Aug 2 1996, 3:00 pm - Ruapehu Volcano

Tremor levels at Dome have been low since about 11:00 on August 1. Bad weather this morning has prevented observations of the volcano, but if any eruptions have occurred they will have been minor.

The following information updates reports of activity on July 31/August 1. A moderate ash eruption occurred at 19:30 on July 31, accompanied by the resumption of moderate level intermittent tremor on Dome. This tremor is eruption-driven. Another eruption was observed at about 22:00, and at 03:00 and 06:30 on August 1. These eruptions occurred in strong northerly winds, which kept the plume at low elevation and caused most ash to fall out on southern slopes of Ruapehu. The 0300h eruption caused moderate ash fall on Turoa skifield, closing it for skiing. Moderate gas emission with intermittent ash eruptions was observed by GNS staff on August 1. A helicopter crater inspection at ~11:00 observed that the active pit crater in the south of the crater basin may be deepening as intermittent ash eruptions excavate the tip of the magma column. (There has been no seismic indication of recent magma recharge in the volcano). Jetting onsets of ash emission during the inspection ejected blocks vertically above the vent on well-collimated trajectories, so that most blocks fell back into the vent. Impact craters in the thick ash on the crater margins do not extend beyond the old lake basin. No incandescence could be detected at base of the eruption column. All these features suggest the active vent is deep. No significant changes were seen in the crater area.

Ash layers from most of the June - July eruptions were sampled from the snowpack at Turoa on August 1, as was the August 1 ash on the surface. This ash is dominated by mm-size blocky clasts of solid magmatic material and crystals, with little scoria, and is interpreted as derived by frittering from a near-solid (cooling?) magma body. The COSPEC gas flight planned for August 1 was precluded by rain, and is now planned for the first suitable weather.

The Alert Level remains at 3.

Ian Nairn
for Volcano Operations Manager