The typical cycle of volcanic activity is
- Filling of a deep magma chamber
The deep magma source is controlled by regional tectonics and how it occurs is not well understood in detail.
- Movement of the magma to a reservoir at shallow depth
This is often accompanied by increased seismic activity, ground levels rising, changes in ground water levels and changes in heat flow.
- Extrusion of the magma (now called lava once it erupts) and emptying of the shallow reservoir
If the magma comes in contact with ground water the eruption may be explosive. The release in pressure allows gasses dissolved in the magma to come out of solution (like taking the cap off a seltzer bottle) and blow huge amounts of rock into the sky. Lava that erupts through dry rocks, or lava without a lot of dissolved gasses, will mostly flow out with some fountaining at the site of the eruption.
There may, during the course of the eruption, be further filling of the shallow reservoir and later phases of the eruption sequence. Looking at the gas being emitted by the volcano is a common way of detecting the introduction of fresh magma at depth.
- A winding down phase where the lava solidifies, seismic activity drops, emission of sulpher dioxide goes down, and ground levels return to pre-eruption levels.
This is interpreted as an exhaustion of the shallow magma reservoir. Until the links to the deep magma source are closed the volcano may erupt again. Prediction of such renewed activity decades, even centuries, in the future is not within current capabilities.