A seamount is an undersea mountain which has not breached the ocean surface. Most seamounts are extinct volcanoes, but some, such as Lohi in the Hawai'ian archipelago, are still active. If a seamount grows above the ocean surface, it becomes an oceanic island.
Strictly speaking, a seamount must have fairly steep, cone-shaped sides, with a limited summit area. Undersea elevations which are more pillar-like are properly called pinnacles.
A seamount must also be at least 3,281 feet tall. If it is lower than that but has the same general shape, it is properly called a knoll.
All seamounts are isolated from the surrounding seafloor. They are not part of an undersea range, but they may be grouped together in a ridge.
However, in practice, the term is used much more loosely to define any isolated undersea mountain. For this reason, there may be as few as 100,000 seamounts or as many as 2 million. Although seamounts can be found in every ocean, nearly half of all seamounts are in the Pacific Ocean, mostly in the southern hemisphere.
All seamounts are caused by magmatic processes. Nearly all are formed by 1 of 2 volcanic processes, although the origins of a few, such as the Christmas Island Seamount Province, are not so clear. The type of process determines its composition.
Some are undersea volcanoes which have formed near subducting zones, the same way as Mount St. Helens and other surface subduction volcanoes. However, the high pressures in the ocean depths mean that most undersea volcanoes are not explosive in the same way as land stratovolcanoes.
The other common way in which seamounts are formed is through decompression melting of mantle rock along plate boundaries and mid-ocean ridges. This is what is responsible for hotspots such as Hawai'i.
Magmatic regions such as seamounts are often rich in heavy metals from the Earth's mantle and core. This makes shallow seamounts excellent candidates for future deep sea mining.
Seamounts are rich biotic environments, partly because a tall seamount offers a wide range of seafloor depths along its slopes. This may enable coastal species to use seamounts as intermediate stepping stones when dispersing between land masses. These rich habitats may end up supporting large numbers of fish, shellfish, and marine mammals. Even seabirds may be more common above a shallow seamount.
The biodoversity of seamounts varies greatly depending on their geographical location relative to other seamounts, land masses, and ocean currents. Some have high biodiversity, while others have very few species. The seamount itself can also create currents which affect the ocean life around and above it.
Some seamounts have become important commercial fisheries, especially for longer-lived fish such as orange roughy. However, many of these species live only at a few seamounts, or even just a single one. This is known as endism. Overharvesting or bottom trawling of these regions can eradicate the entire local ecosystem.
Most active seamounts erupt at low levels which can be identified easily and pose little danger to surface activities. However, continued extrusions can weaken the volcano's flanks until they collapse. The amount of seawater displaced by this kind of collapse can cause a tsunami.
Extinct seamounts also pose a danger to surface activities. Seamounts which have not yet been mapped pose a danger to shipping, but this is decreasing with satellite mapping. Deeper unmapped seamounts pose a constant danger to submarines, although these collisions have also been caused by human error.