May 20 1996, 1:00 pm - White Island Volcano

For some time now White Island volcano has been showing signs that another eruption episode will occur. As part of our regular surveillance programme we visited the island on Thursday 16 May with the support of the RNZAF No.3 Squadron, to make a detailed inspection. The results available to-date from that visit confirm that White Island is continuing to show signs which indicate another eruptive episode is approaching. However, we now believe eruptive activity will occur later, rather than sooner as was earlier expected. This interpretation is primarily based on the rates at which our monitoring parameters are changing, and the lack of significant seismic activity.

During the surveillance visit on May 16 we completed deformation and magnetic surveys. The seismic installation was also serviced during this visit and gas and water samples were collected from fumaroles, hot springs and streams.

The deformation survey has shown continuing uplift over most of the Main Crater floor. The survey pegs near Donald Mound are rising the fastest at about 170 mm/year, while more distant pegs are rising at about 55 mm/year. Presently the uplift rates are steady. They did increase at the beginning of the year, but have not shown any changes in rate recently.

The magnetic survey, which reflects subsurface temperatures, has also shown continuing heating at depth (c.f. the previous survey in January 1996). However, as with the deformation survey, the rate of change has not increased. Towards the end of April temperatures of fumaroles about Donald Mound and at Noisy Nellie started to increase reflecting the heating at depth. Temperatures at some fumaroles have risen from about 105°C to over 400°C.

The SO₂ output has increased slightly, but not significantly. The total output was about 180 tons/day when last measured. In the past, SO₂ outputs in excess of 1000 tons/day have been measured during eruptive activity. Other geochemical data has shown that equilibrium temperatures (ie. those factors which control certain chemical reactions) increased in 1995 but have now stabilised at the higher values. There is no apparent change in the rate that the equilibrium temperatures are presently increasing.

These observations taken together continue to indicate the probability of an eruption starting at White Island. Our measurements indicate a longer (months) rather than shorter time (weeks) to the commencement of significant eruptive activity. However, past experience with active volcanoes has shown that this situation could change rapidly. The Scientific Alert Level remains at 1, but this could quickly change to 2, or higher.

The Scientific Alert Level remains at 1, but will be revised if/when eruptive activity starts.

B.J. Scott (for C.J.N. Wilson)
Programme Leader