Volcano 'recharging' can give clues about its next eruption

Volcanoes which spend longer filling with magma have been linked with having more explosive eruptions.

Researchers from the Museum and Durham University found that the type of eruption from the Popocatépetl volcano in Mexico can be linked to the number and timing of magma recharge events in the decades preceding an eruption. These can be identified from the chemical makeup of crystals left behind in the volcanic rock.

Dr Martin Mangler, a co-author from Durham University, says, 'Volcanoes like Popocatépetl are notorious for their wide range in eruption styles and sizes, and it’s been all but impossible to tell when and how they will erupt next.

'Our results are promising because they suggest that there are indeed precursory magmatic processes and signals that point towards a specific eruptive style.'

The findings of the study were published in Geology.

A force of nature

Volcanoes are the Earth's pressure release valves, releasing magma, ash and gases from inside the planet through gaps in the crust. These materials cool and build up over time, forming the mountain-like structures we are familiar with.

Volcanoes offer a range of benefits to humanity, with the ejected materials containing a wealth of minerals which make volcanic soils some of the most fertile areas in the world. They can also be tapped for geothermal energy and harvested for materials such as pumice, while volcanoes like Mount Fuji can play an important cultural role in our societies.

However, they can also be incredibly destructive. Volcanoes can eject liquid magma and solid rocks, as well as superheated gases known as a pyroclastic flow that can travel at hundreds of kilometres an hour.

The largest eruption in recent history was that of Mount Tambora in 1815, which is believed to have killed over 70,000 people through the eruption itself and the ensuing famine that followed. The explosion was so violent it caused a six-kilometre wide area of the volcano to collapse, and is blamed for lowering the world's temperature by at least one degree the following year.

While this eruption is estimated to have ejected around 41 cubic kilometres of magma – enough to fill Sydney harbour around 70 times over – other volcanoes can be much less violent. As opposed to Tambora's explosive eruption, other volcanoes can produce steady flows of magma known as effusive eruptions.

Understanding the type of volcanic eruption can be crucial to respond to it in the right way. While effusive eruptions can occasionally be controlled by digging trenches, building barriers or spraying with water, explosive eruptions sometimes require temporary evacuations or the abandoning of towns and cities altogether.

To find out more about the different types of eruption, researchers turned to Popocatépetl, one of the most active volcanoes in North America. They compared details from the known eruptions of the volcano with samples from the rocks which were ejected from it to look for any clues that it might offer. 

Reading the crystals

The researchers sampled rocks from the volcano between 2013 and 2016, separating them by age. The rocks span a period of 40,000 to around 1000 years old, representing seven eruptions of which four were explosive and the rest were effusive.

These rocks were then analysed to investigate their chemical makeup, and the composition of the crystals contained inside them. These crystals are made of pyroxenes, a group of minerals that are mostly found in volcanic rocks.

As the volcano's magma cools within it, these crystals begin to form from elements contained within the magma such as magnesium and iron. Over time, the volcano can be 'recharged' when new magma is brought up from the Earth's mantle, which often has a different composition of elements and minerals within it to the magma already in the chamber.

When the two different magmas meet, the crystal develops a distinct band, like a tree ring. From this, the researchers were able to establish when each band formed, as well as the temperature and pressure at the time.

By comparing this to the time and type of the known eruptions, this allows them to understand how the behaviour of the magma influences which type of eruption will occur.

They found that at Popocatépetl, magma recharge periods lasting between nine and 15 years often led to effusive eruptions, while those longer than 15 years were often explosive. The most explosive also had a period of around a year before the eruption occurred where magma was not recharging the chamber.

However, magma recharging doesn't always lead to an eruption. While it can help create the conditions required, a complex mix of other factors are involved before a volcano erupts.

Dr Chiara Maria Petrone, a Principal Researcher in earth sciences at the Museum and co-author, says, 'We find that magma recharge patterns control the style and intensity of eruptions at Popocatépetl.

'This is an important step forward toward a better understanding of how this extremely active volcano switches between different types of volcanic activity, from effusive to highly explosive.

'This, alongside our estimate of timescales of magma recharge events, have important implications on understanding how the current phase of activity might evolve.'

The researchers now hope to gather more information on other eruptions at Popocatépetl and at other volcanoes to confirm their findings, which has the potential to help with eruption forecasting around the world.