The ocean floor is home to submerged mountains known as seamounts. These underwater landforms rise abruptly from the abyssal plains, yet their peaks never break the surface of the water, distinguishing them from islands. Scientists estimate there are over 100,000 seamounts that meet the standard height definition across the globe, in every ocean basin. Despite their sheer number and global distribution, less than one-tenth of one percent of these features have been explored in detail. This lack of exploration means the majority of these geological structures remain poorly understood.
Defining Seamounts and Their Physical Characteristics
Seamounts are typically defined by oceanographers as independent features that rise at least 1,000 meters (3,300 feet) above the surrounding seafloor. They generally possess a conical or peaked shape, resembling mountains found on land. Their flanks are often steep, and their summits can be found thousands of meters below the ocean surface.
A key distinction is made between a standard seamount and a guyot, which is a flat-topped submarine mountain. Guyots were once tall enough to reach the surface, where wave action eroded their peaks into a flat surface before they subsided and sank back into the deep ocean.
The Geological Process of Formation
The vast majority of seamounts are remnants of extinct volcanoes, forged through distinct magmatic processes tied to plate tectonics. One primary mechanism occurs at mid-ocean ridges, where tectonic plates spread apart, allowing magma from the mantle to rise and solidify into new crust, sometimes forming discrete volcanic peaks. Another major formation site is near subduction zones, where one plate slides beneath another, generating magma that rises to form volcanic arcs.
The most recognized seamount structures, such as the Hawaiian-Emperor Seamount Chain, form above stationary mantle hot spots. A plume of superheated rock rises from deep within the mantle, creating a volcano through the overlying tectonic plate. As the plate slowly moves over the fixed hot spot, the volcano is carried away, becomes extinct, and a new volcano begins to form in its place, creating a long chain of progressively older features.
A seamount’s life cycle follows a pattern of build-up, erosion, and eventual subsidence. During the initial period of intense volcanism, a single seamount can grow tall enough to briefly become an oceanic island. Once volcanic activity ceases, the entire structure begins to sink as the underlying oceanic crust cools, contracts, and becomes denser. This long-term sinking, combined with erosion, ensures that ancient volcanoes eventually become submerged seamounts or flat-topped guyots.
Seamounts as Biological Hotspots
The physical presence of a seamount alters the flow of deep-ocean currents, creating localized oceanographic phenomena that enhance biological productivity. As deep-sea currents encounter the slopes, they are forced to accelerate upward in a process called upwelling, bringing nutrient-rich water from the deep ocean toward the summit. Under specific conditions, a phenomenon known as a Taylor column can form, which is a persistent, rotating vortex of trapped water that circulates above the seamount’s peak.
This hydrodynamic trapping mechanism retains plankton and larval stages near the summit, while the upwelling current provides a constant stream of food particles and nutrients. This combination transforms the seamount into an oasis of life in the otherwise barren deep ocean. This localized productivity supports dense and unique biological communities that are rarely seen on the flat abyssal plains.
The hard, volcanic rock of the seamount slopes provides a stable attachment point for organisms, a substrate that is absent across the muddy seafloor. Deep-sea coral and sponge communities flourish here, forming complex, three-dimensional habitats. Many of these species are exceptionally slow-growing and long-lived; for instance, some black coral species have been aged at over 4,000 years. This extreme longevity and isolation results in a high degree of endemism, meaning many species found on a seamount occur nowhere else on Earth.
Exploration and Conservation Challenges
The remote nature of seamounts, often located in international waters and thousands of meters below the surface, makes them challenging to study, requiring specialized tools such as remotely operated vehicles (ROVs) and advanced bathymetric mapping. This lack of accessibility means that human activities can cause significant, undetected harm.
The primary threat comes from deep-sea bottom trawling, a destructive fishing practice where heavy nets are dragged across the seafloor. Trawling physically destroys the fragile, centuries-old deep-sea coral and sponge habitats that form the foundation of the seamount ecosystem. Studies have shown that a single trawl can severely reduce the coverage of habitat-forming corals. Due to the slow growth rates of these organisms, damaged areas show no clear signs of ecological recovery even a decade after trawling activity has ceased.
Another element is the growing commercial interest in deep-sea mining, particularly for cobalt-rich ferromanganese crusts, which form pavements on the rock outcrops of seamounts. These mineral deposits often co-occur at the same depths as the slow-growing deep-sea coral communities, creating a new threat to these vulnerable marine ecosystems.