The Reykjanes Peninsula, in the southwest Iceland, shook with an intense swarm of earthquakes on the afternoon of Friday, November 10. Regional networks of seismometers detected hundreds of tremors and several were strong enough to be felt in Reykjavík, 50 kilometers away.

A civil protection alert warning of the risk of an eruption, which would be the fourth since 2021. Why is it repeated and what could happen next?

Iceland stretches along the mid-Atlantic ridge, where the North American and Eurasian plates move apart by about 2 cm per year. In the Earth’s mantle, where rocks behave like very rigid candy, plates can continually expand.

But close to the surface, The rocks of the Earth’s crust are cold and brittle, and can only be stretched by breaking. Like pulling on the ends of a chocolate bar with a soft interior but a hard shell, the tension built up as the plates separate is released in bursts as the shell breaks.

The Reykjanes Peninsula forms the southwestern tip of Iceland, where the Mid-Atlantic Ridge rises from the sea. Here, the crust responds to inexorable tectonic forces by breaking apart every hundreds of years, forming a crack.

The last sequence of cracks and eruptions occurred more than 800 years ago. Since then, the plates have moved about 16 meters apart.

We are now in another phase of rifting marked by hundreds or thousands of earthquakes, many of them large enough to be felt throughout southwestern Iceland, and all driven by the arrival of magma near the surface.

Each earthquake and eruption releases a little more pent-up motion in these tectonic plates and, Finally, when the tension has been released, the eruptions will cease. In the last 50 years, we have seen similar rashes a couple of times around the world.

Between 1975 and 1984, 18 seismic swarms and Nine lava eruptions devastated northern Iceland during the Krafla fires. Between 2005 and 2010, 14 seismic swarms and three eruptions occurred along an 80 km section of the Rift Valley in Afar, northern Ethiopia.

As in all mid-ocean ridges, the rifting process is lubricated by magma. Magma forms continually at depth, and its buoyancy indicates that it is destined to rise.

In brittle crust, magma can only rise when there are fractures to follow. But once you start to ascend, you will make your way to increasingly shallower depthsincreasing the risk of rashes.

The view from above

The scientists of the Iceland Meteorological Office can detect what happens in depth and locate the smallest shocks using networks of seismometers. These alert the team to rock breaking in the crust and pinpoint where it is occurring.

Sensors that communicate with navigation satellite constellations can provide punctual measurements of the tiny movements of the Earth’s surface, and satellite radar images can be used to map and measure the three-dimensional shape of the changing surface.

The earthquake swarm that began at the end of October It is the latest in a sequence of events that began in early 2020 and has so far culminated in three eruptions in the Fagradalsfjall volcanic system in southwestern Iceland, in 2021, 2022, and most recently in the summer of 2023. .

When the earthquakes started this time, They were grouped around and under another volcanic system: Thorbjörn, 10 kilometers west of Fagradalsfjall. At first, no deformation of the Earth’s surface was visible and it was not clear whether it was just a readjustment of the crust after the previous episode of rupture.

But once signals showed that the Earth’s surface was beginning to bulge, this indicated that new magma was entering the crust. Over the past weekend, things evolved quickly. The size, number and location of the earthquakes indicated that a fracture in the crust was filling with magma about 5 km deep.

As the magma continued Flowing into it, the fracture tips opened, running through the crust until the new dam was about 15 km long. The magma has not yet reached the surface, but ground movement patterns and computer models suggest that a pool of magma has already accumulated less than a kilometer from the surface.

Is an eruption imminent?

At the time of this writing, it seems quite likely that this magma will break the surface and initiate an eruption. But surveillance teams will only know when and where it is about to happen when they detect the telltale signs of moving magma. These signals could include the repetitive “buzz” of a volcanic tremor.a sign that magma can erupt in a matter of hours, or the proliferation of earthquakes at very shallow depths.

For now, the dam appears to extend directly beneath the town of Grindavik, a fishing community near the southwestern tip of Iceland. If an eruption occurs on the earth’s surface, It is likely to be similar to the 2021-2023 eruptions at Fagradalsfjall, with a crack or fissure opening in the Earth’s surface and fountains of red-hot molten rock, with lava flowing downhill and away from the eruption site.

This will pose a threat depending on where the eruption starts and how far the lava flows. gas vapors released by erupting magma Combined with the burning of peat and vegetation they could create toxic air depending on the speed of the eruption and the direction of the wind.

If an eruption occurs in the city of Grindavik, the effects could be similar to those of Eldfell eruption which buried a part of the city of Heimaey in 1973. Hence the preventive evacuation of the city, the nearby Svartsengi geothermal power plant and the blue lagoon, one of Iceland’s best-known tourist attractions.

If an eruption begins at the southern end of the dyke, which extends out to sea, the encounter of hot lava with seawater in an underwater eruption could generate small-scale explosions and local ash clouds, and release more harmful gases. coming from the boiling sea water.

Although would probably not have effects as widespread as those of the Eyjafjallajökull eruption of 2010which closed airspace over a large area of ​​northern Europe For several weeks, even a small underwater eruption would add to the challenges that authorities must manage even in a well-prepared country like Iceland.

David PyleProfessor of Earth Sciences, University of Oxford and Tamsin MatherProfessor of Earth Sciences, University of Oxford

This article was originally published in The Conversation. read the original.