A seamount is an underwater mountain with steep sides that rises from the seafloor but does not reach the surface. To count, it needs at least 1,000 metres of relief above the surrounding seabed, and most are extinct volcanoes.
Key takeaways
- NOAA defines a seamount as an undersea mountain with at least 1,000 m of vertical relief, and most are extinct volcanoes.
- Our list has 420 named seamounts, and the depth we give for each is its summit.
- Seamounts often support more species and more biomass than the flat seabed around them.
- Estimates suggest over 100,000 seamounts exist, and fewer than 0.1% have been explored.
How a seamount works
A seamount is a mountain that begins on the deep seafloor and ends below the sea surface. NOAA gives a size rule: a seamount rises at least 1,000 m above the seabed around it [1]. Smaller bumps are not seamounts, and mountains that rise above the water are islands.
Most seamounts are what is left of volcanoes that have stopped erupting. They are usually cone-shaped, though some have craters, long ridges or wide flat tops [1]. The flat-topped ones have their own name, the guyot.
Volcanoes form on the ocean floor in a few settings. Some build above a hotspot, where heat rises from deep inside the Earth, and as the plate moves on, the volcano is carried away from its source and a new one forms, leaving a trail of islands and seamounts [2]. Others build along the edges of plates, in a volcanic island arc.
Seamounts matter because they change the water around them. NOAA describes them as oases of life: they create complex currents, offer surfaces for animals to settle on and often host species found nowhere else, so their waters and summits often hold more species and more biomass than the flat seabed [1].
Seamounts are also still poorly known. Estimates from satellite altimetry and ship mapping suggest more than 100,000 seamounts at least 1,000 m tall, and fewer than 0.1% have been explored [1]. Much of the seabed in the grid is filled in from satellite estimates and not direct survey, so a seamount's exact summit depth can be off. The homepage explains how the seabed is mapped.
An example from our data
Our list has 420 named seamounts, and each page gives the depth to the summit, the shallowest point. Davidson Seamount has a summit 1,393 m down, 98 km from United States of America. Cobb Seamount is much shallower at 73 m, and Bowie Seamount is 83 m.
Because their summits are so shallow, the drain slider can bring the tops of seamounts out of the water. In the simple bathtub model, lowering the sea by 80 m would leave a summit as shallow as Cobb's above the surface. The model drops the sea equally everywhere on today's seabed, so treat it as an illustration. See the 80 m step for the land it exposes.
The thing people get wrong
People often think a seamount is an island that sank, or that all underwater mountains are the same. In fact a seamount can be a perfectly ordinary volcano that never reached the surface. And a seamount is not a ridge. A ridge is a long chain of raised seabed, such as the Mid-Atlantic Ridge, while a seamount stands alone or in a short chain.
A second mistake is to assume that all seamounts are deep in the ocean. As the examples show, some have summits that sit close to the surface, and a few break through it.
Seamount vs guyot
Frequently asked questions
How tall is a seamount?
NOAA says a feature needs at least 1,000 m of vertical relief above the surrounding seafloor to be called a seamount. Smaller undersea hills are called knolls or banks.
Are seamounts always volcanoes?
Most are the remains of extinct volcanoes, according to NOAA. They are usually cone-shaped, though some have craters, linear ridges or flat summits.
Sources
- What is a seamount?, NOAA Ocean Service.
- Hotspots, USGS (This Dynamic Earth).
- Guyot, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).