Report on White Island (New Zealand) — April 1996
Bulletin of the Global Volcanism Network, vol. 21, no. 4 (April 1996)
Managing Editor: Richard Wunderman.
White Island (New Zealand) Uplift of the main crater floor and changes in the hydrothermal system
Please cite this report as:
Global Volcanism Program, 1996. Report on White Island (New Zealand). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 21:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199604-241040.
37.52°S, 177.18°E; summit elev. 321 m
All times are local (unless otherwise noted)
There has been no eruptive activity at White Island since the minor ash emission of 28-29 June 1995 ([corrected from] BGVN 20:07). A vent formed in August but no tephra was erupted (BGVN 20:11/12). Ground-deformation surveys continued to record uplift of the main crater floor. Magnetics showed a persistent decrease N of Donald Mound and an increase S of it. Changes were noticed in the hydrothermal system, but no significant variations were observed in seismicity. This report briefly summarizes several visits to the island from 27 December 1995 through 17 March 1996.
On 27 December, a slow but continuous rise in the water level of the combined crater lake was observed. Strong and audible fumarole activity was concentrated in the May 91 embayment area, on the N side of 1978/90 Crater Complex. During 22-24 January, scientists noted enhanced steam emissions, which may have been due to falling atmospheric pressure before a storm. The Dragon's Foot spring below fumarole 1 (SE of Donald Mound) reverted to being a fumarole after being a hot pool during most of 1995. Comparison of photographs suggested that the lake level had risen by 1-2 m since 12 December 1995.
During 9-12 February, fumarole temperatures in the May 91 area were 143°C, the lake level had risen, and the water temperature was 53°C. More intense fume emissions were observed from Donald Mound. In March, a new fumarole with a temperature of 100°C was found on Donald Mound and Noisy Nellie's temperature rose from 89°C on 7 March to 124°C on 17 March. Although there was no major increase in fumarole temperatures, heat flow had increased, as evident from expanded areas of sublimation, steaming ground, and mud pots. Gas discharge was very strong in the fumaroles along the S crater wall. Partially completed analysis from fumarole 13 showed increased HCl discharge.
Water analyses from the crater lake carried out between November 1995 and February 1996 showed decreasing pH values (from 0.58 to 0.23), increasing Cl concentrations (63,600 to 68,100 ppm), and decreasing Mg concentrations (6,060 to 5,600 ppm).
Observed ground deformation on 29 February suggested continuing strong inflation centered on Donald Mound, where the uplift rate since December had almost doubled compared to the previous five-month period (from 92 to 171 mm/year at peg F). Moreover, the uplift rate across the entire crater floor was twice that observed over the past two years (55 mm/year at peg C since December 1995, 28 mm/year at peg C from December 1993 to December 1995). This significant increase in the rate of inflation under both Donald Mound and the crater floor was interpreted as an indication that shallow (~100 m deep) heating is increasing under Donald Mound, and intrusion and/or heating is occurring in the whole area at a deeper level (~500 m). These observations suggested that water table changes were not the cause of the surface deformation (BGVN 20:11/12).
The magnetic survey on 23 January focused near Donald Mound where most of the recent changes took place. The dominant trends, decrease on the N side of Donald Mound and increase to the S, indicated shallow (50-100 m deep) demagnetization in this area.
No significant seismicity has occurred since September 1995. However, since 15 January 1996, high-frequency microearthquakes have appeared on the records in low numbers (50-300/ day). The occurrence of two E-type events did not produce any surface manifestation.
Geologic Background. Uninhabited 2 x 2.4 km White Island, one of New Zealand's most active volcanoes, is the 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, four sea stacks that are remnants of a lava dome, lie 5 km NNE. Intermittent moderate phreatomagmatic and strombolian eruptions have occurred throughout the short historical period beginning in 1826, but its activity 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.
Information Contacts: B.J. Scott, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.