Submarine volcanoes are fissures or vents on the ocean floor through which molten rock, gases, and other materials are released from the Earth’s interior. They are the most common type of volcano on the planet, with estimates suggesting over one million exist across the global seafloor. The immense hydrostatic pressure from the overlying water column significantly modifies their eruptive behavior compared to land-based counterparts.
Geological Environments and Distribution
The distribution of submarine volcanoes is closely tied to the boundaries of the Earth’s tectonic plates. The vast majority are found along the mid-ocean ridge system, a colossal chain of mountains where tectonic plates are pulling apart. This divergent boundary allows magma to rise and solidify, continuously creating new oceanic crust and accounting for approximately 75% of the planet’s annual magmatic output.
Submarine volcanism is also widespread in subduction zones, where one plate slides beneath another, often leading to the formation of volcanic island arcs. In these convergent settings, the melting of the subducting plate generates magma that rises to the surface, creating volcanoes that can be highly explosive due to the magma’s higher gas content.
A third significant environment is found within the middle of tectonic plates, where stationary columns of hot mantle material called plumes or hotspots create volcanoes. As the overlying plate moves across this plume, a chain of volcanoes and seamounts forms, exemplified by the Hawaiian and Emperor Seamount chains.
Eruption Dynamics and Products
The interaction between erupting magma and the cold, high-pressure seawater dictates the unique style of a submarine eruption. In the deep ocean, the sheer weight of the water suppresses the explosive conversion of water to steam, often resulting in effusive eruptions where lava flows relatively calmly. However, in shallower waters, the hydrostatic pressure is less effective, allowing for highly explosive steam-blast eruptions, which can breach the surface of the sea.
The most distinctive product of deep-sea eruptions is pillow lava, which forms when hot magma extrudes into the frigid water. The outer surface of the lava instantly chills and solidifies into a glassy crust, while the molten interior continues to advance, repeatedly breaking through the crust to form bulbous, rounded shapes that stack up like a pile of pillows.
The circulation of seawater through the hot volcanic rock drives the formation of hydrothermal vents. These vents expel superheated, mineral-rich water that appears as dark clouds, known as “black smokers,” or lighter clouds, known as “white smokers,” providing the chemical energy for unique deep-sea ecosystems.
From Seamount to Volcanic Island
Sustained submarine volcanic activity builds massive structures on the seafloor, known as seamounts. A seamount is an underwater mountain that rises at least 1,000 meters above the surrounding seabed but does not break the ocean surface. Most seamounts are extinct volcanoes.
Over millions of years, if the volcanic activity continues, the seamount can accumulate enough material to breach the surface and become a volcanic island, such as the island of Hawai‘i. Once above the surface, the island is subject to wave and wind erosion. As the oceanic crust underneath it cools and subsides, the island can eventually sink back down. If an island sinks and its top is eroded flat by wave action, it is classified as a guyot or tablemount.
Monitoring and Detecting Activity
Studying these remote, underwater geological features requires specialized technology for detection and monitoring. Hydrophones, sensitive underwater microphones, are widely used to listen for distinct acoustic signals, known as T-phases, generated by submarine explosions or seismic activity. These sound waves can travel vast distances through the ocean’s sound channel, alerting scientists to an eruption far from any monitoring station.
Mapping technologies like multibeam sonar are used to create detailed bathymetric maps of the seafloor, which can reveal changes in the volcano’s shape or the presence of new lava flows. Scientists also use ocean-bottom pressure sensors to detect changes in seafloor elevation caused by the inflation or deflation of an underlying magma chamber. Furthermore, analyzing the water column for plumes of increased turbidity, temperature, or unusual chemical signatures can indicate an active eruption or a release of hydrothermal fluids.