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Volcanic eruptions are among nature’s most forceful events. While iconic eruptions such as Mount Vesuvius, Krakatoa, and Mount St. Helens capture our imagination, the majority—about 80 %—take place beneath the ocean’s surface.
For most of human history, the workings of submerged volcanoes remained a mystery because they lie thousands of feet below sea level. Today, advances in sonar mapping, deep‑sea submersibles, and autonomous underwater vehicles (AUVs) allow scientists to locate, monitor, and even film these eruptions in real time.
In all volcanoes, magma rises until it breaks a vent, a process known as an explosion. Underwater, however, the surrounding water exerts more than 100 times the pressure of a volcano at sea level. This hydrostatic pressure suppresses the eruption’s upward force, turning it into a fissure rather than a classic lava flow.
Cold, high‑pressure water cools the molten rock almost instantly, a process called quenching. As a result, the lava solidifies quickly, producing hard rock rather than the flowing lava seen on land. If the vent lies close to the surface, the collision of molten rock with water vaporizes the water and produces a plume that can rise into the atmosphere—a phenomenon known as a volcanic‑efflorescence or fissile plume.
In many cases, underwater volcanoes emit streams of hot water and steam—called hydrothermal vents—without breaking the surface. These vents can create a subtle, steaming glow that signals hidden activity beneath the waves.
Volcanoes form at tectonic plate boundaries, where the movement of Earth’s crust creates the energy that drives magma. In the Pacific “Ring of Fire,” over 90 % of the world’s earthquakes and 75 % of its volcanoes are located along these boundaries.
Magmatic activity can trigger earthquakes, which in turn can trigger eruptions. The relationship is so tight that scientists often forecast eruptions by monitoring seismic swarms. For instance, a pair of magnitude 5.9 and 6.0 quakes struck Antarctica in 2020 after the dormant Mount Erebus re‑awakened.
When an underwater eruption displaces enough water to generate a tsunami, the resulting wave can devastate coastal communities. A 2019 study in Scientific Reports found that tsunamis caused 20 % of all deaths linked to volcanic eruptions over the last four centuries.
While many eruptions are silent, some create new land. The process begins when a volcanic vent builds a “cone” that gradually rises above the sea floor, forming a seamount. Over millions of years, continuous lava flows can push the structure above the waterline, giving rise to islands such as Hawai‘i, Samoa, and Iceland.
When a vent erupts near the surface, it can expel ash, rocks, and organic material—collectively called anastomosed reef—into the atmosphere. This “mushroom” of life forms new ecosystems and can seed entire biomes.
However, without sustained volcanic activity, the newly formed islands can erode and sink back beneath the waves, a process called subaerial denudation.
Underwater eruptions can be deadly, destroying fish and other marine organisms instantly. Yet the same vents provide a unique environmental niche that supports diverse communities. Hydrothermal vents, for instance, are rich in minerals and gases—so‑called chemosynthetic ecosystems—that can host species found nowhere else.
Research suggests that hydrothermal vents may have been the cradle of life on Earth. The extreme conditions—high temperatures, low pH, and abundant chemical energy—provide the perfect template for the earliest life forms.
Scientists locate volcanoes by measuring seismic waves on the ocean floor and by mapping water‑pressure changes with bottom‑pressure recorders. These tools reveal subtle rises in the seafloor that indicate hidden volcanic structures.
A landmark achievement came in 2009, when the first live video footage of an underwater eruption was captured at West Mata in the South Pacific. The footage showed molten lava erupting like a “fiery bubble” and has since guided subsequent studies.
Although rarely hazardous to humans, the last decade has seen several spectacular eruptions. In 2022, the Hunga Tonga–Hunga Ha’apai volcano in the South Pacific produced the most powerful underwater eruption recorded—its plume released enough vapor to fill 58,000 Olympic‑size swimming pools and even lowered atmospheric ozone temporarily.
In 2023, a new island briefly emerged off the coast of Iwo Jima, Japan, after a 10‑day eruption. By mid‑2024 the island had largely submerged again, illustrating the transient nature of volcanic islands.
Today, the Axial Seamount off Oregon’s coast is an active submarine volcano. While its depth—about 1 mile below the surface—keeps it far from populated shores, scientists monitor it closely for potential seismic or tsunami activity.
