The ocean floor is far from the flat expanse many people imagine, instead featuring a complex topography of plains, canyons, and vast mountain systems. These submerged landscapes are some of the most numerous and least explored features on our planet, shaping deep-sea environments. Understanding the geography of the ocean basin requires specialized terms to describe its varied structures. The most common term for an isolated underwater mountain is a seamount.
Defining the Isolated Underwater Mountain
An isolated underwater mountain that does not reach the ocean’s surface is formally called a seamount. To be classified as a seamount, the feature must rise at least 1,000 meters above the surrounding seafloor. Scientists estimate that there are over 100,000 such submerged mountains globally, though only a small fraction have been thoroughly explored.
Seamounts are typically steep-sided and conical or elliptical in shape, resembling terrestrial mountain peaks beneath the waves. They are usually formed from extinct volcanoes, representing the remnants of past volcanic activity on the ocean floor. The sheer size of these features is immense; for example, the base of Hawaii’s Mauna Kea rises over 9,100 meters from the seafloor, making it taller than Mount Everest when measured from base to peak.
The vast majority of seamounts remain hundreds to thousands of meters below the sea surface, placing them within the deep-sea environment. The volcanic rock that makes up these mountains is predominantly oceanic basalt, often covered with only a thin layer of marine sediment.
Major Classifications and Chains
While the conical seamount is the general classification, a flat-topped variation is known as a guyot, sometimes called a tablemount. Guyots are distinct because their broad, flat summits result from erosion that occurred when the volcanic peak was once above or near sea level.
As the tectonic plate carrying the volcanic island moves away from its heat source, the oceanic crust cools and contracts, causing the entire structure to subside and sink. Wave action flattens the top while it is near the surface, and the resulting structure becomes a guyot as it continues to sink.
In contrast to these isolated peaks, the ocean floor also features continuous, linear mountain systems known as oceanic ridges. These ridges form a global system that stretches approximately 80,000 kilometers across the world’s oceans. The Mid-Atlantic Ridge is a well-known example running down the center of the Atlantic Ocean. Oceanic ridges are fundamentally different from seamounts and guyots, as they represent a constructive plate boundary.
How Underwater Mountains Are Formed
The formation of underwater mountains is driven by volcanism and the movement of Earth’s tectonic plates. Oceanic ridges are formed at divergent plate boundaries, where two plates are separating. Magma rises from the mantle to fill the gap, solidifying to create new oceanic crust and gradually building the mountain chain. This process, known as seafloor spreading, continuously creates new crust along the ridge axis.
Isolated seamounts and seamount chains, such as the Hawaiian-Emperor chain, often form away from plate boundaries above stationary “hot spots.” Hot spots are areas where plumes of hot mantle material rise toward the crust. As the tectonic plate moves slowly over this fixed plume, magma punches through the crust to form a volcano, which grows into a seamount.
The continuous movement of the plate carries the newly formed mountain away from the heat source, causing the volcano to become extinct. A new volcano then begins to form over the hot spot, resulting in a distinct trail of increasingly older seamounts and islands on the ocean floor.
Ecological Significance
Beyond their geological importance, underwater mountains serve as biological oases in the vast, nutrient-poor deep sea. Their sheer size and steep slopes influence ocean currents, forcing deep water upward in a process called upwelling. This upwelling brings nutrient-rich water from the depths toward the surface.
This infusion of nutrients fuels the growth of phytoplankton, forming the base of a complex food web that supports a high density of marine life. The currents also sweep away fine sediment, exposing hard, rocky surfaces. This hard substrate is necessary for sessile organisms like deep-sea corals and sponges to attach and grow, creating complex, three-dimensional habitats.
Seamounts are recognized as biodiversity hotspots, attracting migratory species, including fish, sharks, and whales, that use the area for feeding, breeding, or as a navigational landmark. Researchers estimate that these features may host a high percentage of endemic species, which are found nowhere else on Earth, due to the physical isolation they provide.