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The pressure from magma can fold solid rock in less than one day

The consequences of volcanic eruptions can be severe. Researchers have now made a surprising discovery. They found the evidence on a mountainside.

Jagged mountain ridge in Sarek National Park, northern Sweden, under a cloudy sky.

This mountain is part of the Sarektjåhkkå massif in Sarek National Park in Northern Sweden. This is where the researchers discovered a window into the deep roots of a continental rift that saw massive volcanic activity 600 million years ago.

(Photo: Drone / Research team)

Gunn Kristin Tjoflot Gunn Kristin Tjoflot SENIOR ADVISER

Presented by: Presented by: University of Oslo

Published 15 May 2026 - 00:01

Volcanoes have frightened and fascinated humans throughout the ages.

Researchers are continually gaining more knowledge about the natural and violent volcanic events caused by magmatic forces deep within the Earth.

Most people are familiar with the Roman cities of Pompeii and Herculaneum, which were destroyed during the eruption of Vesuvius in 79 CE.

The eruption could be heard 10,000 kilometres away

"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.

He is a geologist and researcher at the University of Oslo's Department of Geosciences.

People walk at the base of a steep mountain cliff with visible magma dikes in Sarek National Park. — The magma dikes exposed at the foot of the mountain in the Sarek National Park bear witness to the powerful forces that took place in the depths 600 million years ago.

Kjøll and his research colleagues Olivier Galland at the University of Oslo and Thomas Scheiber at the Western Norway University of Applied Sciences have recently investigated the forces of magma and how magma moves through the Earth’s crust on its way to the surface.

The path of magma to the surface is often viewed as passive. We have imagined that it flows through already existing cracks, but that is rarely the truth, Kjøll notes.

High pressure can create lava jets

During the Fagradalsfjall eruption in Iceland in 2021, lava fountains were measured shooting lava up to 460 metres straight into the air.

The same was observed at the Kīlauea volcano in Hawaii during the ongoing eruption there. For comparison, the Eifel tower is approximately 330 metres 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 forwards, opens new cracks, and deforms the surrounding rocks,” explains Kjøll.

So what actually happens on the way up – through the many kilometres of crust the magma must travel before reaching the surface?

Magma's journey to the surface can happen quickly

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

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

The researcher explains that deeper down in the crust, the rocks tend to be softer and normally deform through ductile processes.

Ductile deformation means that rocks deform permantently, gradually changing shape without cracking.

“But because the magma moves so quickly, the slower ductile deformation cannot keep up. The rocks surrounding the magma end up fracturing,” Kjøll says.

Useful concepts

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.

Enormous pressure can deform rocks

The new study shows that this is not the full picture. The force of magma can be so great that it can deform the surrounding rocks.

This can be seen in the mountain massif in the Sarek National Park, which has given researchers a new understanding of how magma 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.

He adds that they can show that this deformation happened extremely rapidly, because it must have taken place before the magma solidified.

Geological outcrop showing a thin magma dyke cutting through folded marble layers beside a marker pen. — A thin magma dyke (up to 8 metres thick in this area) has intruded into the soft marble and deformed it, causing it to be shortened by folding.

The 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.

Millions of years in a single day

By calculating how long the magma in the fractures remained molten, 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," says volcanologist Olivier Galland.

He explains that 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,” he says.

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

Instead, the researchers show that the magma pressure is powerful enough to physically compress the rocks surrounding the magma conduit – much like squeezing a soft lump of clay.

The rocks yield and permanently change shape, instead of springing back as previously assumed, Galland elaborates.

Geologists on a glacier below a steep cliff with dark rock bands and pale marble layers. — To access the best outcrops, the researchers had to climb vertical walls. Access to the walls were gained by first ascending the glaciers. The dark rocks in the central and left are the magma conduits, and the light-coloured rocks are marble. Geologists for scale in the middle of the picture.

A rare window into the Earth's interior

The observations were made in Sarek National Park in northern Sweden. In this area, deep sections of a continental rift similar to the East African Rift are exposed.

Here, they were able to study volcanic processes that normally take place 10 to 15 kilometres underground. These have now reached 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.

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 the surrounding rock.

Normally, such rocks are transformed on their way to the surface, but in Sarek they have survived an extraordinary geological odyssey.

A film from the field work in Sarek can be seen on YouTube: