Crater Lake dry, voluminous ash produced

Ruapehu Bulletin, Volume 13, Issue 608, 17 October 1995, Page 5

Crater Lake dry, voluminous ash produced

The period of tremors under Mt Ruapehu' s crater, recorded Saturday night between 3.50pm and 8.30pm, was one of the most intense intervals of activity during the 1995 eruption to date, observations by the Institute of Geological and Nuclear Sciences have confirmed. Tremor levels were 2-3 times that recorded on the evening of 1 1 October when the largest ash fall to date was recorded. On Saturday afternoon the tremor record from the Dome Shelter instrument was strongly banded into intervals of moderate and low tremor. Two small volcanic earthquakes were recorded at 4.34am and 6.34am Saturday and a "period of elevated tremor" started at 3.50pm and increased to intense levels by 4.30pm. This intense tremor was accompanied by voluminous ash emissions, with ash plumes rising to almost 12,000 metres and a continuous ash cloud was reported on weather satellite imagery, extending downward to cross the coast between Napier and Waipukurau.

Ash emission appeared to decline after 8.30pm as did tremor levels, which were replaced by numerous separate volcanic earthquakes, the largest about magnitude 3. At least one of the quakes (at 1 0.00pm) produced a loud boom heard by IGNS staff collecting ash at the Wahianoa Aqueduct on the Desert Road. The volcanic earthquakes "appear to record individual explosions in the crater," reports IGNS. Observations on Friday showed intermittent ash eruptions from the deep southern vent in "the now-dry crater basin" with mostly steam and gas emitted from the northern vent. "This was the reverse of the situation observed during an over flight on Friday morning when ash was being emitted from the northern vent and steam/gas from the southern vent," states IGNS. The eruptions on Saturday night produced "voluminous ash" blown in Turn to Page 7

Eruption update

From Page 5 a narrow tongue to the south-southeast by strong northnorthwesterly winds. This tongue was centred north of Waiouru and ash fall was recorded at Dannevirke, Waipukurau, Waipawa, and Takapau. The Institute has field parties collecting the Saturday ash and analysing it for comparison with ash from last Wednesday. On Saturday and Sunday the volcanic alert level remained at three but was under hourly review.