Volcanism is the surface manifestation of internal heat-producing processes on a rocky planet or moon, with the release of solid, liquid or gaseous products (Francis in A Planetary perspective. Clarendon Press, Oxford University Press, 443pp, (1993)). On Earth, igneous processes including volcanic activity are of considerable interest to geologists because they generate new crust, form deposits of gold, silver and copper, contribute to fertile soil formation, and perturb the atmosphere and climate. At a more immediately practical level, an understanding of how volcanoesVolcano (definition), volcanoes work and what triggers eruptions is essential in order to mitigate their negative effects on human society.