Over the past 50 yr the risk to society from volcanic eruptions has increased sharply due to an increased population, more developed and diversified economies, and a more technologically advanced infrastructure. This fact is demonstrated vividly by the impacts from the two largest eruptions of the twentieth century from the cone volcano of Ruapehu, which suggest that the vulnerability of key sectors in New Zealand society has increased by one to two orders of magnitude over this period. Both the 1945 and 1995–1996 eruptions included explosive phases that dispersed ash over a wide area of the North Island for a period of several months. Individual ash falls were only a few millimeters thick in communities within 150 km of the volcano and only trace amounts were found in communities farther away. The 1995–1996 eruption caused similar physical effects to the 1945 eruption but had considerably greater social and economic impacts. The greatest contribution to the cost of the 1995–1996 eruption, estimated in excess of $130 million (New Zealand dollars), was the impact on the alpine tourist industry in the central North Island, essentially nonexistent in 1945. Other significant impacts were felt by the rapidly growing aviation and electricity-generation sectors. The cost of any future eruption of the same magnitude is likely to grow as the vulnerability of our society increases at a rapid rate.

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