Iceland volcano baffles volcanologists – Vajradalsvilla eruption reveals unique ‘on the fly’ change

A sudden change: Iceland’s Fagradalsfjall volcano amazes not only with its suddenly renewed activity – its behavior is also unique in the world. Because the composition of the lava changed over the course of a few weeks drastically and rapidly as had not been observed before in erupting volcanoes, as volcanologists reported in “Nature”. This sheds entirely new light on subterranean magma flows and volcanic “supply lines”.

According to conventional wisdom, a volcanic eruption is fed by the magma chamber of Fire Mountain: when this reservoir in the Earth’s crust fills and pressure increases from magma and volcanic gases, volcanic material makes its way to the surface – the volcano erupts. It will take thousands of years for the magma in this subterranean chamber to cool, partially crystallized It mixes with the incoming magma.

When a volcano erupts, the magma chamber empties and fuels lava flows emerging at the surface. So their chemical makeup reflects the conditions in the volcanic magma chamber and constitutes the typical “chemical signature” of each volcano – so presumably.

eruption after 800 years

But the Fagradalsfjall eruption in southwest Iceland contradicts this scenario, as Sæmundur Halldórsson of the University of Iceland and his team found. Fagradalsfjall outbreak started In March 2021, when a fissure opened in this volcanic region about 40 kilometers south of Reykjavik and glowing lava began to emerge. Some of the lava fountains rose up to 450 meters in the air. This was the first volcanic eruption on the Reykjanes Peninsula in about 800 years.

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Volcanologists were more curious as to what caused this eruption and what was going on underground. “To understand the driver of the activity, we monitored the chemical composition of the lava, its crystals, and volcanic gases throughout the eruption period,” the researchers said. They took weekly samples of the excavated material.

Amazing change

The result: In the early days of the eruption, Fagradalsfjall produced lava that was fairly low in gas and minerals—like most other Icelandic volcanoes. This lava showed the expected formation of magma from a magma chamber about 15 kilometers from the surface. “It corresponds to the long-term average of mantle melt that has been stored in the Earth’s lower crust,” the volcanologists explain.

But then something surprising happened: within a few weeks after the start of the eruption, the chemistry of the lava changed drastically: “When we saw the values, it soon became clear that this new lava was chemically unusual: it belonged to a very primitive class of lava type Rich in magnesium that we had never seen before in Iceland in the entire modern era,” says co-author Imo Ranta from the University of Iceland.

Unique worldwide watch

Such a rapid and sharp change in lava composition during eruptions came as a surprise to volcanologists: “In just one month, the Vagradalsvial eruption showed greater chemical variation than Kilauea eruptions in Hawaii in decades,” says co-author Matthew Jackson of the University of California, Santa Fe. Barbara. “Iceland’s volcano has shown rates of change thousands of times higher than Kilauea, with lava changing slightly even over the years.”

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Changes in magma composition and origin during the Fagradalsfjall eruption. © Halldórsson et al./Nature, CC by 4.0

Such a strong change is also unusual for Iceland and the volcanic region of Reykjanes: “The range of values ​​measured at Fagradalsfjall only in the first month is as large as all the lava compositions measured in southwest Iceland in the last 10,000 years,” says Jackson. No such rapid and extreme change in the magma chemistry of a hot volcano has been observed in real time.

A direct line in the Earth’s mantle

But what is behind that? As volcanologists have discovered, the chemical change in Fagradalsfjall’s lava reflects a drastic change in lava regeneration: Instead of “old” magma from the magma chamber, the volcano increasingly produced fresh magma directly from the Earth’s mantle. “The eruption was fed directly from the mantle reservoir without long delay in a shallow area of ​​the Earth’s crust,” Halldorsson and his team report. It has always been suspected that something like this is possible, but it has not been directly observed.

“The Fagradalsfjall eruption demonstrates how quickly a deep magma channel system can re-form itself on the fly,” volcanologists say. The new observations thus provide unique insight into the hidden processes under volcanic systems. It can also help improve volcanic models and thus possibly also risk predictions for active volcanic areas. (Nature, 2022; doi: 10.1038/s41586-022-04981-x)

Source: Nature, University of California-Santa Barbara

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