Volcanic activity is very diverse, with three fundamentally different causes and corresponding effects. Qualitatively different phenomena are observed at ocean ridge spreading centres, at subduction zones and at isolated hot spots, such as Hawaii. At any time, there are typically about 50 volcanoes in various states and stages of eruption and a much larger number described as dormant (but not extinct). Eruptions are assigned volcanic explosivity indices (VEIs, Table 15.1), numbers that are measures of the volumes of pyroclastic material emitted, on a logarithmic scale, that is, 8 for volumes exceeding 1000 km3, 7 for 100–1000 km3, and so on down to 2. This scale is attributed to a 1982 proposal by C. Newhall and S. Self of USA, but is just a minor rescaling of the volcanic intensity scale of Tsuya (1955) of Japan. It is not to be confused with an index of volcanic explosiveness proposed in 1927 by K. Sapper, a number between 0 and 100 that is the percentage of the total eruptive material that is fragmented (pyroclastic).