The volcano that could devastate Florida isn’t in America – and scientists say its catastrophic collapse is a matter of when, not if.
The prospect of a 20m-high tsunami slamming into the United States’ East Coast sounds like the stuff of science fiction.
But it’s happened before.
And it’s going to happen again.
“The bottom line is that we just can’t say where the next volcanic tsunami will happen, nor how big it will be, but happen it will,” warns volcanologist Professor Bill McGuire.
He’s pointing to the risk posed by the Cumbre Vieja volcano on La Palma, one of the Canary Islands off the coast of Morocco.
“During the 1990s, my research team confirmed that this huge mass of rock – adding up to hundreds of cubic kilometres in volume – was creeping slowly seawards, at perhaps a centimetre or two a year,” he writes for Science Focus.
“I expect this part of the flank will eventually crash into the North Atlantic, launching a megatsunami in the process.”
A set of fresh studies support his fears.
In 2022, the small Hunga volcano in Tonga erupted. It was underwater. And its 4km-wide caldera – solidified magma cliffs – collapsed 1000m into the chamber below.
The resulting tsunami was unexpectedly severe.
Another underwater Kikai-Akahoya volcano erupted near Japan some 7300 years ago. It changed the archipelago’s landscape for decades to come.
And its tsunami was just one of the destructive forces unleashed.
Cumbre Vieja is different because it sits above sea level. But it is an island. And its unstable flanks are poised to slip into the Atlantic. That would cause a 1km-high splash of water. Waves hundreds of meters in height would radiate out into the broad ocean.
This would devastate the neighbouring islands. And, nine hours later, a 20m-tall wave could crash through the Bahamas and into Florida.
Canary in the caldera
“Huge boulders dumped on shore in the Bahamas are believed to have been carried there by ancient tsunamis sourced in the Canary Islands,” says Professor McGuire.
Experts dispute exactly how destructive such a wave would be.
“I have no doubt this will be a cataclysmic event, but have no idea when it will happen – it could easily be centuries or even millennia down the line,” he adds.
Rubble from at least five past volcanic tsunamis has been found in the Canary Islands up to 300m above sea level.
Most of these are believed to have been caused by enormous landslides.
“Major flank collapse is a normal part of the life cycles of large island volcanoes, and is certainly not going to stop happening just because we are around,” Prof McGuire warns.
Research shows similar coastal volcano eruptions and collapses have produced nine tsunamis within the past 300 years, killing at least 55,000 people.
The famous 1883 eruption of Krakatoa is the most devastating example.
The volcano sat in the Sunda Strait, between the islands of Java and Sumatra in Indonesia.
When it erupted, half of its 300m-high edifice collapsed into the sea, causing surges of water up to 45m.
The surrounding coastline was scoured clear of all life – including 33,000 people.
Its offspring is still there.
Anak Krakatoa erupted in 2018. That tsunami killed 400 and injured 15,000.
Prof McGuire says volcano collapses that generate 100m-tall megatsunamis average roughly one every 10,000 years.
The most recent example was Réunion Island in the Indian Ocean. That happened about 5000 years ago.
Unseen. Unexpected
About 7300 years ago, a natural disaster changed the prehistoric landscape of the Japanese archipelago.
“This eruption must have been devastating, exploding out of the seabed with little warning,” writes Lund University professor Peter Jordan.
“It battered the Japanese islands with powerful earthquakes and enormous tsunamis, and then blanketed the landscape with meters of volcanic ash.”
His international research team has just published a study that seeks to understand the fallout from the Kikai-Akahoya eruption. So it sought out evidence in layers of soil, bones, ruins and debris throughout the region.
They found evidence of a devastated environment.
Some regions became uninhabitable. The climate changed for decades to come. Survivors fled.
It wasn’t just the volcanic gases, lava and tsunamis.
Skulls from ancient deer and wild boar reveal teeth severely worn down by abrasive volcanic ash that collected on, and in, the plants they ate.
Human survivors migrated. Adapted. Changed.
New societies and economies grew out of the world-shattering event.
Auckland University of Technology researcher Marta Ribó published the results of a study on the 2022 Hunga volcano in the Pacific Tonga Islands last month.
It was a rare opportunity.
The volcano, some 200m beneath sea level, had been extensively surveyed in 2015-16.
That allowed researchers to compare the before-and-after of an eruption that sent an enormous column of vaporised seawater 55km high into the stratosphere.
The tsunami reached 45m high some 100km from its source.
It was a surprising impact.
Hunga volcano is relatively small. Its caldera was about 4km in diameter.
But the volcanic chamber itself was relatively deep. It plunged 1000m beneath the ocean surface.
When Hunga erupted, the entire caldera collapsed into the chamber at once.
That caused a suction surge loosely comparable to suddenly pushing a cupped hand down into a basin.
Researchers call this a “piston-style collapse”.
The Tonga-Kermadec range has 74 underwater volcanoes. About 20 of these have calderas.
Aftershock
Florida isn’t the only heavily populated US state at risk of volcanic-slip tsunamis.
The Hawaiian Islands are also subject to instability.
The Nu’uanu slide, off the north of O’ahu, is 230km wide. Some 3000 cubic kilometres shifted beneath the ocean. Just one piece is 20km long and 2km wide.
It let go more than a million years ago. Scientists have found tsunami deposits linked to the event as high as 400m above sea level.
Simulations project the wave impacting California would have remained up to 70m high after travelling five hours across the Eastern Pacific.
We can’t stop them.
But we can expect them.
Modern seismic and space-based surveys can reveal which calderas are unstable. They can measure their slippage in real time.
“Our study underscores the urgent need for high-resolution repeat sea floor mapping,” Ms Ribó says of the Tonga eruption. This would improve “tsunami early-warning systems and protect critical sea floor infrastructure, particularly in poorly surveyed regions such as the southwest Pacific”.
Enormous tsunamis and eruptions can be catastrophic.
But not world-ending.
“Even in the most impossible situations, survivors and communities have choices, however limited: they need to act and make decisions; they need to adjust, collaborate, and find new connections and social networks,” Professor Jordan concludes of his Kikai-Akahoya research.
Forewarned is forearmed.
“Just because a volcano isn’t erupting doesn’t mean a guarantee of safety,” Professor McGuire concludes.
“[They are] just tottering piles of rubble, rotted internally by seeping volcanic gases and looking for any excuse to collapse.”