Eruption triggering

Volcanic systems behave episodically and require mechanisms for magma segregation, instability and ascent. Here, we discuss the processes that promote the ascent of magma to the Earth's surface and prepare the onset of an eruption, thus acting as triggers, as well as the factors that prevent eruption. We describe the various petrological, geochemical and geophysical observations that reveal the triggering processes and events. Based on emerging concepts of magma plumbing systems, we consider the mechanisms involved, the buoyancy and overpressure of magmas and associated fluids, using modelling approaches. The transient increase in magma inflow at depth, leading to magma recharge at shallow levels, and the increase in volatile content during ascent are key ingredients of internal triggering. Events, such as earthquakes, changes in surface loading and interactions with external water and hydrothermal systems, perturb the stress field, change magma and fluid pressures to act as external triggers.