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What Does It Take for a Volcano to Blow?

Predicting the world’s most dangerous, spectacular explosions is no easy feat. A new review of the geosciences literature unearths the conditions and triggers of volcanoes’ activity.

Though the world’s oldest volcanoes erupt infrequently, those eruptions are larger and more dangerous than younger volcanoes’ emissions, according to a new review of the scientific literature on volcanic activity that reveals the how and why of our planet’s most explosive events.

The paper, published Tuesday in the scientific journal Nature Reviews Earth and Environment, contributes to scientists’ quest to accurately predict when volcanoes will blow, how frequently they will erupt and how dangerous these eruptions will be — knowledge that would come as a great help for the 800 million humans around the world who live near active volcanoes.

Most magma — the molten hot rock that flows like a fluid miles below the Earth’s surface — does not finish its journey from the inner earth to the planet’s surface, the researchers wrote.

“During its journey, magma can get trapped in reservoirs within the Earth’s crust, where it may stagnate for thousands of years and potentially never erupt,” said Meredith Townsend, an earth scientist at the University of Oregon who participated in the research, in a statement to the press.

Magma is called lava when it flows out from the planet’s depths and reaches the air, which cools the lava into “extrusive” igneous rocks such as basalt, obsidian and pumice.

Magma frequently cools before it reaches the surface, and the minerals within form “intrusive” igneous rocks like granite and peridotite. These crystals begin taking shape at temperatures as high as 2372 degrees Fahrenheit.

Townsend determined how much pressure it took for magma to pierce the rocks surrounding these reservoirs and reach the surface as lava. The viscosity of the magma at hand makes a great difference as to whether it will make it to the surface.

“If it is runny enough, that is if it does not contain too many crystals, magma can rise very quickly by a sort of self-propelled fracking,” said Eleonora Rivalta, a researcher at Germany’s Potsdam Research Centre for Geosciences, in a statement.

The scientists found that if more than half the magma crystallizes underground, it will be too viscous to be completely propelled to the planet’s surface. Much of this, it turns out, depends on the chemical makeup of the molten rock.

“The chemistry of magma and the crystals it contains provide vital information on the sequence of events leading to a volcanic eruption, which is valuable to better interpret the monitoring signals of active volcanoes and anticipate whether an eruption might occur,” said Luca Caricchi, a geoscientist studying volcanoes and formation of rocks at the University of Geneva in Switzerland, in a press statement. 

Some volcanic eruptions follow seismic events, such as severe earthquakes of magnitude 6 or greater, but earthquakes do not outright cause volcanoes to erupt: the underground magma must already feel sufficient pressure and be poised to blow.

“These alone cannot cause an eruption, the magma has to be ready and awaiting a trigger,” said Nagoya University environmental scientist Atsuko Namiki in a press release. 

When less than half of the magma has crystallized into igneous rock, and the reservoir hosting the magma is excessively pressurized, the magma may begin to rise. External factors such as earthquakes can spur this process along, but do not alone cause any earthquakes.

The age of a volcano is a major factor in predicting how dangerous and how frequently it will erupt, the scientists found. 

“When a volcano is just starting to be active, its reservoir is rather small (a few km3) and the surrounding crust is relatively cold, which leads to many frequent, but small and rather predictable eruptions,” said Caricchi, who is also the study’s lead author.

But older volcanoes enjoy much larger reservoirs and have long heated the earth around them.

“Their reservoir is bigger and the rocks around them are hotter. When new magma is injected, it does not generate much overpressure because the rocks around the reservoir deform and the growth continues,” Caricchi continued.

A young volcano might have begun erupting as early as 40,000 years ago, as in the case of Washington state’s Mt. St. Helens, whereas volcanic activity at Mt. Etna, near the coast of Sicily, began 500,000 years ago.

As volcanoes age, the magma available to erupt mounts in the Earth’s crust.

“Numerical models for magma reservoir growth demonstrate that the rate of magma supply is a key parameter that governs the accumulation and evolution of eruptible magma,” the group writes in the study. “As magma reservoirs grow, crystallizing magma becomes volatile saturated and host rocks become warmer and weaker.”

Magma supply rate, reservoir growth, the age of the volcano, the temperature of the surrounding rocks — all these are helpful indicators in determining the frequency and severity of volcanic eruptions. But precisely predicting future eruptions remains an elusive goal.

“The warning signs are very difficult to detect because the high temperatures decrease seismic activity and the interaction between gases and magma modifies their composition, making it harder to understand what is going on underneath,” Caricchi said. “There are currently 1,500 active volcanoes, and about 50 of them erupt each year. Knowing whether or not to evacuate the population is crucial and we hope that our study will contribute to decrease the impact of volcanic activity on our society.”

The paper ends on a hopeful note, describing new advances in technology that will help monitor magma reservoirs, underground pressure and magmatic crystallization. New tools and further research will be critical in developing and testing models for predicting future volcanic activity.

“Hopefully our findings will be tested on volcanoes that have been studied extensively, such as those in Italy, USA and Japan, and transferred to other volcanoes for which there are less data, such as in Indonesia or South America,” Caricchi said.

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