Large lahars & gas clouds feature of eruption

Ruapehu Bulletin, Volume 13, Issue 628, 19 March 1996, Page 6

Large lahars & gas clouds feature of eruption

Lahars, massive gas emissions and changes to the Crater Basin landscape were major features of the early part of the current eruption of Ruapehu. Despite having a lower peak flo w than the T angiwai Disaster lahar, the 1995 event produced the largest lahars in terms of number, frequency and magnitude down the Whangaehu Valley in more than 134 years. Following is an abridged report on the lahar activity, as well as other major eruption effects, written by Department of Conservation scientist Harry Keys. In the 24 days between the first on 18 September and the last from the lake on 11/12 October, there had

been at least 1 8 lahars down the valley although some had merged with others creating multiple peaks in some of the flood hydrographs. Seven lahars were recorded in 58 days during the 1971 eruption sequence which is the only other eruption on Ruapehu in the last 134 years known to have produced more than two lahars in total and three in a single week. On Monday 25 September a huge lahar developed in the valley, observed from the air by Peter Otway of the Institute of Geological and Nuclear Sciences (GNS). He reported two huge standing waves moving down the Whangaehu Glacier fed by lake water. Lahar velocity was prob-

ably more than 60 kilometres per hour. At the aqueduct/ford site in the Rangipo Desert the ice and mud-laden lahar rose up four metres above the ford level by 8am. The powerful slurry swept past with a loud banging roar at a speed of up to 36km/h, rolling two-metre boulders and shaking the ground. The lahar generated between 24 and 26 September 1995 had a volume of about six million cubic metres. This is on a par with the largest known historic lahar which occurred on 1 3 February 1 86 1 . It was o ver three-and-a-half times larger than the lahar on 24 April 1975 (previously the largest recorded this century) and about four

times larger than the lahar which caused the Tangiwai disaster. However, peak flow was probably much less than the T angiwai lahar. The cumulative volume of lahars down the Whangaehu . (probably more than 95 per cent of the total volume of lahars produced during this eruption went down this river) was greater than the volume of Crater Lake. This was due to the large amount of snow, ice, ash and rock debris incorporated in the lahars as they swept down the Whangaehu Valley. Due to the remarkable sequence of powerful explosions, the Crater Lake was substantially emptied by 29 September, the first time this has been seen.

Ash rain The eruption of 1 1 and 12 October produced the greatest amount of ash in New Zealand since the 1945 eruption and possibly since the Tarawera eruption in 1 886. It was difficult to determine what the true thickness was because wind remobilised the ash into drifts after it fell. About 100mm of material fell at Tukinp village, including marble-sized material 10mm in diameter. Sandsized ash rained on the Desert Road, closing it for a second time, and fine ash fell as far away as Gisborne and Whakatane. This event was accompanied by the drying out of the active vents as the last significant portion of Cra-

ter Lake disappeared. A further ash eruption on 14 October was reported as turning day into night in Waiouru and deposited ash as far away as Dannevirke and W aipukurau. The many previous explosions in the 1995 sequence produced very small ashfalls in comparison with these two. More volcanic gas than Pinatubo's eruption After the lake dried out a large amount of volcanic gas was emitted from the crater. Continual strong emission from both large and small gas vents fed blue and brown plumes which were seen stretching from the crater for over 100km at times. These plumes are composed of magmatic gases such as steam, hydrogen chloride and Sulphur dioxide (S02). Bruce Christenson (GNS) measured the production of S02 using a correlation spectrometer (COSPEC) as exceeding 10,000 tonnes per day over a number of days. This is a large amount in global terms, being twice the amount produced during the climax of the Mt Pinatubo eruption in June 1991. Some of this sulphur dioxide is thought to have been derived from the oxidation of sulphur deposited in the volcano conduit system during previous decades, as well as from the magma itself. Gas emission peaked at about 15,000 tonnes of sulphur dioxide per day, equivalent to some hundreds of thousands of tonnes of total gas emission per day. Gas output had declined to about 1000 tonnes by the end of November. Apparently similaramounts were still being emitted by early January 1 996 and certainly enough to be a significant hazard to people venturing close to the crater when the gas cloud drifts over them. Permanent landscape changed The explosions, fall ing water and lahars have wrought considerable permanent changes on the crater rim and Whangaehu Valley, much more so than previous eruptions back to atleast 1945. The crater rim between Pyramid Peak and the (former) lake outlet area has been severely eroded on both inner and outer faces by cascading water, other erupted material and rockfall. As a result, the rim here has been lowered and thinned and its profile has become morejagged. Huge gullies have been eroded in the outer (northeast and south) faces of Pyramid, while its inner (crater) face has had wide terraces carved in it. A portion of the peak on the rim north of Pyramid fell into the crater probably during the afternoon of 20 October. Major rockfall appears also to have occurred on the buttress across the Whangaehu upper gorge opposite Pyramid, and smaller rockfall has oc-

curred on Paretetaitonga and Te Atuaahua. Subsequent ashfall laid what is probably a very superficial cover over some of these changes. New waterfall and a rock pinnacle have been created by lahars in the Whangaehu Valley opposite Tukino skifield. The valley sides here have been turned into cliffs which will be a source of instability and rockfall probably for years to come. The large outwash fan below the lower gorge has been built up by material deposited by the lahars, and water is being forced into channels more to the north than previously. Ice carving changes Ruapehu's glaciers Large changes are also evident in the Crater Basin and Whangaehu glaciers, which have been permanently reduced in volume. The last glacial ice remnant over the outlet stream bed was largely destroyed during the 23 September explosion and the resulting ice canyon was widened further during the following days. Some of the glacial ice has probably been covered by volcanic sediment in the new channel floor. The southeast margin of Crater Basin Glacier has been truncated in this area. Whangaehu Glacier has also been severely eroded by cascading water and lahars which have cut gullies several metres deep below Pyramid. A huge glacier pothole has formed here due to lahars channelling down these gullies and draining into the glacier, Further landscape evolution will occur in these areas in the coming months and years. The Tu wharetoa Glacier has also retreated due to undercutting of the ice cliffs and cascading warm water. With the lowering of the lake the ice cliffs now end on an extensi ve rock bench, as they did in 1953 following the Tangiwai Disaster. Accelerated crevassing and ice caving have been spectacular. When the eruption is over this glacier can be expected, over the coming 10-20 years, to recover its size, first as a slope terminated at the lake shore and then as a new ice cliff. Footnotes: Mr Keys acknowledges scientists from GNS, NIWA, Massey and James Cook universities for sharing their observations with him. The most obvious change in the landscape is the lower Crater Lake level, now 801 00 metres down from prior to the eruption. Assuming that the eruption is on the wane, estimates put the refilling of the lake any where between two and seven years. DoC warns people that "the possibility of eruptions large enough to throw blocks and lake water out into the Crater Basin remains — "as always".