One of White Island's largest explosions of the last 10 years occurred on 25 January at [about] 2040. Ejecta up to 5 cm in longest dimension fell on a boat anchored roughly 1 km ESE of the vent (in Crater Bay); people on board heard rumbling. From another boat 5 km NW, ash and rocks were seen falling into the sea. A large black cloud rose from the main crater and lightning flashed in the eruption column. Seismicity associated with the eruption lasted for ~20 minutes (table 5). [An E-type earthquake accompanying the eruption began at 2044 but was relatively small, suggesting that the eruption was largely open-vent (Latter, 1988)]. There was no recognizable precursor to the eruption which occurred during a period of low seismic activity.

Table 5. Seismic activity during White Island's 25 January 1987 eruption. Times are in seconds after eruption onset (at 2041) except where otherwise noted.

An aerial inspection on 27 January showed that new ash had been dispersed mainly S of Congress Vent; 1-m ballistic blocks were abundant up to 0.5 km from the vent, and more widely dispersed 0.7 km away. The largest blocks appeared to be 1-2 m in diameter. Ash covered the floor of 1978 Crater and the main crater, coated the main crater's S wall, and extended over the rim onto the S flank. Vegetation on the edge of the ash did not appear to have been affected. At 1020 a weak gray ash column rose to 800 m from Congress Vent; fumaroles were emitting steam.

Since mid-November, seismicity has remained dominated by low-frequency (B-type) events. In mid-late November the number of B-type events declined from >20 to

A swarm of A-type earthquakes began at 2100 on 22 December and was followed by strong microearthquake activity (2,000-5,000/day) for 3 days. No eruptive activity was reported. Less intense microearthquakes occurred on 4-5 January. B-type events with anomalously large amplitudes usually numbered >30/day during January. On 20 January [at 0537-0538] a high-amplitude (>110 mm p-p), wide-band, long-duration, eruption (E-type) earthquake sequence was recorded. It is not known if eruptive activity accompanied this event. Weak, medium- to low-frequency volcanic tremor followed until 23 January. Tremor was banded and amplitude decreased with time. The 24-25 January seismic records were very quiet until the eruption.

Further Reference. Latter, J.H., 1988, Shallow seismicity of various volcanic areas during 1987: New Zealand Volcanological Record, no. 16, p. 35-44.

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: I. Nairn and B. Scott, NZGS Rotorua.