Hotspot volcanoes

How are underwater volcanoes formed?


It’s usually the volcanoes that we can see that grab our attention. But a violent underwater volcanic eruption on the Pacific island of Tonga in mid-January has people staring at volcanoes under the ocean. Two-thirds of all volcanic activity occurs on the high seas,” says Christoph Helo, a volcanologist at the University of Mainz in Germany. The underwater eruption in Tonga caused a tsunami that flooded parts of the country’s capital. But usually these underwater explosions come and go without much fanfare.

“Most of the volcanoes on our planet are indeed submarine volcanoes – it’s nothing special. They just erupt very quietly (not explosively) so that no one notices,” Helo told DW The exact number of active submarine or submarine volcanoes is not known, but estimates range from hundreds to thousands, said Tamsin Mather, a volcanologist and professor of earth sciences at the University of Oxford. “There is no specific difference in the formation of submarine (underwater) and subaerial (on land) volcanoes,” Helo told DW Volcanoes form when molten rock is produced into the second layer of the Earth’s interior – the mostly solid upper mantle – and work its way through the crust.

“Most submarine volcanism is associated with continuously active volcanism along mid-ocean ridges, where two tectonic plates are pulling apart,” Mather said. The collision of two plates can also cause a volcano. If both tectonic plates are under the ocean, then the volcano will grow underwater, Helo said. Over time, they can expand to form volcanic islands, he added. Volcanic activity within a single tectonic plate can also lead to the formation of a volcano. This can happen when there is a hot spot under an oceanic plate, creating a chain of volcanic islands like Hawaii.

The impact of an underwater volcanic eruption depends on its proximity to the surface of the water. If the eruption occurs at very great depths under water, the weight of the overlying water acts as a pressure cap,” said David Pyle, volcanologist and professor of Earth sciences at the University of Oxford. If a piece of molten rock enters the sea two kilometers (1.24 miles) below the surface, it will come into contact with cold sea water and cool very quickly. The water will become very hot, but it will not turn into steam. But if the water is shallow enough, the magma begins to heat the water, which is then converted to steam. This creates a large change in volume. Steam explosions are really destructive because a small volume of water turns into a huge volume of steam,” Pyle told DW. In addition to the tsunami risk, the mass of ash ejected into the air during the eruption of an underwater volcano in shallow water can have serious consequences for the health of populations. Falling ash and emitted gases not only pollute the air but can affect access to electricity and water, Pyle said.

Unsurprisingly, the fact that they are underwater makes underwater volcanoes difficult to study. Only a few active sites have been studied in detail due to their inherent inaccessibility,” Mather told DW. Scientists working on land can learn more about a volcano’s history by visiting the volcano site and collecting data. This can be done using cliff sequences or by digging holes and collecting materials. For submarine volcanoes, scientists usually have to rely on marine surveys and mapping technologies like sonar. It’s like a very complicated layered cake,” says Pyle, “someone will bake this pretty complicated cake, then punch it and put a hole in it, and if it’s above water, you can just go watch it, and if they dropped it in the bath, then you’re really stuck.