Detectives in search of underwater volcanoes

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During the summer of 1883, a caldera in the Sunda Strait, located between the islands of Java and Sumatra, became increasingly turbulent, releasing huge plumes of ash and steam into the sky. Then, on August 26, an undersea volcano ejected about 25 km3 (six cubic miles) of debris, hurling pumice ash and boiling lava flows through nearby settlements. The eruption killed tens of thousands of people. Krakatoa remains one of the deadliest underwater eruptions in history.

Almost a century and a half later, on January 15, 2022, another underwater giant woke up from its slumber, this time off the coast of Tonga. The Hunga Tonga-Hunga Ha’apai eruption and resulting tsunami were different, however. Volcanologists were able to document the violent release of the undereating mountain in real time, and what they found baffled their expectations.

The South Pacific nation was virtually cut off from the rest of the world after an undersea communications cable was severed by the blast, but satellites captured hundreds of lightning discharges from ash clouds in the volcano. Remote sensors recorded powerful shock waves rippling through the world for days. A column of ash rose to heights never seen before, lingering in the far reaches of the planet’s atmosphere.

The Hunga Tonga eruption remains a humanitarian disaster for the nearly 100,000 people who live in Tonga – and a story of mystery and caution for the world. This has prompted scientists to rethink their ideas about the dangers posed by the many undersea volcanoes lurking beneath the oceans. Now the hunt is on to find these seamounts to protect the land and the ocean.

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With increasingly sophisticated detection methods, volcanologists hope to improve early warning systems, determine environmental impact, mitigate the risks posed by eruptions and help restore ecosystems. Who are the people trying to find where the next underwater volcano is hiding? And where do they look next?

Volcanoes in the open sea are much more difficult to locate than those at ground level; indeed, we know more about the surface of the Moon than about the bottom of the ocean. But the Hunga Tonga eruption has galvanized the scientific community and underscored the need for further exploration of this uncharted realm. In April 2022, New Zealand’s National Institute for Water and Atmospheric Research (Niwa) launched an ocean voyage to the site of the dramatic Tonga eruption. Their vessel, the RV Tangaroa, surveyed thousands of square miles of seabed and collected video footage and physical samples, which are currently being studied ashore.

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