Volcanoes form underwater, where they are known as submarine volcanoes. The majority of the planet’s volcanic activity takes place beneath the ocean surface, largely hidden from view. This vast, global system of underwater volcanism creates new crust and shapes the ocean floor. These submerged mountains are far more numerous than their land-based counterparts.
Tectonic Environments Where Underwater Volcanoes Form
The largest concentration of submarine volcanism occurs along the boundaries of Earth’s tectonic plates. The mid-ocean ridge system, a massive chain of mountains running through all major ocean basins, represents a divergent boundary where plates pull apart. As the plates separate, magma rises from the mantle to fill the void, creating new oceanic crust. This process accounts for an estimated 75% of the annual magma output on Earth.
Volcanism also takes place at convergent boundaries, where one plate slides beneath another in a process called subduction. As the sinking plate descends, water is released, lowering the melting point of the overlying mantle rock and generating magma. This magma often forms volcanic arcs, many of which begin underwater, such as those found along the Pacific Ring of Fire.
A third setting for underwater volcanoes is the oceanic hotspot, which is independent of plate boundaries. Hotspots are caused by mantle plumes rising from deep within the Earth. As the tectonic plate moves over this stationary plume, a chain of volcanoes is created. The active volcano is situated directly above the plume, illustrated by the Hawaiian Islands and the Emperor Seamount chain.
How Water Pressure Affects Eruption Style
The presence of immense volumes of seawater profoundly changes how magma erupts due to hydrostatic pressure. At great depths, the overwhelming pressure suppresses the explosive conversion of water to steam. This forces dissolved gas within the magma to remain in a dense, supercritical fluid state, limiting explosive fragmentation.
Deep-sea eruptions are effusive, meaning the lava flows out slowly and smoothly. When the hot magma meets the frigid seawater, its surface cools instantly, forming a glassy, solid skin. Continuous pressure from the lava inside breaks this skin, pushing out a new bulbous lobe that immediately cools, resulting in characteristic pillow lavas. These rounded, stacked structures are the most common volcanic rock found on the ocean floor.
The eruption style changes dramatically in shallower waters, generally less than 500 meters deep. Here, the hydrostatic pressure is low enough to allow the explosive interaction of magma and water, creating phreatomagmatic eruptions. The rapid flash of water to steam causes violent fragmentation of the magma, resulting in explosive outbursts that can breach the ocean surface. Shallower volcanoes, such as those near the Tonga Islands, can temporarily form new volcanic islands that are often quickly eroded.
Unique Ecosystems Powered by Hydrothermal Vents
The heat from submarine volcanoes drives a chemical process that supports unique ecosystems in the dark ocean. Cold seawater seeps into cracks in the ocean floor and is heated by the underlying magma chamber. This superheated water, which can reach temperatures over 350 degrees Celsius, dissolves minerals and chemical compounds from the surrounding rock.
The hot, mineral-rich fluid is then expelled back into the ocean through chimney-like structures called hydrothermal vents. These vents are often referred to as “black smokers” because the fluid plume contains fine particles of metal sulfides, which precipitate upon contact with the cold seawater. White smokers expel lower-temperature fluids, often rich in compounds like barium, calcium, and silicon.
These vents create oases of life that do not rely on sunlight for energy. Specialized microorganisms, including bacteria and archaea, perform chemosynthesis, using chemical energy from compounds like hydrogen sulfide to produce organic matter. These chemosynthetic microbes form the base of the food web, supporting dense communities of unique organisms, including giant tube worms, specialized shrimp, and clams.
The Lifecycle of a Submarine Volcano
Submarine volcanoes follow a life cycle of growth and eventual decline, shaping the topography of the seafloor. A volcano that forms on the ocean floor but does not reach the surface is defined as a seamount. Seamounts are numerous, with tens of thousands identified in the Pacific Ocean basin alone.
Over millions of years, if the eruption rate is sufficient, a seamount may grow to breach the ocean surface, temporarily becoming a volcanic island. Once the volcano becomes inactive and the tectonic plate moves it away from its magma source, the island begins to erode due to wind, rain, and wave action. If erosion levels the top of the extinct volcano and it sinks below the surface due to the cooling and subsidence of the oceanic crust, it becomes a flat-topped seamount known as a guyot.
The size and steep slopes of these features can present geological hazards, even when they are extinct. Large seamounts and volcanic islands can become unstable, and massive flank collapses or landslides can occur. If such a landslide happens rapidly, it can displace a significant volume of water, generating destructive tsunamis.