Tonga eruption was a ‘record atmospheric explosion’

Tonga eruption was a 'record atmospheric explosion'
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The eruption of the Tonga volcano in January was confirmed as the largest explosion ever recorded in the atmosphere by modern instruments.

It was far larger than any volcanic event of the 20th century, or even any atomic bomb test conducted after World War II.

The assessment comes in a pair of scientific papers in the journal Science that reviewed all the data.

From recent history, it is likely that only the Krakatoa eruption of 1883 rivaled the atmospheric disturbance produced.

This catastrophic event in Indonesia is said to have caused more than 30,000 deaths. Fortunately, the January 15 climatic eruption of the Hunga Tonga-Hunga Ha’apai (HTHH) submarine volcano in the South Pacific resulted in very few fatalities, although it also caused large tsunamis.

“Tonga was a truly global event, just like Krakatau, but now we have all these geophysical observing systems and they recorded something that was really unprecedented in modern data,” said Dr Robin Matoza, from the ‘University of California, Santa Barbara. , told BBC News. He is the main author of one of the papers.

Scientists now have access to an extraordinary range of ground and space-based instruments, including atmospheric pressure sensors, seismometers, hydrophones and a fleet of satellites that monitor Earth across the entire light spectrum.

The colossal Tonga explosion, which occurred after several weeks of activity on the seamount, produced several types of atmospheric pressure waves that traveled great distances.

In the audible frequency range, people 10,000 km away in Alaska reported hearing repeated booms.

The global network of detectors set up to monitor compliance with the Comprehensive Nuclear Test Ban Treaty picked up the infrasonic signal. Infrasound has frequencies that are just below what humans are able to hear.

Network data indicated that the explosion of the Tonga volcano produced an atmospheric pressure wave comparable to that of the largest nuclear explosion of all time – the Tsar Bomb detonated by the Soviets in 1961 – but lasted four times longer. long time.

Graph showing the extent of the ash cloud.

The articles discuss in detail the disturbances caused by so-called Lamb waves, named after the early 20th century mathematician Horace Lamb.

They are energetic waves in the air that propagate at the speed of sound, along a trajectory guided by the surface of the planet. They are also non-dispersive, i.e. they retain their shape when moving and are therefore visible for a long time.

The Lamb wave pulses produced by the Tonga eruption circled the Earth at least four times.

In the UK, about 16,500 km from Tonga, these pulses started arriving on the evening of the 15th, about 14 hours after the climatic eruption on the other side of the planet.

They lifted the clouds over the UK.

“At the time, we had a cloud-based laser recorder that looked at the base of the clouds and when the wave passed through the cloud, it was perturbed,” recalls physicist Professor Giles Harrison of the atmosphere at the University of Reading and co-author of one of the papers.

“If you ever wanted to prove that the atmosphere is a remarkably interconnected thing, this was it. And what happens on one side of the planet can spread to the other side at the speed of sound.”

Where Lamb’s waves coupled with ocean waves, they were able to generate a tsunami – not only in the Pacific Ocean, but also in the Atlantic Ocean and the Mediterranean Sea.

Scientists are still studying the generation of near-field tsunamis that traveled the coasts of the Tonga archipelago. Some were undoubtedly created by pressure waves from the volcano pushing on the surface of the water, but investigations are ongoing to determine if the collapse of part of the volcano also made a significant contribution.

This will be evident from the seabed mapping projects which must report their results in the coming weeks.

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