Breakout Lahar from New Zealand's Crater Lake

Abstract

Lahars, an Indonesian word referring to a volcanic mudflow carrying up to 80 percent solid material by weight, are highly lethal, having caused more than 30,000 deaths since the late 1700s. Although they are a major hazard, particularly in the developing world where population growth and settlement of river terraces in volcanic areas are increasing societal vulnerability, their study is hindered by their unpredictable timing, capricious travel paths, dramatic changes in flow behavior as they erode and deposit sediment [Pierson and Scott, 1985], and the fact that these high‐energy flows are dangerous to observers and measuring equipment.A unique opportunity to capture a complete data set on a single, discrete lahar event was presented following the 1995–1996 eruptions of Mount Ruapehu in the central North Island of New Zealand (Figure 1). This activity expelled the water in the summit Crater Lake, generating a series of eruption‐ and later rain‐triggered lahars [Cronin et al., 1997], and deposited about 8 meters of tephra (volcanic ash and rock fragments ranging from pebbles to carsized blocks) over its former overflow channel. On 18 March 2007, this fragile tephra dam, which had been impounding extra water in the refilling lake basin, failed, releasing 1.3 million cubic meters of water in less than 90 minutes and generating the largest lahar at Ruapehu in 100 years.