When magma prevents volcanic eruptions

November 28, 2017, University of Geneva
Following a large caldera-forming eruption some magma remains in the magma reservoir.This magma cools, its viscosity increases, and when new magma is injected, the magma left over after the caldera-forming eruption stops the fresh magma from propagating to the surface and promotes caldera resurgence. Credit: UNIGE / Roma Tre

A spectacular proof of our planet's activity, calderas are huge topographic depressions, similar to flat-bottomed craters, with a diameter of several tens of kilometres. They are formed by large volcanic eruptions, and sometimes experience an inflation of their floor of up to a kilometre, caused by magma injection. This well-known process, dubbed "caldera resurgence," has been observed several times and yet remains one of the least understood in volcanology. But why, after an eruption, does the arrival of new magma not produce another major eruption, but instead resurgence? A team of researchers from the University of Roma Tre, Italy, and the University of Geneva (UNIGE), Switzerland, shows that the non-erupted magma left after the caldera-forming eruption behaves as a "rubber sheet" that inhibits the rise to the surface of the newly injected magma. The research is published in Nature Communications.

A caldera forms when a magma chamber is partially emptied by a large and its roof collapses, producing a depression at the surface. After this catastrophic event, in a slow process that can last thousands of years, the caldera floor may start to lift disproportionately but without eruption. Resurgence does not immediately follow caldera formation, suggesting that it is not driven by the residual magma left in the reservoir after collapse, but rather by the injection of new magma.

The magma behaves as a rubber sheet

"The magma is not entirely removed from the magma chamber during the caldera-forming eruption. We used thermal modelling to determine how this residual magma evolves over time, and what role it plays in the resurgence process," explains Luca Caricchi, associate professor at the Department of Earth Sciences of the UNIGE Faculty of Science. The magma, hotter than the rocks surrounding the magma chamber, cools progressively and its viscosity increases. The higher viscosity of the leftover magma, with respect to the newly injected magma, makes it behave as a rubber sheet, stopping the propagation of the new magma to the surface.

These results were corroborated by experiments. The leftover magma was replaced by a silicone layer and the newly injected magma by less viscous vegetable oil. The contrast in viscosity between these two materials is equivalent to the contrast observed between the two magmas in nature. "Independently of the depth of the silicone layer, its presence always impedes the propagation of the newly injected magma to the surface," says Federico Galetto, researcher at the Department of Science of the University of Roma Tre.

The model developed by the researchers provides a theoretical framework to account for the transition from magma eruption to accumulation. Valerio Acocella, associate professor at the Department of Eart Sciences of the University of Roma Tre, adds, "The process we discuss is essential not only to develop resurgence, but also for the formation of the reservoirs responsible for the largest eruptions on Earth."

Explore further: Scientists locate potential magma source in Italian supervolcano

More information: Federico Galetto et al, Caldera resurgence driven by magma viscosity contrasts, Nature Communications (2017). DOI: 10.1038/s41467-017-01632-y

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5 / 5 (1) Nov 28, 2017
Does this account for oft-puzzling 'flank eruptions' ?
Captain Stumpy
5 / 5 (1) Nov 28, 2017
Does this account for oft-puzzling 'flank eruptions' ?
Hmm ...
check out the models section and reference number 50 - listed here
Norini, G. & Acocella, V. Analogue modeling of flank instability at Mount Etna: understanding the driving factors. J. Geophys. Res. 116, B07206 (2011).

5 / 5 (1) Nov 28, 2017
The Nature article is 'open source' !!! So many journals are pay-walled, I often don't bother looking...
Wow, that is one hefty report. I'll need some time to digest it.
Thank you.
Captain Stumpy
not rated yet Nov 30, 2017
The Nature article is 'open source' !!! So many journals are pay-walled, I often don't bother looking...
Wow, that is one hefty report. I'll need some time to digest it.
Thank you.
you're welcome

Usually, I read the title and first paragraph or so and skip to the study before reading the rest.

I was just as surprised as you.

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