Most volcanic activity occurs beneath the ocean – but not everyone knows that.
Typically, the volcanoes we can see attract the most attention. However, the powerful eruption of an underwater volcano in the Pacific island nation of Tonga in mid-January 2021 sparked curiosity about the volcanoes at 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.
Satellite image from SkySat shows smoke rising from the underwater volcano Hunga Tonga-Hunga Ha’apai in Tonga about a week before the eruption on January 15, 2022. (Photo: Planet Lab/Reuters).
The eruption of the underwater volcano in Tonga caused a tsunami that flooded areas in the capital, Nuku’alofa. However, generally, underwater eruptions tend to occur quite “quietly” and go unnoticed.
“Most volcanoes on our planet are actually underwater volcanoes, and this is not unusual. They tend to erupt very quietly rather than violently, so people often overlook them,” Helo stated.
According to Tamsin Mather, a volcanologist and Earth sciences expert at the University of Oxford, it is currently unclear exactly how many active underwater volcanoes there are, but estimates range from several hundred to several thousand.
How are underwater volcanoes formed?
“There is no specific difference in the formation of underwater volcanoes or seafloor volcanoes compared to land-based volcanoes,” Helo noted.
Volcanoes form when molten rock is generated in the second layer of the Earth’s core – the upper mantle – and breaches the crust.
“Most underwater volcanic activity is associated with volcanoes that continuously erupt along mid-ocean ridges, where two tectonic plates are pulling apart,” Mather explained.
The collision of two plates can also cause volcanic activity. According to Helo, if both tectonic plates are located beneath the ocean, the volcano will form underwater. Over time, they can develop into volcanic islands. Volcanic activity within a single tectonic plate can also lead to the formation of volcanoes. This can occur when a hotspot exists beneath an oceanic plate, creating a chain of volcanic islands similar to Hawaii.
What happens when an underwater volcano erupts?
The impact of an underwater volcano’s eruption depends on its proximity to the water’s surface.
“If the eruption occurs very deep underwater, the weight of the water above acts like a pressure cap,” said David Pyle, a volcanologist and Professor of Earth Sciences at the University of Oxford.
If molten rock flows into the ocean 2 kilometers (1.24 miles) below the surface, it will come into contact with cold seawater and cool down rapidly. The water will heat up, but it will not turn into steam.
However, in shallow water, magma begins to heat the water, which then turns into steam. This creates a significant change in volume.
“Steam explosions can be truly devastating because even a small volume of water can turn into a massive volume of steam,” Pyle stated.
Besides the risk of tsunamis, the volume of ash released into the atmosphere when underwater volcanoes erupt in shallow waters can severely impact human health.
According to Pyle, ash and gases released not only pollute the air but can also affect access to power and water supplies.
Studying underwater volcanoes is quite challenging
In reality, because underwater volcanoes are submerged, they are very difficult to study.
“Only a few active areas have been studied in detail due to the inaccessibility of underwater volcanoes,” Mather noted.
Scientists working on land can study the history of a volcano by directly visiting the volcanic area and collecting data. They can use rock formations or drill deep holes to gather samples. In contrast, for underwater volcanoes, scientists often rely on maritime surveys and mapping technologies like sonar.
“Underwater volcanoes are like a cake with complex layers. If it were above water, you could slice it to see the inside, but if it’s submerged, there’s really nothing you can do,” Pyle explained.
