Dec 3 1998, 3:00 pm - White Island Volcano

Summary

There is evidence that molten magma is now being tapped directly to drive the eruptive activity, although only very weak volcanic tremor accompanies the current ash eruptions. A surveillance visit was made on Tuesday December 1 to assess the ongoing activity, conduct deformation and magnetic surveys, and collect ash and gas samples. The results available to date from that visit are reported below.

In summary, minor eruptive activity continues at White Island. The level of activity has varied with time, however the level of current activity and volcanic indicators are now sufficient to raise the Alert Level from 1 to 2. The current style of activity may continue for some time and will vary in intensity with time.

Observations

The active vent at the base of the north-west wall of 1978/90 Crater continues to erupt fine grained volcanic ash. The vent size has not changed since the November visit. During our visit an ash-charged, tan-brown, convecting plume was being emitted from the vent and rose to about 800 m before trailing off down wind for 10 - 15 km.

The volume of ash contained in the plume was greater than observed over the past month. The eruptive activity over the previous month has deposited up to 45 mm of fine dark grey and brown volcanic ash at the crater rim. Samples of ash falling on 1 December 1998 showed a significant change from ash collected on 23 November 1998 and earlier. The 1 December ashes contained fresh, vesiculated glass, suggesting that molten magma may have risen in the vent and is now contributing directly to the eruption discharge. Previously the ash was derived from solidified lava covering the magma.

The ground deformation survey shows a consistent trend of minor inflation across the main crater floor, with continued subsidence near the rim of 1978/90 Crater, (Fig. 1). Shown in figure 2 are time series plots for four selected marks, the large scale inflation post 1990 is evident at the more distal sites (Pegs C and J) with only minor changes over the last 2 - 3 months. A small scale deflation-inflation event occurred 1996 - 97. Collapse about the crater rim which started in July is continuing, but now at a lesser rate (Pegs M and W). The provisional results from the magnetic survey indicate heating at depth, with shallow cooling about the crater rim area.

Fumarolic discharge pressures from sites 1, Donald Mound and 13a were not significantly stronger than those observed during 2 recent visits (November 2 and 16), and temperatures remain high for these features (1=124°C; 6a=107°C; 13a=1200°C).

Molten sulphur was found in vents at sites 1 and 13a, which is consistent with these temperatures being in excess of 119°C. The sulphur mound at site 1 has grown considerably over the vent over the last month, suggesting that sulphur is being remobilised from depth in response to the elevated temperatures.

Access to the floor of 1978/90 Crater allowed for a visit to the fumarole at the base of Donald Mound. This discharge (6a) was mildly superheated, but of high pressure indicating a relatively high gas content. These observations are all consistent with general heating of the hydrothermal system in response to the present activity. The lake which has reformed in the main crater is the likely result of recent heavy rains. It was cool (ca. 20°C), and had the brown colour of the ash which was falling into it.

As eruptive activity is now essentially continuous at White Island and there is evidence that molten magma is being tapped directly to drive the eruptive activity we advise the Alert Level is now set at Level 2. The current style of activity may continue for some time and will vary in intensity with time.

Brad Scott
Volcano Surveillance Manager

Figure 1: Contour plot showing the height changes (mm) between November 2 and December 1 1998.

Figure 2: Time series plots showing the height changes at selected pegs (1970-1998), height in m ASL.