A spectacular explosion (Figure 1) beneath Ruapehu Crater Lake on September 23, 1995, ejected lake water, rocks, and scoria bombs onto the summit of snow-covered Ruapehu volcano and sent lahars (mudflows) down its outer slopes. Eruptions continued into October, completely emptying the ˜107 m3 lake, and dispersing ˜0.1 km3 of ash and scoria downwind. While this ejecta volume is relatively small, Ruapehu is the only major ski resort in the North Island of New Zealand and it attracts up to 10,000 skiers on some days. Chair lifts and tows (Figure 2) carry skiers to within 2 km of the summit crater through areas affected by lahars during eruptions in 1969 and 1975. The 1995 eruption episode dramatically demonstrated volcanic hazards at Ruapehu to a largely unaware public.