Earthquakes and volcanoes: what’s happening in Iceland?

An intense series of earthquakes shook the Reykjanes Peninsula in southwestern Iceland on Friday, November 10. Hundreds of earthquakes were detected by regional seismometer networks and several were strong enough to be felt in Reykjavik, 50 kilometers away.

A civil protection alert was issued warning of the risk of a volcanic eruption – it would be the fourth since 2021.

Why does this phenomenon repeat itself? What could happen?

Iceland lies on either side of a collapsing ditch

Iceland lies on the Mid-Atlantic Ridge, where the North American and Eurasian plates move apart about 2 centimeters per year. In the Earth’s mantle, where rocks behave like very stiff caramels, the plates can continually deform.

But at the surface, the rocks of the Earth’s crust are cold and brittle: they stretch only when they break. Like pulling on the ends of a chocolate bar where the inside is soft but the shell is hard, the tension that builds as the plates move apart is released in fits and starts when the coating breaks.

The Reykjanes Peninsula forms the southwestern tip of Iceland, where the Mid-Atlantic Rift leaves the sea. There the Earth’s crust responds to inexorable tectonic forces, breaking apart every few hundred years to form a collapse moat (or gap).

The last series of ruptures and eruptions of the Earth’s crust occurred more than 800 years ago. Since then, the plates are said to have shifted about sixteen meters apart.

We are currently in a new phase of rupture, marked by hundreds or thousands of earthquakes, many of which are large enough to be felt in southwestern Iceland. They are all caused by the arrival of magma near the surface.

Every earthquake and eruption releases some of the energy stored in these tectonic plates, and eventually, when this tension is completely released, the eruptions will stop. Over the past 50 years, we have seen several similar eruptions and eruptions around the world.

From 1975 to 1984, 18 series of earthquakes and nine lava eruptions hit northern Iceland – an episode dubbed the “Krafla fires”. Between 2005 and 2010, fourteen series of earthquakes and three eruptions occurred along an 80-kilometer stretch of a rift valley in Afar, northern Ethiopia.

Fractures in the crust are ‘lubricated’ by the presence of magma – just like other oceanic ridges. Magma is constantly forming at depth and due to its density it will rise again.

Within the stiff and brittle crust, magma can only propagate by following faults – if there are any, that is. But once it starts to rise, it makes its way into increasingly shallow areas, increasing the risk of an eruption.

The view from above

Scientists at the Icelandic Meteorological Office can detect what’s happening at depth and pinpoint the smallest tremors using networks of seismometers. These instruments alert the team to new rock fractures in the Earth’s crust and their location.

Sensors communicating with constellations of navigation satellites can make local measurements of small movements of the Earth’s surface. Radar satellite images make it possible to map and measure the shape of this evolving surface in 3D.

The series of earthquakes that began in late October is the latest in a series of events that began in early 2020 and culminated so far in the three eruptions of the Fagradalsfjall volcanic system in southwestern Iceland, in 2021, 2022 and the summer of 2023.

When the earthquakes started this time, they were centered around another volcanic system: Thorbjörn, 10 kilometers west of Fagradalsfjall. Initially, there was no visible deformation of the Earth’s surface, and it was unclear whether it was ‘just’ an adjustment of the Earth’s crust from the previous episode of rifting.

Then the signals showed the Earth’s surface beginning to swell, indicating new magma was invading the crust. The situation evolved rapidly over the past weekend. The size, number and location of the earthquakes all indicated the filling of a fault in the crust with magma, at a depth of about 5 kilometers.

The magma continued to flow in and the tips of the fault opened, clearing a path through the crust until the ‘dyke’ – that vein of rock that seeped into a fissure – reached a length of about 15 kilometers. The magma has not yet reached the surface, but ground movements and computer models suggest that a magma pool has collected within a kilometer of the surface.

Is an eruption coming?

At the time of writing, it seems very likely that this magma will reach the surface and cause an eruption. But monitoring teams won’t know when and where this will happen until they detect specific signals of moving magma. These signs may include the repeated ‘humming’ of volcanic tremors, indicating that magma may erupt within hours, or earthquakes multiplying at very shallow depths.

For now, the dike appears to extend directly beneath the town of Grindavik, a fishing community in southwestern Iceland. If an eruption occurs at the surface, it will likely be similar to the 2021-2023 eruptions at Fagradalsfjall: a fissure opening in the Earth’s surface, fountains of hot red molten rock, lava flowing down the hill away from the eruption site .

The threat will therefore depend on where the eruption starts and how far the lava travels. The fumes released by erupting magma combined with the burning of peat and vegetation can also create toxic air, depending on the speed of the eruptions and the direction of the wind.

If an eruption occurs in the town of Grindavik, the effects could be similar to those of the 1973 Eldfell eruption, which buried part of the town of Heimaey. Hence the preventive evacuation of the city, the nearby Svartsengi geothermal power plant and the Blue Lagoon, one of Iceland’s most famous tourist attractions.

If an eruption occurs on the south side of the dike, which extends offshore, the eruption will occur underwater: the encounter between hot lava and seawater could cause small-scale explosions and local ash clouds, and the boiling sea could release toxic gases.

While the effects of such an eruption were unlikely to be as widespread as those of the 2010 Eyjafjallajökull eruption – which resulted in the closure of airspace over much of northern Europe for several weeks – even a small underwater eruption would contribute to the consequences of the eruption. the challenges the authorities have to deal with… even in a country as well prepared as Iceland.

Leave a Comment