La Soufrière’s Pyroclastic Flows

La Soufrière's Volcanic Eruption on April 9th 2021 seen from Fort Charlotte with Mt. St. Andrew in the middle for perspective. (Stephan Hornsey)

La Soufrière‘s Volcanic Eruption on April 9th 2021 seen from Fort Charlotte with Mt. St. Andrew in the middle for perspective. (Stephan Hornsey)

Present La Soufrière Alert Level:ORANGEThere is a highly elevated level of seismic and/or fumarolic activity or other unusual activity. An eruption may begin with less than twenty-four hours’ notice.

Beginning April 11th, 2021, scientists in St. Vincent believe pyroclastic flows have begun across northern areas of the island, but what are they and how do they work?

Pyroclastic flows, which are extremely fast “moving masses of destruction” containing hot gas, ash, and rocks, and which kill and destroy everything in their path, even out into the sea. The lead scientist on the ground, Professor Richard Robertson, gave an update on the ongoing explosive phase of the eruption while on NBC St. Vincent and the Grenadines Radio on April 11th and 12th, 2021

Robertson informed that there had been evidence of the pyroclastic flows, also called pyroclastic density currents, revealing, “There is one on a video that we got access to that happened on the Leeward side,” on April 11th. At 4:15 AM April 12th, 2021, one of the largest eruptions to date occurred at the La Soufrière Volcano, accompanied by numerous pyroclastic flows on both sides of the islands.

From 6:00 PM on April 11th, 2021, Robertson said that is when they think the first of four pyroclastic density currents or pyroclastic flows occurred. He also added that because it was overnight, it was not very clear, but the pyroclastic flows were incandescent, so it glowed as it raced down the mountain. “We could see glows going down areas like Walliabou, upper parts of Rabacca, and we could see it going down valleys a little bit north of that.”

Robertson explained that the La Soufrière Volcano, after expelling copious amounts of ash, the volcano has moved into a new pattern where eruptions, in a sense, “has slowed down” calling the activity “sluggish.”

“When this new pattern started, the volcano had in a sense slowed down. It became a bit more sluggish, […] it becomes a bit more pressurized.” Robertson added, when an eruption occurs now, it breaks up the rock into larger pieces which causes the rock to fall to the ground in a process called column collapse. “Column collapse, not dome collapse is the column of air and gas, rocks and boulders, instead of going up in the air, it collapses on itself.

While the ash has been wreaking its own havoc throughout St. Vincent and throwing neighboring islands into darkness; the pyroclastic flows are in a league on their own.

Robertson explained, “When it collapses, it goes down the mountain and follows the valleys, but it is not confined to the valleys. It’s like a river but moving much faster than a river. Because it is hot because it has different sizes of rock with gas in it and is moving very fast, virtually there are very few structures in the world that could stand up to the forces of that material going down the mountainside. It just destroys everything. Trees and buildings get mashed up; things get bulldozed out of the way.”

“So it means that the amount of devastation you’re probably going to have, unfortunately, on the volcano itself, when this thing is stopped, we go back and see – it’s going to be significant. Not only because of the ash but because of these flows,” the scientist concluded.

However, column collapse isn’t the only way a pyroclastic flow can occur. According to Robertson, “The other way in which these would form, and we think this is how the earlier ones that happened, the material just foamed over. Sometimes when the volcano explodes, it doesn’t have enough energy to put everything into the air to allow it to collapse. Instead, it’s like a foam that comes over the sides of the mountain. We think the earlier ones, the weaker ones, based on satellite imagery from yesterday, these foaming kind of pyroclastic density currents (PDC’s) didn’t go very far going down the valleys.”

Coupled with this, the explosive phase has not yet shown signs of letting up. Scientists expect more pyroclastic flows through both column collapse and through “foaming.”

The pyroclastic flows are one of the most dangerous things that the volcano can do, and it is one of the things that killed people during the 1902 eruption. Robertson added on April 12th, the particularly energetic flows that occurred at 4:15 AM, “Essentially what all of that means is because of the strength of the explosion and the strength of the PDC’s, I expect quite a bit of the mountain now, the communities, the buildings and the structures that are on the mountain are destroyed, damaged or mashed up. I struggle to think of any living creatures that were on the mountain because any man, animal, anything, in those PDC’s or part of those PDC’s, and its a terrible thing,”

Volcanic Hazard Map for the La Soufrière Volcano, St. Vincent (UWI SRC/NEMO)
Volcanic Hazard Map for the La Soufrière Volcano, St. Vincent (UWI SRC/NEMO)

Official information will originate from St. Vincent and the Grenadines National Emergency Management Organization and the University of the West Indies Seismic Research Center.

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