Why does the Earth have so many underwater volcanoes?

Not only on land, volcanoes are found scattered on the deep ocean floor. According to estimates, the world has about 1 - 3 million underwater volcanoes.

Volcanoes are not evenly distributed across the Earth, but are often located 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 raises the question not "why are there so many underwater volcanoes?" but "why are there so many underwater plate edges?" Photo: NSF and NOAA via Flickr (CC BY 2.0).

First, the process of subduction (in which one tectonic plate slides under another as they collide) is responsible for much of the volcanic activity in the Pacific Ring of Fire—which requires the presence of water to soften the mantle enough to accommodate the subducting plate. Photo: NOAA/NSF/WHOI.

Second, the Wilson cycle explains how supercontinents are created and broken up by tectonic activity, when two plates separate, they form a large basin that leads to the appearance of an ocean, even if they were originally connected to a large land mass. Photo: Alexis ROSENFELD – UNESCO – @1ocean_exploration.

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

Volcanoes under the ocean look very different from volcanoes on land. Specifically, volcanoes on land look like large mountains with red-hot lava and explosive activity like Mount Etna or Mount Rainier or less steep like volcanoes in Hawaii or Iceland. Photo: WHOI.

But at the bottom of the ocean, where temperatures typically reach just 4 degrees Celsius, an underwater volcanic eruption is very different. According to the Smithsonian Institution’s Ocean Center, most scientists don’t understand how underwater volcanoes work because the eruptions are hidden from view under thousands of meters of water. Photo: ARoxoPT/Shutterstock.

When the West Mata volcano, whose base lies 3km below the Pacific Ocean near Fiji, erupted, a bright plume of hot magma was blown into the water before settling on the seafloor. The eruption released ash and rock into the water, with molten lava glowing below. Photo: Rebecca Carey, University of Tasmania/Adam Soule, WHOI.

Many underwater volcanoes, however, are not as violent. Sometimes, only bubbles appear on the ocean’s surface, but underwater, the magma is still under the pressure of tons of ocean water as it sinks to the seafloor. Photo: Rebecca Carey, University of Tasmania/Adam Soule, WHOI.

That means the lava solidifies into different shapes than it does on land. Because there is so much water pressing down and cooling it, the lava from an underwater volcano can’t explode everywhere like it does in the air, but instead freezes rapidly into volcanic glass or pillow lava. Photo: oregonstate.

According to the Smithsonian Institution, below about 7,000 feet, where the pressure is too great for water to boil, when water comes into contact with magma that is 1,600 degrees Fahrenheit, it immediately vaporizes. The rapid expansion into steam can be powerful enough to rupture the lava. Conversely, when magma comes into contact with water, the temperature change is so sudden that the magma instantly solidifies in a process called quenching. Photo: The Daily Galaxy --Great Discoveries Channel.

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

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