Grey and yellow crater lakes form on Ruapehu

Ruapehu Bulletin, Volume 13, Issue 613, 21 November 1995, Page 7

Grey and yellow crater lakes form on Ruapehu

New lakes have formed in the crater of Mt Ruapehu, one grey and one bright yellow reports the Institute of Geological and Nuclear Sciences (GNS), as part of their ongoing study of the million-year-old volcano. Following is a summary of the eruption activity on Mt Ruapehu since Monday last week, taken from media releases from W airakei . The Alert Level remained at Three throughout the week. Monday (13/11/95) Seismic activity contin-

ued on a similar level to the previous week with tremor accompanying significant gas emissions and minor ash. Near-continuous gas emissions were reported, with the gas forming a ground-hugging fog that rolled down the Waihianoa Valley. Minor ash fall was reported to the south of the vent to a distance of about lkm. Wednesday New lakes startedto form in the former Crater Lake basin fed by ice melt and rain on the summit. The re-

cent activity was not sufficient to eject or evaporate all inflowing water. A grey lake about 50m across covered the Southern vent separated from a similar-sized bright yellow lake at the northern vent. The volume of the lakes was estimated at about a million cubic metres, much smaller than the original lake's 100 million m3. The surface of the lakes was about 100m lower than the old lake. GNS staff warned that growth of the lakes would increase the risk of further lahars. Gas continued to emit from the northern vent, which was carried away as a brown gas layer at a low

elevation for many kilometres. New ballistic ejecta reported in the crater area by pilots appeared to GNS staff to be old blocks revealed by recent snow melt. They saw no evidence of ash or block eruptions in the previous 24 hours. Volcanic tremor continued at a similar level, and a few high frequency vol-cano-tectonic earthquakes were recorded. Thursday Flights over the mountain showed the southern lake to be lower than the northern one. A small gas plume emitted from the northern vent similar to Wednesday. Tremor activity was as for

Wednesday. GNS staff carried out deformation surveys and examined recent ejecta on the volcano which was feeding secondary lahars. Friday Tremor activity was the same as for Wednesday and Thursday. Gas plumes were reported up to 4000 metres and 3300 metres by aircraft 20 miles west of Napier. A GNS party carried out a deformation survey at Tukino on Friday completing surveys around the volcano. Tilt-levelling surveys at Knoll Ridge and the Dome on Thursday found slight but statistically insignificant inflation of the mountain had occurred during the previous 10 days.

GNS staff said a repeat of the survey 7-10 days later was required to establish a

trend for the site. Saturday & Sunday No reports.