Why do some volcanoes erupt explosively?
Glen S. Mattioli, associate professor of geosciences, replies: Volcanoes are surface manifestations of processes that originate deep within the Earth. As depth increases in the Earth, temperature also increases. At Earth’s surface pressure, one atmosphere, most rocks melt at about 700 to 1200 degrees centigrade, and their melting points rise with increasing pressure. Given that the rate of temperature and pressure increase starts at one atmosphere pressure — well within the solid state field — and that its slope remains below that of the melting curve, this implies that common rocks melt within the Earth only under special conditions. Three scenarios can lead to melting: the temperature or pressure at some fixed depth increases, the pressure decreases so that the melting curve is crossed from a higher pressure/temperature regime, or the melting curve is depressed by the addition of water. All cases can lead to the formation of magmas, or molten rock. The exact melting temperature and p
