Researchers have uncovered a geological process that could help explain why some volcanic eruptions are more explosive than others. The findings shed new light on why the infamous eruption of Mt. Vesuvius in 79 A.D., which destroyed the Roman town of Pompeii, was so destructive.
Most volcanic eruptions involve magma—the name for lava when it is underground—being released from vents. These eruptions can generally be categorized into two main groups: effusive and explosive.
Effusive eruptions—the most common form—tend to release magma, which is relatively thick, sticky and low in gas content, in a relatively tame manner. Explosive eruptions shoot out fragmented magma in a much more explosive and destructive way.
The geological processes behind why some eruptions are effusive and others are explosive remain poorly understood. However, a paper published in the journal Nature Geoscience suggests it may be something to do with the rapid crystallization of magma.
A team of scientists analyzed explosive eruptions using a combination of numerical modeling and experiments conducted in the lab, as well as on site at volcanoes. They also examined samples of volcanic rock collected from highly explosive eruptions, including the 1886 Tarawera eruption in New Zealand.
The researchers found that a rapid ascent of magma during explosive eruptions produces a cooling effect. The cooling leads to the rapid formation of crystals in the magma which very quickly increases its viscosity causing it to fragment. This produces a highly explosive eruption.
"We found that, under certain conditions consistent with highly explosive eruptions, crystallization can occur within a couple of minutes during magma ascent," Fabio Arzilli, lead author of the study from the University of Manchester in the U.K., said in a statement.
In light of the findings, Arzilli argues that all volcanic systems on Earth which involve the most abundant type of lava—the kind which forms basalt on the surface when it cools—have the potential to produce powerful explosive eruptions.
"This has important implications for the volcanic hazard and risk, on not only the regional, but also the global scale," he said.
Other scientists have studied magma crystallization before, but generally they came to the conclusion that the process was too slow to be the cause of highly explosive eruptions.
Both effusive and explosive eruptions can be further subdivided into six major subcategories depending on their specific characteristics: Icelandic, Hawaiian, Strombolian, Vulcanian, Pelean and Plinian.
"Plinian eruptions" are a form of violent, explosive eruptions named after the ancient Roman historian Pliny the Younger, who was the creator of the only surviving eyewitness account of the Vesuvius disaster. Like the Vesuvius event of 79 A.D., Plinian eruptions involve an intense explosive blast directed vertically upward which shoot out volcanic material.