Volcanoes have frightened and fascinated humans throughout the ages, and researchers are continually gaining greater knowledge of the natural and violent volcanic events driven by magmatic forces deep within the Earth.

"Most people are familiar with the Roman cities of Pompeii and Herculaneum, destroyed during the eruption of Vesuvius in 79 AD. Other more recent eruptions include the South Pacific island of Hunga Tonga–Hunga Haʻapai in 2022, which was heard in Alaska seven hours later – 10,000 kilometres away," says Hans Jørgen Kjøll, who is a geologist and researcher at the Department of Geosciences at the University of Oslo (UiO).

Kjøll and his research colleagues Olivier Galland at University of Oslo and Thomas Scheiber at the Western Norway University of Applied Sciences are behind a recently published study in Nature Communications. They have investigated the forces of magma and how magma moves through the Earth’s crust on its way toward the surface.”

The path of magma to the surface is often viewed as passive – flowing through already existing cracks – but this is rarely the truth. During the Fagradalsfjall eruption in Iceland in 2021, lava fountains were recorded shooting lava up to 460 metres straight into the air; the same has been observed at the Kīlauea volcano in Hawaii during the ongoing eruption there. For comparison, the Eifel tower is approximately 330 meters tall.

"The force behind such lava jets comes from the enormous pressure within the magma. The pressure is so intense that the magma deep in the Earth's crust pushes forward, opens new cracks, and deforms the surrounding rocks," explains Kjøll.

So what actually happens – across the many kilometres of crust that magma must travel before reaching the surface?

Before magma reaches the surface, it must find its way through the roughly 15 to 30 kilometre-thick Earth's crust.

The uppermost ten kilometres are relatively cold, and the rocks fracture as the magma pressure builds and forces new cracks forward in short cycles.

"Deeper down in the crust, the rocks tend to be 'softer' and are usually deformed through ductile processes. But because the magma moves so quickly, the slower ductile deformation cannot keep up – and the rocks surrounding the magma end up fracturing," Kjøll explains further.

The new study shows, however, that this is not the full picture. The force of magma can be so great that it can deform the surrounding rocks. This is what can be seen in the mountain massif in the Sarek National Park, and it is what has given the researchers their key insight to understand how magma moves in the deep crust and can affect the surrounding rocks.

"When we were out in the field, we could see that the rocks around the magma conduits had been squeezed so hard that they were folded! This tells us that the magma pressure must have been enormous – and that the surrounding rocks must have been surprisingly weak," says Kjøll.

"Furthermore, we can show that this deformation happened extremely rapidly, because it must have occurred before the magma solidified.”

The UiO researchers see evidence of this in rocks that originally formed at depths of around 10–15 kilometres in the Earth's crust, but are now exposed in mountain formations in Sarek National Park in Sweden.

"The fieldwork for this study has been exceptional. Getting to spend two weeks in the national park for research purposes, climbing this fascinating mountain massif alongside a professional climber, has been an incredibly cool experience," says Kjøll.

By calculating how long the magma in the fractures remained liquid, the researchers were able to estimate how quickly the ductile folding occurred. The result was remarkable:

"By examining the magma conduits in the rocks, we found that processes which normally take millions of years beneath large mountain ranges had occurred in under a year – and in some places within a single day. From a geological perspective, this is extremely rapid. These fast processes are driven by the high temperatures that can arise in areas where the crust is being stretched and there is a great deal of volcanism," says Olivier Galland, volcanologist and co-author of the study.

These observations challenge a long-held hypothesis that the stresses generated by magma moving toward the surface are absorbed primarily by elastic mechanisms in the Earth's crust.

The researchers instead demonstrate in their new study that the magma pressure is powerful enough to physically shorten the rocks surrounding the magma conduit – much like squeezing a soft lump of clay. The rocks yield and permanently change shape, rather than springing back as previously assumed.

The researchers' observations were made in Sarek National Park, which is located in the north of Sweden. The area exposes deep sections of a continental rift similar to the East African Rift of today. Here they were able to study volcanic processes that normally take place 10–15 kilometres underground, but which have now been brought to the surface after millions of years of erosion.

"Sarek is like a geological laboratory where the end products of processes that tear a continent apart and form new ocean lie right before us," says geologist and mountain guide Thomas Scheiber, co-author of the study, who also researches geohazards at the Western Norway University of Applied Sciences.

The researchers emphasise that it is extremely rare in the geological record to find such large areas exposed from such great depths, that still preserve the primary contacts between magma and host rock. Normally, such rocks are transformed on their way to the surface, but in Sarek they have survived an extraordinary geological odyssey.

Fact box:

Lava: Molten rock from a volcanic eruption on or above the Earth’s surface.

Magma: Molten rock associated with volcanic activity below the Earth’s surface.

Brittle deformation: Rocks fracture along faults or fractures when the stress becomes high enough.

Ductile deformation: Rocks undergo permanent deformation and gradually change shape without fracturing.

Kjøll, H.J., Scheiber, T. & Galland, O. Rapid viscous flow of crustal rocks controls dyke emplacement in the ductile crust. Nat Commun 17, 785 (2026).

Research project: Beyond Elasticity – How inelastic properties of crustal rocks control the propagation of dykes and sills in vulcanic plumbing, a FRIPRO-project supported by The Norwegian Research Council.