Mount Etna has fascinated geologists for decades. The towering volcano on the Italian island of Sicily is the most active in Europe, erupting several times a year, yet scientists have never fully understood how it formed.
Now, researchers from the University of Lausanne (UNIL) have proposed a new explanation that could change that. Their study suggests Mount Etna may have formed through a rare volcanic process unlike the one behind any other large volcano on Earth, making it potentially one of a kind.
More than 500,000 years old and rising more than 3,000 meters (9,800 feet) above sea level, Mount Etna has long resisted attempts to fit it into existing models of volcano formation. The new findings, published in the Journal of Geophysical Research -- Solid Earth, were developed in collaboration with Anna Rosa Corsaro of the Istituto Nazionale di Geofisica e Vulcanologia in Catania. The research may also help scientists improve volcanic hazard assessments conducted by researchers at INGV in Catania, Italy.
Volcanoes form when molten rock from Earth's mantle rises to the surface and hardens. Traditionally, geologists have grouped volcanoes into three main types based on how that magma is generated:
Mount Etna does not neatly fit any of these categories.
Although it sits near a subduction zone, the chemical makeup of its lava more closely resembles volcanoes formed above hotspots, even though no hotspot exists beneath the region.
The researchers propose that Etna is supplied by small pockets of magma that already exist in the upper mantle about 80 kilometers (50 miles) below the surface. Instead of forming shortly before eruptions, as is typical for many volcanoes, this magma may have remained in place for long periods before being pushed upward.
According to the study, the collision between the African and Eurasian tectonic plates gradually transports these pockets of magma toward the surface. As the tectonic plate bends near the subduction zone, fractures develop, allowing the magma to rise through the crust, much like liquid being squeezed from a sponge.
This mechanism could explain both Mount Etna's unusual chemistry and its long history of frequent eruptions.
The team believes Mount Etna may belong to a little known fourth category known as "petit-spot" volcanoes. First identified by Japanese geologists in 2006, petit-spot volcanoes are small submarine volcanoes that provide evidence for pockets of magma already present near the top of Earth's mantle, an idea originally proposed in the 1960s.
Until now, this process had only been associated with relatively tiny volcanic structures.
"Our study suggests that Etna may have formed through a mechanism similar to the one that generates petit-spot submarine volcanoes," explains Sébastien Pilet, Professor at the Faculty of Geosciences and Environment at the University of Lausanne and lead author of the study. "This is unexpected, as such processes had previously only been observed in very small volcanic structures, typically rising no more than a few hundred meters. Mount Etna, by contrast, is a large stratovolcano, whose activity began around 500,000 years ago and which now towers more than 3,000 meters above sea level."
If the hypothesis is correct, it could expand scientists' understanding of how volcanoes form and encourage researchers to look for similar geological processes elsewhere in the world.
To investigate Mount Etna's history, the researchers analyzed rock samples spanning roughly 500,000 years of volcanic activity. By reconstructing the chemical evolution of Etna's lava and comparing it with experimental data, they found that the composition of the volcano's magma has remained remarkably stable despite changes in the surrounding tectonic environment.
Those results support the idea that the magma feeding Mount Etna already exists in the upper mantle and that the amount reaching the surface is largely controlled by the movement of tectonic plates. Together, the findings strengthen the case that Mount Etna's volcanism is driven by the same underlying process responsible for petit-spot volcanoes.
Materials provided by University of Lausanne. Note: Content may be edited for style and length.
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