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Volcano study shows timeline of magma behaviour before eruption

A new approach for analysing minerals in volcanic rocks is helping scientists work out what happens inside a volcano before it erupts.

The research is an important step towards a better understanding of how much time minerals spend in a magma chamber before an eruption - which could help scientists work towards a way of forecasting eruptions.

A team of researchers, including Museum volcanologist Dr Chiara Maria Petrone, studied rocks that erupted from Stromboli volcano in Sicily, Italy. Stromboli currently erupts three to four times every hour. 

By accounting for the effect of temperature change in a magma chamber, they were able to unravel the sequence of events occurring inside the volcano.

Dr Petrone says, 'Our research is helping the scientific community to learn more about how volcanoes behave. In the long run, we hope this extra knowledge will help efforts to understand magma chamber dynamics, which could contribute to volcano hazard assessment and protect the people who live in its shadow.'

Dr Petrone intends to use the new method to examine Popocatépetl volcano in Mexico. About 30 million people live within 70 kilometres of the volcano's summit.

The life of a mineral

As magma - hot molten rock - inside a magma chamber begins to cool, it forms a sequence of minerals. These solid minerals continue to grow, layer by layer, and their composition depends on the magma they crystallise from. 

New magma - with a different composition and temperature - may enter the chamber periodically. It mixes with the existing magma, changing the chemical and physical conditions inside the chamber. As a mineral grows by forming a new layer it develops bands with different compositions, preserving a record of these changing conditions.

Because we know that one trigger of volcanic eruptions is the arrival of new magma in a magma chamber, scientists can look at these compositional bands - known as mineral zoning - to work out the timescales of these events.

Multiple magmas

Scientists study mineral zoning to estimate the amount of time a mineral spent in a magma chamber before being erupted - its residence time. They are particularly interested in the timescale from the new injections of magma into the chamber until the eruption.

These estimates are tricky to calculate, especially for complex minerals that have experienced multiple injections of new magma and temperature changes within the chamber. These minerals have developed multiple bands with different compositions.

Yellow mineral crystal with zoning

The team studied minerals with chemically different bands, such as this pyroxene crystal, from volcanic rocks

In the zone

The team's study, published in Nature Communications, used a new approach to a method known as diffusion chronometry - a kind of internal clock for minerals - to analyse the minerals of volcanic rocks from Stromboli.  

The method, undertaken in the Museum's Imaging and Analysis Centre, allowed the scientists to identify the chemically different zones within minerals that had formed under specific temperatures.

The team could then calculate the most reliable estimates yet of the entire history of mineral growth within the magma chamber, and therefore the timescale of new magma injections feeding the volcano.

Dr Petrone, who is also the lead author of the study, says, '[The new model] means we can see more accurately how and on what timescales the new injections of hotter magma affect magma chamber dynamics.'

This will allow scientists to better understand the full life history of complex zoned minerals, revealing a record of magmatic changes that occurred within the volcano before it erupted. 

The team's analysis showed that at Stromboli, between new magma entering the chamber and the eruption, minerals resided in the magma chamber for three to 12 years. While this may seem like a long time, it is merely the blink of an eye in the lifetime of a volcano.

Stromboli volcano smoking

Stromboli volcano

Dr Petrone's next task is to use this method to examine Popocatépetl, the most active volcano in Mexico. Only 35 miles from Mexico City, this volcano is less well understood and threatens about 30 million people.

A glimpse into the future

Dr Petrone and her colleagues have made an important contribution to our understanding of how volcanoes work, but there's a lot more to do before we can forecast a volcanic eruption.

'This study isn't a tool to forecast when a volcanic eruption is going to happen - we are a long way off being able to do that,' says Dr Petrone.

'But by improving scientists' understanding of magma chamber dynamics, it is moving us closer to this goal.'

Many high-risk volcanoes are under constant examination as scientists try to monitor changes in the volcano's shape or surrounding environment that might indicate an eruption is imminent. 

Dr Petrone adds, 'Only with a good understanding of past eruptions, will we be able to combine this data with physical and chemical signals we get from volcanoes - such as gas emissions and earthquakes - and be in a better position to forecast when they will erupt.'

For now, the team's new analysis will help other volcanologists to untangle the complex history of magma chambers inside volcanoes around the world, while Dr Petrone is busy disentangling the magmatic history of Popocatépetl volcano.