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The Cumbre Vieja volcano of La Palma Island, Spain, started to erupt 3 months ago.

This week, the volcano stopped emitting lava and there remains only degassing processes. According to this news story, the Spanish National Geographic Institute (IGN) plans to declare the end of the eruption on 25th December if no more activity occurs.

That make me wonder: what determines the end of a volcanic eruption and how can it be predicted?

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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.

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