50 years since century's biggest North Island eruption

Ruapehu Bulletin, Volume 12, Issue 577, 14 March 1995, Page 6

50 years since century's biggest North Island eruption

Bx

Dave

Johnston,

Dept. ofSoil Science,

Massey University. Reprinted from Tongariro, The Annual, December 1994 Vol.3 published by the Department of Conservation. The 1945 eruption of Mount Ruapehu is still the largest eruption in the North Island this century. Activity was first noticed on 8 March, when the pilot of a United Airways plane, en route to Wellington, reported a long plume of steam coming from the crater and blowing to the east. On the same day staff at the Waiouru Military camp also noticed the steam issuing from the crater. Further steam discharges were observed up to 19 March when a lava dome was first seen as it emerged from the Crater Lake. Activity increased with the gro wth of this dome, accompanied by large steam explosions observed as far away as Palmerston North and Wanganui.

Spectacular eruptions were seen on 26 March with Taupo residenfs describing a large explosion in the afternoon sending a column of steam to 8000 ft (2437m) above the mountain. The following day the Crater Lake was reported to be steaming but the lava dome had disappeared. Activity remained relatively quiet throughout April with sporadic expulsions of water, "ash and mud from the lake. A second larger dome was observed on 7 May, growing at the same location as the first. During

the first two weeks of May steam and ash were emitted in a continuous column and loud rumblings were heard by communities around the mountain. The growth of the lava dome appeared to stop during the latter half of May. Visitors to

the Crater Lake, on 2 June, described the oncegrowing lava dome only as a circular reef in the middle of the lake. Activity increased on 17 Jurte with a large ash and steam cloud observed above the crater from Ohakune in the evening. Ash falls The first ash falls to occur away from the mountain were reported lightly coating Taupo on 19 June. The lava dome grew steadily through the rest of June entirely displacing the Crater Lake. Mr A Mason observed the dome on 24 June, reporting

that "the outside of the island appeared to be solid, but the centre was shaking like a jelly .. periodically steam pressure beneath would lift a great lid of viscid lava aside and dense black smoke with steam would issue with a roar ". The most serious injuries of the entire eruption occurred on 2 July when two trampers, who had camped the night near the crater, were caught by an explosion which showered them with hot rocks. Both received burns and one was knocked unconscious. Turn to Page 8

50 years since century's biggest North Island eruption

FROMPAGE6 Explosive style Activity changed its style during July with the series of explosive eruptions producing high columns of steam and ash. The large explosive eruptions in August and early September sent an ash and steam column high above the mountain and the prevailing winds dispersed ash as far afield as Upper Hutt, Wanganui, Whakatane and Opotiki. Numerous ash falls coated Ohakune, Waiouru and Hawkes Bay, Accompanying rumbling noises were heard far from the mountain during a number of the explosive events. The increased activity in the fxrst week of August caused some concern in communities around the mountain. Authorities in Ohakune and Waiouru made arrangements for a possible evacuation, although it was never necessary. The ash was mostly a nuisance in affected communities, causing eye and throat irritations, entering and soiling interiors of houses and damaging paint work on cars. Crop damage was reported around Ohakune, in particular, causing rafting among cabbages. Farmers in the Taihape area reported that ash found in the wool of sheep during shearing had reduced its quality. The ashfalls severely disrupted skiing on

the mountain for the entire season. Visitors to the crater area on 17 August described three vents, a main vent towards the eastern rim of the crater, emitting ash and steam and two smaller steam vents. Intermittent explosions were observed ejecting ash, rocks and steam. Columns of ash and steam rose several thousand metres above the crater and lightning discharges were common within the rising plume. Activity appeared to reach a peak on 22 August. During September activity remained at a moderate level with the emission of steam and ash and the enlargement of the main vent. On 27 September activity increased with a number of large explosions, again producing a high eruption column and dispersing ash far afield. During October Taupo, Rotorua, Whakatane, Napier - Hastings, as well as communities around the base of the mountain all received ash falls. Clock damage Explosive eruptions continued into November and early December with Waiouru, Ohakune, the Chateau and Hawkes Bay receiving most of the ash falls. Some Hawkes Bay residents reported that watches, docks and fine machinery were suffering irreparable damage from the repeated ash coat-

ings. During November the New Zealand Army moved some 700 vehicles from Waiouru to Linton due to the deleterious effect of the acidic ash on the canvas tops of vehicles and its reported corrosive effect on metal. On 23 November Taumarunui's water supply was disrupted by the large quantity of ash being carried in the Wanganui River blocking water intake filters. Pumping was reduced from 20,000 gallons/hour to 7000 gallons/hour. At the Chateau, which was a psychiatric hospital at the time, numerous disruptions to both water and electricity supply were reported over several months due to ash in the streams feeding the water supply and electricity generating plant. The hospital was eventually forced to close in December, resulting in the relocation of its 1 80 patients. The reworking of ash further affected the water supply of the Chateau in 1946 and during the warm summer of 1955. The last reported ash falls to affect Napier and Hastings were reported on 6 December and only intermittent activity occurred after that, with the last strong explosive event on 15 December. After the cessation of explosive activity the crater consisted of a steep walled vent about 300m deep and occupied by a boiling lake. The Crater Lake re-

filled and by 1 949 it had returned approximately to its previous level. A failure of the blocked Crater Lake outlet occurred on 24 December 1 953, resulting in the Tangiwai Disaster and the loss of 151 lives. Small eruption The 1945 eruption was relatively small in comparison to many of the past Mt Ruapehu eruptions but serves as a reminder of the hazards that exist in the vicinity of the central North Island volcanoes. As part of my research I am keen to hear from anyone who remembers the eruption, especially where ash fell and if any photos of the eruption or ash samples exist. By August 1953 the level of the Crater Lake stabilised at eight metres higher than the preeruption level. A barrier of ice and debris was blocking the previous lake outlet. The collapse of this barrier on 24 December 1953 resulted in a lahar (a torrent of water, volcanic debris, sand and rocks) from the overfilled lake. The lahar swept down the Whangaehu Valley and washed out the Main Trunk Railway Line bridge at Tangiwai only minutes before the northbound train arrived. That night 151 lives were lost when the train plunged into the swollen river.