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Report on Whakaari/White Island (New Zealand) — February 1987

Whakaari/White Island

Scientific Event Alert Network Bulletin, vol. 12, no. 2 (February 1987)
Managing Editor: Lindsay McClelland.

Whakaari/White Island (New Zealand) Phreatic eruption ejected lapilli and blocks

Please cite this report as:

Global Volcanism Program, 1987. Report on Whakaari/White Island (New Zealand) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198702-241040

Whakaari/White Island

New Zealand

37.52°S, 177.18°E; summit elev. 294 m

All times are local (unless otherwise noted)

The NZGS reported that the 25 January eruption that directionally ejected lithic lapilli and blocks over the SE part of the island appears to have been dominantly phreatic in origin. The NZGS noted that this interpretation is consistent with the lack of change in seismicity prior to the eruption, the continuing decline in fumarole temperatures (measured 13 days before the eruption), the apparent lack of associated inflation of the main crater floor (from surveys at 3-month intervals), and significant weekly rainfall (62 mm) that ended a period of drought before the eruption.

About 10 hours before the eruption (at 1000) a white steam plume was issuing from Congress Vent and no fresh ejecta were visible on the main crater floor. Comparison of photos taken before and after the eruption show that the mouth of Congress Vent approximately doubled in diameter and elongated N-S.

The January deposits had two main components: 1) a continuous tephra blanket of blocks and lapilli and 2) numerous large scattered ballistic blocks that fell up to 750 m from the vent. The lapilli and blocks were dispersed along a narrow SSE- trending axis with a maximum thickness of 0.4 m. Ash coated the main crater walls and outer flanks. Stratigraphic position indicated that the ballistic blocks were erupted after the lapilli/block bed. Ejecta volume was estimated at roughly 40,000 m3. Most clasts were andesitic/dacitic as in earlier eruptions.

December-February subsidence rates decreased substantially (to -4 mm at one site) and only very minor subsidence has occurred in the Donald Mound-Congress Vent area. Fumarole temperatures measured on 2 March ranged from 117 to >530°C. Since 1983 there has been a general trend of crater floor deflation and declining fumarole temperatures.

A M 4.7 earthquake occurred ~430 km N of White Island at 2037 on 25 January, 4 minutes before the onset of local seismicity and seven minutes before the eruption [at 2044]. The early phase that emplaced lapilli and blocks on the main crater floor is thought to have been associated with 8 minutes of continuous medium-frequency tremor (185-685 seconds after 2041 in table 5). About one minute of discrete (C-type) events soon followed, when most of the ballistic blocks may have been explosively erupted. Seismicity of higher amplitude and longer duration has accompanied smaller past eruptions and sometimes has occurred without an associated eruption.

Since 25 January, B-type events have continued to dominate the records, numbering 28-42/day. Small, high-frequency (A-type), volcano-tectonic events have numbered 1-3/day. Wide-band (E-type) 'eruption earthquake' sequences were recorded on 29 January at 1920, 1 February at 0214, and 2 February at 0958. The first was followed by 29 hours of low-amplitude, medium-frequency, volcanic tremor accompanied by moderate ash emissions. The other two events were unusually short and dominated by low frequencies. A M 6.2 earthquake centered ~60 km NNE of White Island on 2 March was not associated with any obvious change in volcanic activity.

Geological Summary. 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: A. Cody, B. Houghton, I. Nairn, P. Otway, B. Scott, and C. Wood, NZGS Rotorua; J. Latter, DSIR Geophysics, Wellington.