Looking for "clock bombs" on the seabed.  Who are the detectives looking for underwater supervolcanoes

In January 2022, a huge eruption of a submarine volcano shook Tonga, in the Pacific, with disastrous consequences. Scientists are now trying to figure out where and when the next “supervolcano” will come to life, the BBC reports.

During the summer of 1883, a volcano in the Sunda Strait, located between the islands of Java and Sumatra, became increasingly turbulent, releasing huge amounts of ash and steam. Then, on August 26, the underwater volcano erupted, throwing hot ash and lava over nearby settlements. The eruption killed tens of thousands of people. Krakatoa remains one of the deadliest underwater eruptions in history.

Nearly a century and a half later, on January 15, 2022, another underwater giant awoke from its slumber, this time off the coast of Tonga. However, the eruption of the Hunga Tonga-Hunga Ha’apai volcano and the resulting tsunami were different. Volcanologists have been able to document the violent eruption of the underwater mountain in real time, and what they found was surprising.

The country was nearly isolated from the rest of the world after an underwater communications cable was blown up, but satellites captured hundreds of lightning bolts from the volcano’s ash clouds. Remote sensors recorded strong shock waves that reverberated around the globe for days. A column of ash has risen to unprecedented heights.

The Hunga Tonga eruption remains a humanitarian disaster for the nearly 100,000 people living in Tonga – and a warning to the whole world. This has led scientists to rethink their ideas about the dangers posed by numerous submarine volcanoes. Now they are trying to find these underwater mountains to protect the land and the ocean.

With increasingly sophisticated methods of detection, volcanologists hope to improve warning systems, determine the impact on the environment, mitigate the dangers posed by eruptions and help restore the ecosystem.

Volcanoes in the depths of the sea are much harder to locate than those at ground level. The truth is, we know more about the surface of the moon than we do about the bottom of the ocean. But the eruption of Hunga Tonga mobilized the scientific community and stressed the need for further exploration of this unexplored land. In April 2022, the New Zealand National Institute of Water and Atmosphere Research (Niwa) launched an ocean voyage to the site of the dramatic eruption in Tonga. Their ship, the RV Tangaroa, surveyed thousands of square kilometers of the seabed and collected images and physical samples, which are now being studied.

Because the region is so seismically active, Niwa is in a unique position to investigate the dramatic impact of the Hunga Tonga eruption.

“Before our trip, we only had information from small boats that had left the islands of Tonga,” says Mike Williams, a researcher at Niwa.

Underwater volcanoes often collapse under their own weight. When seawater mixes with magma, it can lead to an explosive collapse, which in turn can cause a tsunami and create harmful vapors on land.

Researchers aboard the Tangaroa had several immediate goals: mapping the site, collecting volcanic samples for chemical analysis, and analyzing the impact of the eruption on the seabed.

“I reached the volcano at dawn and saw the sun rise over two jagged peaks from which came fire and disaster,” says Kevin Mackay, a veteran volcanologist.

“Before the eruption, the volcano’s caldera was about 120 meters high. It’s a mile deep now. In addition, we found pyroclastic flows – turbulent, dense, violent rivers that flowed on the seabed – at least 60 kilometers away, “says Mackay.

Few submarine eruptions have been documented, as they are usually hidden under ocean water, miles deep. The ones that scientists have observed, however, can give us clues about future disasters. In 2018, for example, an underwater eruption off the French island of Mayotte gave rise to a colossal new submarine mountain, revealing a high level of seismicity in the region. Mayotte is now continuously monitored, and its work is regularly updated by a group of scientists on Revosima, a platform that monitors volcanic hazards, such as magma flow, water temperature and acidity, and seismicity.

Projects like Revosima are extremely important, but they are extremely expensive. Shipping can cost up to 50,000 euros a day, and securing a cable near a volcanic site, which allows data to be collected on the spot, can cost millions of euros.

But research is essential – not only to determine the dangers of active volcanoes, but also to develop a better understanding of their impact on the environment.

“People are interested in volcanoes because they pose a risk,” says Javier Escartin, a researcher at the geology lab at the Ecole Normale Supérieure in Paris.

“In general, deep volcanoes are not very dangerous; more dangerous are those close to the surface “, he explains.

There are about 1,500 potentially active volcanoes in the world, of which about 500 are known to have erupted, but these do not include ocean floor volcanoes – and there could be several hundred. Many of them are probably located along the Pacific, in the so-called Ring of Fire that surrounds the Pacific Ocean. The most dangerous are the volcanic islands inhabited by humans.

“Imagine a Tonga-scale explosion in the Mediterranean or Hawaii. Death and destruction, damage to the economy and transportation systems … but, of course, we can’t study volcanoes if we don’t know where they are, ”says Escartin.

How do volcanologists look for these as yet undiscovered submarine volcanoes? Hydroacoustic monitoring is one answer. When a volcano erupts underwater, it produces acoustic energy: when hot lava at 1,200 degrees Celsius interacts with seawater, which is close to freezing temperatures, it vaporizes, causing an explosion of sounds. Seismic energy is converted into underwater acoustic energy at the bottom of the sea.

At depths of about 1,000 meters, pressure, temperature and salinity combine to slow down the movement of sound through water, making it easier to transmit. This area is known as the Sound Fixing and Amplitude Channel, or Sofar (whales use the same acoustic channel to communicate with each other underwater). Underwater hydrophones or microphones can detect the acoustic signal from sound waves produced by a seismic energy conversion along the Sofar channel. These acoustic data provide clues as to the location of lava flows, a potential warning of imminent or ongoing eruptions.

Editor: MB

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