The claim: Experts predict La Palma eruption to create tsunami that would reach the U.S.
A decades-old theory has resurfaced online after a volcanic eruption on a Spanish island that caught the world's attention.
On the island of La Palma, in the Canary Islands, a volcano erupted in the afternoon of Sept. 19 after a weeklong buildup of seismic activity.
Days later, the lava continued to flow through the four mouths of the volcano toward the island's western coast. By Sept. 20 it had already forced the evacuation and relocation of over 5,000 residents.
As the latest developments are shared online, many social media users are also sharing a longtime theory that the eruption could cause a tsunami that would reach the East Coast of the United States.
"The things I've read they are expecting it to erupt and a piece of land would slide into the ocean and it would cause a tsunami for the USA east coast!" the post says. "They have been predicting this for years."
The accompanying image is a map of the eastern part of the United States. Text laid over it claims that a "50 meter tsunami after Canary Island volcano eruption and landslide puts THIS under water." Florida, New York, Washington, D.C., Boston, and parts of southern Texas and Louisiana are among the areas that would be purportedly affected by the tsunami, according to the image.
The image has spread widely on social media. Another version, posted on Facebook on Sept. 19, accrued more than 800 shares in two days before it was deleted.
Nick Knowles, an English television presenter, also shared a version of the claim on Twitter, where he has almost 160,000 followers.
While the theory being shared exists, social media users are presenting it as if it is a likely scenario or represents an expert consensus of some kind.
In reality, it's a hypothesis that both American and Spanish officials have debunked, saying the conditions required for a tsunami big enough to submerge part of the country's coasts are extremely unlikely to happen.
USA TODAY reached out to the users who shared the image for comment.
Volcanic eruption not big enough to create tsunami
As of Sept. 21, two days after the initial spew of lava, the Cumbre Vieja volcanic eruption had covered around 254 acres and destroyed 166 buildings, according to the Canary Islands Volcanology Institute, which has been sharing news and analysis on the eruption.
The chances of acid rain and toxic clouds remain low, though the volcanic ash in the air could pose a risk to residents of the island.
And there's a vastly lower chance of a landslide causing a tsunami big enough to reach the coast of the United States.
The theory has been around since 2001, when two college professors – Steven Ward from University of California, Santa Cruz and Simon Day from University College in London – published a study about the possibility of a tsunami originating in the Canary Islands later reaching American coasts and other parts of the world.
The four-page paper said that "during a future" eruption of the Cumbre Vieja volcano, a landslide of between 150 and 500 cubic kilometers, which could trigger a tsunami with waves between 10 and 25 meters high hitting North America around nine hours after the hypothesized volcanic eruption.
Spanish and American officials have pushed back on the theory, however.
After announcing the eruption on Twitter on Sept. 19, the U.S. Geological Survey said the tsunami threat remained local, debunking users' claims that a purported "mega-tsunami" would happen.
That same day, the U.S. National Tsunami Warning Center posted on Facebook that the eruption did not pose any tsunami risks to the East Coast.
The islands' volcanology institute has assured that extreme conditions would have to occur for the theory to become a reality.
For instance, the volcano would have to grow by 1,000 meters over its current height, which the institute told Spanish national TV station Antena 3 would take another 40,000 years.
Another way a "mega-tsunami" could occur is if an earthquake of "exceptionally high magnitude" and a high magnitude volcanic eruption were to take place at the same time. Those two phenomena together could lead to a landslide of the Cumbre Vieja flank, which is what Ward and Day's theory hypothesized.
None of these conditions have occurred, however.
'Mega-tsunami' theory resurfaces with seismic activity in La Palma's
The "mega-tsunami" theory often resurfaces with news of a volcanic eruption or seismic activity in the Canary Islands.
In 2016, the Daily Star, categorized by Media Bias Fact Check as a "questionable source" of news based on its "routine publication of conspiracy theories," published an article on the theory. In response, geographer and earth scientist Dave Petley debunked the theory on his blog, "The Landslide Blog."
Petley, whose research at the University of Sheffield in the United Kingdom focuses on landslides , said no previous volcanic flank collapse in the world has led to a tsunami of the magnitude Day and Ward describe. Additionally, he wrote, submarine deposits from volcanic flank collapses suggest they don't happen as one big collapse, like Day and Ward hypothesize.
Rather, they happen in a series of slides, which would make the resulting tsunami "much less significant."
"It really is time that this event was presented for what it is, which is an absolutely extreme scenario based on a very highly unlikely combination of events that is without precedent," Petley wrote.
While the last volcanic eruption in La Palma happened 30 years before Ward and Day's study was published in 2001, the island is of volcanic nature, and seismic activity is often considered normal.
In the last two decades, there has been extensive seismic activity similar to the small earthquakes seen prior to the eruption, according to Spain's National Geographic Institute. This activity was within normal parameters for the area and didn't "present any risks to the residents." Many of the earthquakes registered aren't even felt by residents, as they happen kilometers under the surface.
In October 2017, after more than 100 seismic events were registered within one week in La Palma, the islands' volcanology institute debunked the theory – again.
"Although it's been more than 10 years since this theory, the hypothesis that gave way (to the idea) of a collapse of the Cumbre Vieja is still being considered," the institute said, according to the Spanish online newspaper El Diario.
The institute said the probability of a highly explosive eruption happening at the same time as a large earthquake – which is required for the theory to be true – is "extremely remote, according to geological records of this type of event on the island."
"The Cumbre Vieja is stable even under the effects of eruptions similar to those that have occurred in the last tens of thousands of years," it said.
Our rating: False
Based on our research, we rate FALSE the claim that experts have predicted the eruption in La Palma will cause a tsunami that would reach the U.S. Spanish and American officials have said the volcanic eruption doesn't present any risks to the United States. Experts have thoroughly debunked it, saying the hypothesis is based on unprecedented events and assumptions of a massive landslide. Experts said such a landslide is extremely unlikely.
Our fact-check sources:
The Guardian, Sept. 20, Canary Islands: 5,000 evacuated as La Palma volcano eruptions continue
Nick Knowles, Sept. 21, Tweet (archived)
Involcan, Sept. 21, Tweet
University of California, June 27, 2001, Cumbre Vieja Volcano -- Potential collapse and tsunami at La Palma, Canary Islands
Spain National Geographic Institute, accessed Sept. 22, "New seismic activity localized in the south of the island of La Palma"
USGS, Sept. 19, Tweet
National Tsunami Warning Center, Sept. 19, Facebook post
Antena 3, Sept. 20, Is there a risk of a tsunami after the volcanic eruption in La Palma?
Media Bias Fact Check, Feb. 7, 2020, Daily Star UK
Daily Star, July 20, 2019, MEGA-TSUNAMI hell feared as killer wave could 'hit UK at ANYTIME'
University of Sheffield, accessed Sept. 22, Professor Dave Petley
The Landslide Blog, Sept. 20, 2016, A science story that just won't die: the Canary Island Megatsunami scare rears its head once more
El País, Sept. 19, Teneguía, 1971: This was the last terrestrial volcanic eruption in Spain
Spain National Geographic Institute, accessed Sept. 22, "Earthquake Swarm in the island of La Palma (09-10-2017 and 14-10-2017)"
El Diario, Oct. 19, 2017, The Involcan assures the volcanic building of Cumbre Vieja is 'stable' and it's landslide is very remote
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This article originally appeared on USA TODAY: Fact check: Volcanic eruption in La Palma unlikely to create tsunami