A submarine volcano is a vent or fissure in the Earth’s surface situated beneath the ocean from which magma can erupt. These structures are significant to global geology, as an estimated 75% to 80% of all volcanic activity occurs beneath the ocean surface. The eruption of molten rock from these features is the primary mechanism that shapes the ocean floor and generates new oceanic crust. These volcanic systems range in size from small cones to massive underwater mountain ranges.
Plate Tectonic Settings
The creation of an underwater volcano is controlled by the movement of tectonic plates, which dictates where magma rises from the mantle. Magma is generated in three primary geological environments. The most common setting is the divergent boundary, where plates pull apart, causing hot mantle rock to rise and melt due to a decrease in pressure. This process is called decompression melting.
Volcanism also occurs at convergent boundaries, where one plate sinks beneath another in a subduction zone. As the oceanic plate descends, it releases water trapped within its minerals into the overlying mantle rock. This water lowers the rock’s melting point, leading to flux melting that generates magma. This magma then rises to form volcanic island arcs. A third, non-boundary setting involves fixed plumes of heat that rise through the mantle, creating intraplate volcanoes.
Formation at Spreading Centers
The most prolific site of underwater volcanism is the global Mid-Ocean Ridge system, a continuous divergent boundary spanning over 65,000 kilometers. As the tectonic plates separate, a rift valley forms, allowing magma generated by decompression melting to ascend into the gap. This rising magma is responsible for the continuous creation of new ocean floor in a process known as seafloor spreading.
When basaltic magma is extruded onto the ocean floor, it immediately encounters seawater, causing rapid external cooling. The outer surface of the lava flow solidifies instantly into a glassy crust, while the interior remains molten and pushes forward. This mechanism creates distinctive, rounded structures called “pillow basalts” that are typically one to two meters in diameter.
Successive eruptions stack these pillow basalts, gradually building the primary volcanic edifice of the mid-ocean ridge. Below the seafloor, the slower-cooling magma forms sheeted dikes and gabbro, which together make up the bulk of the oceanic crust. The massive volume of magma extruded along these ridges accounts for the majority of the planet’s annual magma output.
Alternative Formation via Hotspots
Not all submarine volcanoes form at plate boundaries; a significant number are created by fixed sources of heat known as hotspots. These features are fueled by mantle plumes, which are columns of hot rock rising from deep within the Earth’s mantle. The plume head impinges on the underside of the tectonic plate, causing the rock there to melt through decompression.
Since the mantle plume is stationary, the movement of the overlying tectonic plate dictates the pattern of volcanism. As the plate drifts across the fixed magma source, new volcanoes are successively created. This process results in a linear chain of volcanoes. The most active volcano is situated directly over the plume, and the volcanoes become progressively older and extinct further along the chain. The Hawaiian-Emperor Seamount chain is a prominent example of this intraplate formation, tracing the Pacific Plate’s movement.
The Lifecycle: From Seamount to Island
A submarine volcano that rises significantly above the seafloor but remains submerged is defined as a seamount. Many seamounts are formed by extinct volcanoes that failed to grow tall enough to breach the surface. If a seamount’s volcanic activity is prolonged, it can eventually emerge from the water and become a volcanic island, such as those in the Hawaiian chain.
Once a volcano becomes an island, it immediately undergoes sub-aerial erosion from wind, rain, and wave action. Simultaneously, the underlying oceanic crust cools and contracts, causing the entire volcanic structure to slowly subside back into the ocean. If the volcano subsides entirely, and its top has been flattened by wave erosion before sinking into deeper water, it becomes a flat-topped seamount known as a guyot.