Controversial Study Compares Mount Etna To A Gigantic Hot Spring

If you see lava erupting, then it's definitely a volcano. Wead/Shutterstock

Robin Andrews 16 Jan 2018, 16:38

Is everything we knew about the world wrong? Was the Black Death spread by fleas on humans, not rats? Is human memory a function of a virus-like infection 400 million years ago? Is Mount Etna not a proper volcano?

That last one, by the way, is courtesy of a controversial new Earth-Science Reviews paper, first spotted by New Scientist. Penned by the University of Catania’s Carmelo Ferlito, an associate professor of volcanology, it suggests that Etna is more like a hot spring, perhaps like those you’d find in Yellowstone National Park.

One major caveat at this point: the paper doesn’t appear to seriously suggest that Etna isn’t a real volcano, despite its title: Mount Etna volcano (Italy). Just a giant hot spring! It’s a somewhat provocative examination of Etna, one that’s somewhat designed to highlight how little we understand about how it works.

So what does it actually say?

Etna has plenty of eruption styles, including effusive lava flows, sudden water-magma blasts, and ash column-generating explosions. What Ferlito’s paper zeroes in on is that it’s incredibly gassy.

Gas bubbles exsolve from the magma as it rises and the confining pressure lessens. This gas is normally stored as dissolved volatiles – mostly water – in a shallow magma chamber, and there are typical gas-magma ratios that volcanologists expect to see, normally about 50-50, give or take.

Ferlito argues that Etna’s ratio is skewed. His calculations on pre-existing data suggest that “Mount Etna erupts 10 times the maximum H2O that could be dissolved in magma.” Simply put, he suspects there’s not enough magma there to contain that much dissolved gas.

From this, he suggests that the plumbing system beneath Etna is made up of 70 percent supercritical water, with some CO2 and sulfur compounds – where distinct gas and liquid phases don’t exist – and just 30 percent basaltic magma.

To put it crudely, Etna is an extremely burp-prone volcano, one with an “excess degassing problem”.

“The Etnean volcano works in a similar way to a gigantic hot spring, with huge amounts of ‘hot water’ and other gasses constantly rising through the lithosphere of eastern Sicily,” Ferlito added, explaining that this hot water is what transports magma up from the depths.

(He also compared Etna to a “dirty exhaust pipe”, adding that he meant no disrespect.)

It’s a fascinating explanation for Etna’s overly gassy nature, although a few volcanologists I’ve spoken to remain unconvinced. They cite a lack of geophysical and geochemical evidence to back up these claims.

Regardless of its validity, it doesn’t make Etna a non-volcano, though.

Definitions in volcanology can be somewhat debatable; even what’s considered to be an eruption is questionable. You can get a “phreatic eruption” – one in which pressurized steam blasts vapor and debris up into the air – but many would argue that’s not a real eruption.

For it to be real, you need a magmatic component to escape. In a phreatomagmatic eruption, for example, vapor, debris, and lava blebs are observed.

Each volcano is unique, and no two erupt the same. They all do one thing on occasion, though, and that’s erupt some lava or new volcanic products up to the surface. Hot springs – themselves not properly defined – don’t emit lava, even if they are (often) heated by shallow magma in the first place.

University College London’s Professor Bill McGuire, a geophysicist and geological hazards expert, agrees. Although the paper provides “interesting new information,” he told IFLScience that, regardless, “if it looks like a volcano, and behaves like a volcano, it is a volcano!”

“If it erupts molten rock then it is a volcano,” he added.

Ferlito told IFLScience that “the provocative effect must have really worked” for people to be interested in the paper. He said that the aim of the paper was to highlight Etna’s weird gas-magma ratio, and suggest an explanation for it.

“In order to account for those number I had to envisage a totally new concept of magma,” he explained, but acknowledges that more data is needed to make a working model of Etna based on this new paradigm.

“The comparison with the hot spring is mostly to point out the larger amount of water and other gas species [compared to the magma].”

So rest assured: Etna is still a “proper” volcano, albeit one we don’t have a proper understanding of just yet.

The slopes of Etna. Robin Andrews

 

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