New insights into Iceland’s recent bout of eruptions

After almost 800 years of peace and quiet, a new eruption period began on Iceland’s Reykjanes Peninsula on 19 March 2021.

Lava analysis reveals that magma pooled in the Earth’s crust before the eruptions began. The finding disproves the initial hypothesis that magma rose directly from the mantle and may be an important feature for understanding eruption triggering.

Iceland is located on top of the mid-Atlantic ridge where the Eurasian and North American plates pull apart from each other. As the plates diverge, mantle rises towards the surface to fill the gap.

This rift cuts directly through the Reykjanes Peninsula, where 70% of Iceland’s population lives. The region experiences 600 to 1,000 years of inactivity, followed by active eruption periods lasting 300–500 years.

“By collecting lavas at regular intervals, and then measuring their compositions in the laboratory, we can tell what’s feeding the volcano at depth,” says James Day, professor of Geosciences at Scripps Institute of Oceanography, USA, and lead author of the new study in Nature.

“It’s a bit like taking regular measurements of someone’s blood. In this case, the volcano’s ‘blood’ is the molten lavas that emanate so spectacularly from it.”

Since 2021, there have been 3 eruptive events from the Fagradalsfjall volcano: the 2021 Geldingadalir eruption; the 2022 Meradalir eruption; and the 2023 Litli-Hrútur eruption.

Most recently, the 2023–24 Sundhnúkur eruptions near the town of Grindavik remain ongoing.

Using lava samples collected from the Fagradalsfjall volcano in 2021 and 2022, Day and colleagues measured the proportion of different isotopes of the element osmium. Isotopes of an element have the same number of protons but different numbers of neutrons.

“What’s useful about using osmium is that one of its isotopes is produced by the radiogenic decay of another metal, rhenium,” says Day. “Because the elements behave differently during melting, one of the elements, rhenium, is enriched in Earth’s crust.”

“What makes the Iceland eruption so remarkable is the huge signal of crust within the earliest lavas.”

This suggests the start of the eruption was fed by magma that first resided in the crust for some time.

“After that, it appears that the magma of later eruptions used pre-existing pathways to get to the surface,” says Day. “It suggests crustal magma storage may be a common process involved in the run up to larger basaltic eruptions like those in Iceland or the Canary Islands.”

“This information will be important for understanding volcanic hazard in the future as it may help to forecast volcanic activity.”