How does an underwater volcano form?

Most volcanic activity happens beneath the ocean — but we often don't know about it. DW looks at how underwater volcanoes form and what happens when they erupt.

It's usually the volcanoes we can see that get our attention. But a violent underwater volcanic eruption on the Pacific island of Tonga in mid-January has people looking at the volcanoes beneath the ocean.

"Two-thirds of all volcanic activity happens in the deep sea," said 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 generally, these underwater explosions come and go without much fanfare.

"Most volcanoes on our planet are indeed underwater volcanoes — this is nothing peculiar. They just erupt very quietly (not explosively) so nobody takes notice," Helo told DW.

The exact number of active underwater 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.

How do underwater volcanoes form?

"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 in the second layer of the Earth's interior — the mostly solid upper mantle — and makes its way through the crust.

"Most submarine volcanism is associated with the continuously active volcanism along midocean ridges, where two tectonic plates are pulling apart," said Mather.

The collision of two plates can also cause a volcano. If both tectonic plates are beneath the ocean, then the volcano will develop underwater, said Helo. Over time, they may grow to form volcanic islands, he added. Volcanic activity within a single tectonic plate can also result in the formation of a volcano. This can happen when there is a hotspot underneath an oceanic plate, creating a chain of volcanic islands like Hawaii.

What happens when an underwater volcano erupts?

The impact of an underwater volcano eruption depends on its proximity to the water's surface.

"If the eruption happens at very great depths underwater, then the weight of the overlying water acts as a pressure cap," said David Pyle, a volcanologist