Phreatic eruptions of Ruapehu: April 1975

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Abstract
A major phreatic eruption occurred in Ruapehu Crater Lake, North Island, New Zealand, at 1975 April 24d, 03h 59m, N.Z.S.T. Only nine minutes of volcanic-seismic activity preceded the eruption, but crater dilation had been measured by a geodetic survey two weeks earlier. Bad weather prevented observation of the eruption. At least 1· 6 + 106 m3 of Crater Lake water and lake floor deposits were ejected, final lake level falling by more than 8 m. Hot ballistic ejecta fell up to 1· 6 km upwind from the vent; fine ash fell from a narrow plume extending at least 115 km to south-east. Major floods passed down river valleys draining Ruapehu summit, damaging ski installations, bridges, and hydro-electric power scheme constructions. Because of the time of the eruption, no loss of life occurred. Solid ejecta consisted largely of lake floor deposits, some thermally metamorphosed to sanidinite facies (> 750°c) mineral assemblages, plus minor scoriaceous lava blocks. Steam production by self-sustained reaction between hot rock and lake water may have contributed to explosion generation. The explosion effects were of considerably larger magnitude than the 1969 June 22 eruption,previously the largest Ruapehu event since the 1945 lava eruptions. Neither eruption (1969 and 1975) was witnessed, but in both a base-surge mechanism appears to have transported Crater Lake water and mud onto the summit of Ruapehu as an aerosol, with subsequent rainout contributing to the large floods which rushed down the mountain. In several respects the 1975 April 24 eruption appears analogous to the 1946 Bikini underwater nuclear explosion, and to some observed submarine volcanic eruptions.