Why does Earth have so many underwater volcanoes?

Volcanoes are found not only on land, but also scattered across the deep ocean floor. It is estimated that there are between 1 and 3 million underwater volcanoes worldwide.

Volcanoes are not evenly distributed across the Earth but are often found along tectonic fault lines. For example, three out of four volcanoes can be found along the Pacific Ring of Fire, with 10% of the world's volcanic activity occurring in Japan. This leads many to ask not "why are there so many underwater volcanoes?" but "why are there so many underwater tectonic plate edges?". Photo: NSF and NOAA via Flickr (CC BY 2.0).

First, subduction (in which one tectonic plate slides beneath another as they collide) is responsible for much of the volcanic activity in the Pacific Ring of Fire – requiring the presence of water to soften the mantle sufficiently to accommodate the subducting plate. Image: NOAA / NSF / WHOI.

Secondly, the Wilson cycle explains how supercontinents are created and broken apart by tectonic activity: when two plates separate, they form a large basin leading to the emergence of oceans, even if they were initially connected to a large landmass. Photo: Alexis ROSENFELD – UNESCO – @1ocean_exploration.

Essentially, most tectonic plates meet underwater. This is because, in fact, it is very difficult for two large continental plates to break apart without creating an ocean in between. Where tectonic plates move, volcanoes are often found, even if they are located thousands of kilometers below the sea surface. Photo: Alexis ROSENFELD – UNESCO – @1ocean_exploration.

Underwater volcanoes look very different from volcanoes on land. Specifically, land volcanoes resemble large mountains with glowing red lava and erupt when active, like Mount Etna or Rainier, or are less steep, like the volcanoes in Hawaii or Iceland. Photo: WHOI.

However, at the bottom of the ocean, where temperatures often only reach around 4 degrees Celsius, an underwater volcanic eruption is quite different. According to the Smithsonian Institution's Ocean Centre, most scientists don't fully understand underwater volcanic activity because eruptions are obscured from view thousands of meters underwater. Photo: ARoxoPT/Shutterstock.

When the West Mata volcano, whose base lies 3 km below the Pacific Ocean near Fiji, erupts, a bright streak of hot magma is blown into the water before settling to the seabed. The eruption releases ash and rocks into the water with glowing molten lava below. Photo: Rebecca Carey, University of Tasmania/Adam Soule, WHOI.

However, many other underwater volcanoes are not as violent. Sometimes, only bubbles rise to the ocean surface, but underwater, magma is still subjected to the pressure of tons of ocean water as it sinks to the seabed. Photo: Rebecca Carey, University of Tasmania/Adam Soule, WHOI.

This means the lava will solidify into many different shapes compared to on land. Because there is so much water to compress and cool it, the lava from underwater volcanoes cannot erupt everywhere as it would in the air, but instead freezes quickly into volcanic glass or cushion lava. Photo: oregonstate.

According to the Smithsonian Institution, at depths of approximately 2,200 meters, where the pressure is too great for water to boil, when water comes into contact with hot magma at 800 degrees Celsius, it evaporates instantly. This rapid expansion into steam can be powerful enough to break up the lava. Conversely, when magma comes into contact with water, the abrupt temperature change causes the magma to solidify immediately in a process called quenching. (Image: The Daily Galaxy -- Great Discoveries Channel)

Readers are invited to watch the video: Behind the success of scientists. Source: VTV24.

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