Elevated levels of volcanic tremor started in late July, but there were no eruptions at that time (BGVN 21:08). Increased levels of seismicity began again on 2 October, with fluctuations up to 2x typical background levels. Activity increased on 9 October, when ~30 volcanic earthquakes were recorded and visitors to the summit observed small geyser-like eruptions from the crater lake. The amplitude of the tremor was variable; seismicity peaked at M 2.5-3.0. The crater lake turned muddy gray from a dull green color and large volumes of steam rose from the lake surface. The alert level was increased from one to two (indicating the onset of eruptive activity) after the 9 October earthquake swarm; correspondingly, the size of the warning area around the volcano was increased to a 1-km radius from the crater lake.

Over 95 volcanic earthquakes were recorded during 9-10 October before a period of strong seismicity on the night of 10-11 October, thought to represent gas and magma movement. Volcanic seismicity peaked early on the morning of 11 October and then began declining. Minor steam-driven eruptions occurred within the summit crater during the weekend of 11-12 October. The largest event, at 1430 on 12 October, erupted lake-floor mud and rocks ~150-200 m above the crater lake. Some of the eruptions also generated tall steam columns. There is no evidence that the eruptions, believed to be driven by steam and gas, discharged new lava or ash. Water level in the crater lake dropped due to increased evaporation and the small steam eruptions, uncovering large fumarole vents on the crater floor and allowing more volcanic gas to be released.

Seismic activity was declining on the morning of 13 October, but the level remained above the typical background for Ruapehu. By the afternoon of 13 October volcanic tremor levels had stabilized at 2-3x background. During an overflight from 1234-1250 no eruptive activity was observed. The inner crater basin and Pyramid Peak area were totally coated in dark gray mud and ash from the eruptions on 11-12 October. The discolored zone extended from the crater basin to the W and WNW and down off the cone over Tukino Skifield. Impact craters from blocks tossed out on the afternoon of 12 October were visible in the W wall of Pyramid Peak. The crater lake was dark gray and steaming vigorously. An area of high heat and gas flow had intensified on the W side of the lake.

Geological Summary. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: Brad Scott, Wairakei Research Center, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: https://www.gns.cri.nz/).