Crystal timeline from volcanic vents

30 April 2018

Studying volcanos is like detective work. The processes that kick-start an eruption occur deep underground and are impossible to observe directly, so volcanologists look for clues on the surface and work backwards using their knowledge of rock physics and chemistry.

Now, Teresa Ubide at The University of Queensland, Australia and Balz Kamber at Trinity College Dublin, Ireland  have developed a new way to interpret chemical clues from tiny clinopyroxene crystals that grow in the magma chambers of volcanos. As these crystals grow, they build up layers similar to tree rings. The layers contain information about how magma has changed over time, and allow researchers to work out the timing of events that might trigger eruptions.

Ubide and Kamber collected samples from Mount Etna in Sicily, the most active volcano in Europe. Etna has shown increased activity since the 1970s, with eruptions occurring not only from the central vents, but also from the sides of the volcano, which is potentially more hazardous. The eruptions probably occurred soon after new magma was injected into the chamber system, increasing pressure and causing rapid magma ascent to the surface, carrying crystals with it.

“The lava flows we sampled at Etna for the paper are very recent (1974-2014) and detailed geological maps are available,” says Ubide. “We looked for ‘fresh’ rocks, without alteration or weathering, which contained abundant crystals.”

The researchers collected samples from the lava flows of eight eruptions. Back in the lab, they used a  technique called laser ablation mass spectrometry mapping to measure the amounts of trace elements, particularly chromium, in the growth layers of 287 clinopyroxene crystals present in the samples.

“Chromium is abundant in mafic (magnesium and iron rich) magmas and it is readily incorporated into clinopyroxene crystals,” explains Ubide. “We assume that crystals grow from the centre towards the rim, so if we find chromium-rich zones close to the rim of the crystal, it suggests the arrival of mafic magma just before the eruption.”

By measuring the thickness of the chromium-rich outer rims, the researchers estimated that the eruptions tend to happen around two weeks after new magma arrives in chambers ten kilometres below Etna’s surface. This new knowledge of the timescale at which eruptions may be triggered could be extremely useful for future disaster management.

 “Emergency management during volcanic crises, including decisions on evacuation, is guided by monitoring data,” says Ubide. “Studies like ours aim to improve the understanding of the volcanic system in past eruptions, but could also help interpret geophysical signs of unrest in the future.”

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References

1. Nature Communications 9:326 (2018). doi: 10.1038/s41467-017-02274-w