Underwater volcanic eruptions are a common geological process, often unnoticed due to the ocean’s vastness and depth. These events occur when magma emerges from Earth’s crust beneath the ocean surface. Approximately three-quarters of all volcanic activity happens underwater, primarily along mid-ocean ridges where tectonic plates pull apart. Unlike terrestrial eruptions, deep-sea events are largely hidden, yet they fundamentally shape the seafloor and influence ocean chemistry.
How Water Changes Eruptions
The presence of water alters volcanic eruptions compared to those on land. Immense hydrostatic pressure at great depths suppresses gas release, making most deep-sea eruptions effusive rather than violent. Magma cools rapidly upon contact with cold seawater, often transforming into volcanic glass. This rapid cooling prevents the widespread dispersal of ash and volcanic fragments seen in terrestrial eruptions.
In shallower waters, reduced pressure allows for more explosive interactions. Magma interacting with water at these depths generates substantial steam, leading to highly explosive steam-driven eruptions. These events can produce large quantities of pumice and steam plumes that may breach the ocean surface. The rapid cooling and unique pressure conditions underwater dictate the style and intensity of submarine volcanic events.
What Comes Out and What It Forms
Underwater eruptions expel materials that form distinct geological structures. As lava flows into the cold ocean, its surface chills quickly, forming a solid crust while the interior remains molten. This process creates rounded, bulbous shapes known as pillow lavas, the most abundant volcanic rock on Earth’s ocean floor. These pillow lavas accumulate, forming the vast majority of oceanic crust, particularly at mid-ocean ridges.
Another material formed is hyaloclastite, angular glass fragments created when lava is rapidly quenched and shattered by water. This fragmentation results in glassy shards often cemented into breccias. Beyond solid rock, eruptions also release superheated, mineral-rich hydrothermal fluids. These fluids, heated by magma, seep through seafloor cracks and emerge as hydrothermal vents, precipitating minerals to form towering chimney-like structures. Continued volcanism can build massive underwater mountains called seamounts, or contribute to extensive mid-ocean ridges, the longest mountain ranges on Earth.
Effects on Marine Life and Ecosystems
Underwater volcanic eruptions affect the marine environment, influencing water temperature and chemistry. The release of hot, chemical-laden fluids from hydrothermal vents alters seawater, introducing dissolved gases and minerals. These changes include metals like iron and manganese, utilized by deep-sea microbes. While disruptive locally, hydrothermal vents create unique chemosynthetic ecosystems.
These ecosystems are founded on chemosynthesis, where specialized bacteria convert chemicals from vent fluids, such as hydrogen sulfide, into energy. This chemical energy forms the base of a food web supporting diverse organisms, including giant tube worms, vent crabs, and various fish. While large-scale tsunamis are not typical for deep-sea eruptions, shallow-water events or flank collapses of large underwater structures can potentially generate them. The overall impact ranges from localized habitat alterations to the creation of unique biological oases in the deep ocean.
How Scientists Study Them
Studying underwater volcanoes presents challenges due to their remote and extreme environments. Scientists employ various technologies to detect and monitor these submerged features. Seismic monitoring is a primary method, as volcanic activity generates earthquakes recorded by land-based seismometers or ocean-bottom seismographs. Hydrophones, specialized underwater microphones, listen for acoustic energy from volcanic rumblings and eruptions, providing data on intensity and location.
Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) are indispensable for direct observation and sampling. ROVs are tethered to surface ships, controlled remotely for real-time video, detailed imaging, and sample collection. AUVs are pre-programmed to conduct independent surveys, mapping vast seafloor areas with high-resolution sonar and collecting data on bathymetry and water chemistry. Seafloor mapping techniques, particularly multibeam sonar, create detailed 2D and 3D maps, revealing topographic changes caused by lava flows and new structures. Satellite observations, detecting changes in sea surface color or temperature, also indicate underwater volcanic activity, especially for shallower eruptions affecting the water column.