detected, including the eruption event of 8 February (table 8). Weak, low-frequency tremor followed the 14-minute E-type episode on 4 February, which lacked an associated high-frequency cigar-shaped eruption signal. Tremor was strongest (but still only 1-2 mm peak-to-peak) on 6-7 February.
Geologic Background. The uninhabited Whakaari/White Island 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 SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused 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. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.
Information Contacts: I. Nairn, DSIR Geology & Geophysics, Rotorua.