What Are Underwater Volcanoes?
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Underwater volcanoes, also called submarine volcanoes, are volcanoes that erupt on the seafloor. They are far more common than land volcanoes and produce most of Earth's volcanic activity. The Mid-Atlantic Ridge alone has thousands of underwater volcanoes that continuously produce new ocean floor as tectonic plates spread apart.
Most volcanic activity on Earth happens underwater, far from human view. Underwater volcanoes form along mid-ocean ridges where tectonic plates pull apart, around volcanic island chains, and on isolated seamounts. The water above them changes how they erupt: pressure prevents explosive eruptions, and the water rapidly cools and shapes the lava. Despite their abundance, underwater volcanoes are less famous than their land counterparts because we rarely see them.
Where are underwater volcanoes found?
Most underwater volcanoes occur along the mid-ocean ridges, the massive underwater mountain chains that form where tectonic plates are pulling apart. The Mid-Atlantic Ridge alone runs about 10,000 miles from the Arctic to Antarctica. Other major underwater volcanic systems include the East Pacific Rise, the Mid-Indian Ridge, and the Pacific Ring of Fire's deep-sea trenches and arcs. Isolated seamounts (underwater mountains) occur throughout all ocean basins, with many being extinct volcanoes that never reached the surface to become islands.
How do underwater eruptions differ from land ones?
Underwater eruptions behave very differently from land ones because of the water pressure and rapid cooling. The high pressure at deep depths prevents the explosive expansion of gases that drives violent land eruptions. Instead, the lava extrudes more gently, often forming pillow lava (rounded shapes that look like piles of pillows) as the surface cools rapidly while the interior stays molten. At shallow depths (less than about 500 feet), eruptions can still be explosive enough to break the surface, throwing steam and rock into the air. The 2022 Hunga Tonga eruption produced the largest atmospheric explosion ever recorded.
Can underwater volcanoes become islands?
Yes, underwater volcanoes can grow tall enough to reach the surface and become islands. The Hawaiian Islands all started as underwater volcanoes that eventually grew above sea level through accumulated eruptions. Surtsey, off Iceland, emerged from the sea in 1963 during a multi-year eruption and is now a permanent island. The Galápagos, Canary Islands, Reunion, and many other volcanic islands formed through this same process. Some underwater volcanoes briefly appear as islands and then erode away or sink back below sea level after they become inactive.
How are underwater volcanoes studied?
Studying underwater volcanoes is difficult because of the depths involved and the harsh conditions near active eruptions. Scientists use sonar mapping to identify volcanic features on the seafloor, remotely operated vehicles (ROVs) and submarines to take close-up observations, and seafloor seismometers to detect underwater earthquakes that signal magma movement. Hydrophones can pick up the sounds of underwater eruptions. Surface ships sometimes detect underwater eruptions by the discoloration, gas bubbles, or floating pumice they produce at the surface. Despite advances, most underwater volcanoes remain understudied compared to land volcanoes.
Underwater volcanoes are the most common type of volcano on Earth, with the vast majority occurring along mid-ocean ridges. The water above them prevents explosive eruptions at depth, instead producing pillow lava and gentle extrusion. Some grow tall enough to become islands, while others remain hidden seamounts. They produce most of Earth's volcanic activity, though they receive far less attention than land volcanoes.
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