Mysterious aspect of volcanic eruptions observed for the first time

A team of researchers led by the University of California, Santa Barbara has discovered a process far more dynamic than volcanologists had assumed to be possible in the two centuries they have been studying volcanic events. The experts made this discovery while sampling magma from the Fagradalsfjall volcano in Iceland, which erupted in March 2021.

By trying to clarify how deep in the mantle the magma originated, how far beneath the surface it was stored prior to the eruption, and what was happening in the reservoir before and during the eruption, the researchers got “a big surprise” – as study co-author Matthew Jackson put it.

“The assumption was that a magma chamber fills up slowly over time, and the magma becomes well mixed. And then it drains over the course of the eruption.” 

According to Jackson (and most other volcanologists), as a result of this well-defined two step process, there are usually no significant changes in the chemical composition of the magma as it flows out of the earth. 

“This is what we see at Mount Kīlauea, in Hawaii,” said Jackson. “You’ll have eruptions that go on for years, and there will be minor changes over time.  

“But in Iceland, there was more than a factor of 1,000 higher rates of change for key chemical indicators. In a month, the Fagradalsfjall eruption showed more compositional variability than the Kīlauea eruptions showed in decades. The total range of chemical compositions that were sampled at this eruption over the course of the first month span the entire range that has ever erupted in southwest Iceland in the last 10,000 years.”

The experts said this extreme variability is most likely a result of subsequent batches of magma flowing into the chamber from the deeper layers of the mantle. “Picture a lava lamp in your mind. You have a hot lightbulb at the bottom, it heats up a blob and the blob rises, cools and then sinks. We can think of the Earth’s mantle – from the top of the core to under the tectonic plates – operating much like a lava lamp,” explained Jackson.

As the heat causes some regions of the mantle to rise and plumes form and move upwards towards the surface, molten rock accumulates in chambers and crystallizes, gases escape through the crust, and pressure builds allowing the magma to eventually erupt. In the case of the Fagradalsfjall volcano, although what first erupted was the expected “depleted” magma type accumulating in the reservoir, in about a month, the chamber was recharged by deeper, “enriched” type melts with a different composition (higher magnesium and carbon dioxide levels). In yet another month, the magma which dominated the flow was the deeper, enriched type. 

Although such rapid, extreme changes in magma composition have never been observed before – most likely due to the fact that opportunities to sample eruptions at such an early stage are not common – they may not be so rare. Further research is needed to assess the frequency of such phenomena in other volcanic eruptions, and clarify the mysterious ways in which volcanoes work.

The study is published in the journal Nature.

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By Andrei Ionescu, Earth.com Staff Writer