How does a volcano work? A scientific discovery turns our understanding of the process upside down

Sampling magma in full eruption is not an easy task: between the impossibility of predicting the event, the inaccessibility of the place, the risk of asphyxiation with toxic gases and that of being trapped by a flow of lava, one could even say that it is an impossible mission.

And yet. In March 2021, from the moment a fault opened and erupted near the Fagradalsfjall volcano, in the Geldingadalur region located on the Reykjanes peninsula in southwestern Iceland, a series of 50,000 earthquakes – some of magnitude 4 and above – will shake Icelandic soil for several weeks.

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Matthew Jackson, Professor of Vulcanology at the University of California at Santa Barbara (UC Santa Barbara), Sæmundur Halldórsson of the University of Iceland and their colleagues immediately went to the scene. Without knowing that unprecedented circumstances were going to come together and greatly facilitate their work.

Ideal conditions for studying an erupting volcano

Indeed, thanks to the winds which chased the harmful gases from the bowels of the Earth, as well as the slow flow of the lava, these scientists were able to get close enough to sample the magma continuously, and this, throughout the eruption.

Three questions then tormented the researchers: at what depth of the Earth’s mantle did the magma originate, originally? How deep below the surface was it stored just before the eruption? And finally, what was happening in the magma chamber before and during the eruption?

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Published in the prestigious journal Nature (14/9/2022), the study resulting from the analysis of samples collected in Iceland reveals a much more dynamic process than could have been supposed during the last two centuries.

More magma variability in a single month than in 10,000 years!

According to the hypothesis best supported before the publication of this new study, it was customary to consider “that a magma chamber fills slowly over time, and that the magma mixes perfectly” before being “drained (to the surface) when erupting“, explains Matthew Jackson, the main author of the study, quoted in a press release. In such a way that the chemical composition of the magma does not vary – or only slightly – between the beginning and the end of the eruption.

This is what we observe for example at Mount Kilauea, in Hawaii“, illustrates the researcher. “Eruptions last several years, with minor changes (of magma) over time“.

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A hypothesis however incompatible with the recent eruption witnessed by the authors of the study. “In Iceland, the rates of change of the main chemical indicators have increased by more than 1000 times“, says the volcanologist.

In one month, the eruption of Fagradalsfjall showed greater variability in composition (of magma) than the eruptions of Kilauea in several decades“, compares Professor Jackson. “The total range of chemical compositions (of magma) taken from this eruption in the first month, covers the full range for all eruptions that have occurred in southwest Iceland in the past 10,000 last years“, he lets go.

How does a volcano work? Imagine… a lava lamp!

But why such a chemical variability of the magma, and above all, what can we conclude about the functioning of a volcano? According to the authors of the study published in Naturethese measurements prove that the magma coming from the depths of the Earth’s mantle came to flow into the magma chamber in successive “batches” – and not as an already well-brewed mixture.

To understand this, the main author evokes a design object, very fashionable between the 60s and 70s: the “lava lamp”. Who has never let themselves be hypnotized by this glass globe stretched in height, filled with a transparent liquid and in which evolve balls of melted wax?

Picture a lava lamp in your mind“, offers Matthew Jackson. “You have a hot bulb at the bottom, which heats a ball (of wax). This ball rises upwards, cools, and then sinks. We can imagine that the Earth’s mantle – from the top of the core to below the tectonic plates – works a bit like a lava lamp.”

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Thus, when heat causes certain regions of the Earth’s mantle to rise, plumes of molten rock form and rise to the surface, then accumulating in magma chambers. But the comparison with the luminary of the disco years ends there: when the molten rock crystallizes, gas escapes through the earth’s crust, so much so that the pressure increases, until the magma finally finds a way to escape.

A discovery applicable to all volcanoes on Earth?

In their study, the researchers were able to break down the eruption in the Fagradalsfjall volcano area into several stages. During the first weeks, the lava that flowed to the surface came directly from a magma chamber located about 16 km below the surface, where the magma had become depleted in chemical elements.

Then, in April 2021, the composition of the magma was enriched in magnesium and carbon dioxide, testifying to the fact that the magma chamber had “recharged” with molten rock from another plume circulating through the earthly mantle – like another ball of wax rising in a lamp.

If these rapid changes in the composition of the magma “had never before been observed in near real time“According to the authors, this could be explained by the fact that it is generally difficult – if not impossible – to take samples at such an early stage of an eruption cycle, which can last up to several centuries.

And it must be said that the opportunity does not arise so often: on the Reykjanes peninsula where the researchers were, the previous eruption dated from… 800 years ago!

Although the authors are unable to say to what extent this process is representative of the functioning of all the volcanoes on our planet, their discovery nevertheless sheds new light on phenomena which until now remained impossible to verify.

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Proof that the Earth still has secrets to deliver to humanity? In any case, this is what Professor Jackson thinks: “From now on, when I go to sample an ancient lava flow, or when I read or write articles in the future, I will always have this in mind: This may not be the complete story of the lava flow. ‘eruption“, he smiles.

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