Interesting facts about underground volcanoes on Earth

Most volcanic activity happens beneath the ocean – but not everyone knows it.

Usually it's the volcanoes that we can see that attract attention. However, the powerful eruption of a submarine volcano in the Pacific island nation of Tonga in mid-January 2021 has made many people curious about the volcanoes on the ocean floor.

"Two-thirds of all volcanic activity occurs in the deep sea," said Christoph Helo, a volcanologist at the University of Mainz in Germany.

 SkySat satellite image shows smoke rising from the underground volcano Hunga Tonga-Hunga Ha'apai in Tonga about a week before the eruption on January 15, 2022.

The eruption of an underground volcano in Tonga caused a tsunami that flooded areas in the capital Nuku'alofa. However, in general, undersea eruptions usually happen quite 'quietly' and few people notice.

'Most of the volcanoes on our planet are actually subterranean volcanoes and this is not a strange thing. They usually erupt very quietly, not violently, so people often don't notice," Helo said.

According to Tamsin Mather, a volcanologist and expert in earth sciences at the University of Oxford, the exact number of active underground volcanoes is unknown, but estimates range from a few hundred to several thousand.

How do underground volcanoes form?

'There is no specific difference in the formation of subterranean or undersea volcanoes versus surface volcanoes,' Helo said.

Volcanoes form when molten rock is pulverized in the second layer of the Earth's core - the upper mantle - and passes through the crust.

"Most subterranean volcanism is related to continuously active volcanism along mid-ocean ridges where two tectonic plates are separating," said Mather.

The collision of two plates can also cause volcanoes. According to Helo, if both tectonic plates are located below the ocean, then the volcano will form underwater. Over time, they can grow to form volcanic islands. Volcanic activity within a single tectonic plate can also lead to volcanism. This can happen when there is a hot spot beneath an oceanic plate, creating a chain of volcanic islands much like Hawaii.

What happens when an underground volcano erupts?

The impact of an eruption of an underground volcano depends on how close it is to the surface of the water.

"If the eruption happens very deep underwater, the weight of the water above acts as a pressure cap," said David Pyle, a volcanologist and professor of Earth sciences. at the University of Oxford, said.

If molten rock flows into the sea 2 km (1.24 mi) above the surface, it will come into contact with cold seawater and cool very quickly. The water will be heated, but not converted to steam.

But if in shallow water, the magma starts to heat the water, which is then converted to steam. This creates a huge change in volume.

'Steam explosions are really destructive because even a small amount of water turns into a huge mass of steam,' Mr Pyle said.

In addition to the risk of a tsunami, the mass of ash that rises into the air when an underwater volcano erupts in shallow water can have serious effects on human health.

According to Pyle, ash and gas emissions not only pollute the air but can also affect access to electricity and water supplies.

It is very difficult to study underground volcanoes

In fact, because underground volcanoes are all submerged, it is difficult to study.

'Only a few active areas have been studied in detail due to the inaccessible nature of underground volcanoes,' he said.

Scientists working on the ground can study the history of a volcano by going directly to the volcanic site and collecting data. They may use cliff chains or dig deep holes to collect specimens. As for underground volcanoes, scientists often have to rely on maritime surveys and cartographic technology such as sonar.

'Subterranean volcanoes are like cakes with intricate layers. If it's above water, you can slice it to see what's inside, but if it's submerged there's really nothing you can do about it," Pyle said.