Eruptive activity 13-22 May was the most intense of 1987. New ash deposits on the island were voluminous (perhaps 104-10⁵ m³). The largest explosive eruptions threw ballistic blocks S and SSE of Congress Crater, and were similar in magnitude to the 25 January event (SEAN 12:01). On 16 May at 2036, blocks were incandescent and the ejection was accompanied by a shock wave. Major block eruptions appear to have occurred some time after heavy rain on 18-19 May, and have been followed by intermittent ash eruptions. Ash and steam eruptions were reported on 18 and 21 May.

A levelling survey on 12 May showed that inflation had continued E of Congress Crater (in the Donald Mound area), with 7 mm of uplift since mid-April and a total of 28 mm since 3 February. The area NW of Donald Mound, within 100 m of Congress Crater, continued to subside rapidly, falling 58 mm over the last year. Ground oscillations were observed during the levelling measurements, ~1 minute before an eruption. At 1134, an ash pulse was vigorously emitted from a vent in Congress Crater, forming a brown column. Lapilli of up to 50 mm were ejected and fell as far as 50 m from the rim of the 1978 crater complex. Tephra deposited prior to 12 May consisted of well-sorted lithic blocks and lapilli.

An aerial survey on 22 May showed that new, thick, light gray ash covered most of the E end of the island. A lobe of ashfall extended over the N crater wall, into the sea (N of Shark Bay). Blocks littered the flat SE of Donald Mound and were most abundant at the foot of the S crater wall. Impact craters were most abundant on the flatter slopes 60 m above the main crater floor. Blocks and impact craters extended down the S margin of the main crater floor.

Moderate medium-frequency tremor began 3 May and persisted until about 1200 on 22 May. Peak daily amplitude ranged from 11 to 40 mm. Some envelopes of pure harmonic tremor were recorded, especially on 13 May. "Strong" tremor was recorded during the 8.5 hours before tremor ceased on 22 May and larger E-type earthquake sequences occurred at both the commencement and cessation of the strong tremor. In the first 3 hours following the end of tremor, 100 C-type (medium-frequency) earthquakes occurred, the largest about M 1.5. Since 7 April, 2-30 low-frequency (B-type) volcanic earthquakes occurred/day, while high-frequency events (A-type) were rare. E-type eruption sequences had been recorded since 10 April and 18 were recorded 12-22 May, with the largest on 20 May at 2215. Two of the recorded E-type events were correlated with observed eruptions.

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: J. Cole, Univ of Canterbury, Christchurch; I. Nairn and B. Scott, NZGS Rotorua.