Mud rains on Waimarino

Ruapehu Bulletin, Volume 14, Issue 646, 23 July 1996, Page 10

Mud rains on Waimarino

The Waimarino finally experienced the 'raining mud' of Ruapehu's ash falls on Friday night, 10 months into the eruption. Residents awoke to a greyed landscape with a l-2mm coating of ash. Damp, still conditions meant the ash did not cause problems getting into houses, although on Sunday morning it had dried somewhat and winds were redistributing the ash. Ruapehu's volcanic activity increased significantly at around midnight on Friday, back to the moderate levels seen earlier in June and July. Pilots of commercial flights on Saturday morning reported ash-charged plumes rising to heights of 20,000 to 25,000 feet (6100 to 7500m) above Ruapehu. On-going light ash fall was reported from National Park township, Ohakune and Raetihi and there was a significant coating of new ash on the mountain, dashing Whakapapa' s hopes of opening for skiing on Saturday. Institute scientists at the Wairakei research centre recorded high levels of continuous volcanic tremor (interpreted to represent continuous eruptive activity) from midnight until approximately 7.00 am. At 7.00am this tremor was replaced by small eruption earthquakes with airwaves. These airwaves produced sonic booms heard in Ohakune, Waikaremoana, and Turangi. Vulcanologist Bruce Houston said this resump-

tion of activity was anticipated based on recent events. The sustained moderate activity continued through Saturday, although in contrast to the overnight continuous activity the day was characterised by distinct moderate sized events. "These events occurring at a rate of between two and seven per hour rise above a more continuous lower level of activity," said Mr Houghton. Eruption plumes rose up to 35,000ft (10,700m). Institute of Geological and Nuclear Sciences staff overflew the mountain on Saturday afternoon, witnessing one of the more vigorous events when "a series of rapidly expanding, dark brown pulses of ash and gas were emitted from the southern portion of the crater". No large blocks were emitted and the crater appeared to be in a "very open-vent condition". "On this flight the product of the morning' s vigorous fire-fountaining eruptions were clearly visible," said Mr Houghton. "A very high density of impact craters and ballistic bombs was observed within 500m of the vent. Large bombs were still vigorously steaming. Bombs extended up to 1 .4km from the crater, the maximum reached in July. A thick ash cover from this event extended more than lkm even in the cross and upwind directions." The Saturday afternoon activity was the most

sustained and intense since the events of 17 June 1996. Activity changed at around 7.00pm into more continuous sustained tremor with fewer, smaller separate events. From 8.30am Sunday the tremor levels dimin-

ished and large, discrete explosion earthquake events started again. Ash plumes were sent up to 14,000 feet (40004500m) with the ash blown east-south-east. The plumes were brown and clearly ash-bearing "but not ash rich" and only minor ash falls occurred.