All the processes associated with the transfer of magma and volatiles from the interior of the Earth to its surface. Magma beneath the crust is under very great pressure. Deep in the crust, faults and joints develop downward, reach the magma, and allow it to rise up and intrude the crust. The magma then rises in conduits, depressurizes, forms bubbles, and can give rise to explosive volcanism; see Gaonac’h et al. (2003) Geophys. Res. Letters 30, 11, but also see Gonnermann and Manga (2003) Nature 426. Intrusive volcanicity describes magma injected into the crust which fails to reach the surface, and cools slowly underground forming batholiths, laccoliths, dykes, pipes, and sills. Extrusive volcanicity describes magma which forces its way to the Earth’s surface and erupts as lava from volcanoes or fissures, forming cones or plateaux.

Current volcanicity is confined to regions of the Earth where lithospheric plates converge, diverge, or pass over possible mantle hot spots. In a subduction zone, water is trapped in the descending plate. As the plate is heated, water is released, rising into the wedge of mantle above the plume, causing it to melt. This process forms chains of volcanoes; the cordillera of North America is an area of volcanism related to the subducting eastern margin of the Pacific Ocean. Where one oceanic plate is subducted beneath another, a chain of volcanic islands forms; Japan is an example (Bunbury in P. L. Hancock and B. J. Skinner, eds 2000). When a mantle plume rises through the lithosphere, and eventually erupts at the surface, it is often called hot-spot volcanism. About 10% of volcanoes arise through hot-spot activity. Mass movements on volcanic islands may generate large-magnitude tsunamis; see Whelan and Kelletat (2003) PPG 27, 2. Hillier (2007) Geophys. J. Int. 168, in a fairly technical paper, notes that seamounts constitute some of the most direct evidence about intra-plate volcanism, reflecting the ways that the properties of the lithosphere interact with magma generation in the fluid mantle beneath.

In recent years the focus of volcanology has shifted from the physical aspects of why, when, and how volcanoes erupt to incorporate studies on the short- and long-term effects volcanic eruptions have on society and the atmospheric, aquatic, and terrestrial environments; see J. Marti and G. Ernst, eds (2005).