An undersea mountain range is a long chain of mountains rising from the ocean floor. It can be a mid-ocean ridge, where new seafloor forms, or a line of seamounts, which are separate underwater volcanoes. Most of the longest ranges on Earth are underwater.
Key takeaways
- The longest mountain range on Earth, the mid-ocean ridge system, is mostly under water: more than 90 per cent of it lies beneath the sea [1].
- Seamounts are single underwater mountains, mostly extinct volcanoes, and the ones at least 1,000 metres high are thought to number more than 100,000 [2].
- Our list has 112 named ridges and 420 named seamounts, so undersea mountains are a large part of the 950 features on the map.
- Ridges and seamount chains form in different ways, so a long line of underwater peaks is not always a plate boundary.
How undersea mountain ranges form
Mountains on land are mostly made by plates colliding and crumpling the crust. Under the sea there are several other ways to build a range, and they leave different shapes.
Mid-ocean ridges are the biggest. Where two plates pull apart, magma rises, hardens and builds a belt of volcanic mountains. The whole system is the longest mountain range on Earth, with more than 90 per cent of it under water [1]. It has its own entry: see mid-ocean ridge.
Seamount chains are lines of separate underwater volcanoes. NOAA defines a seamount as an underwater mountain with steep sides, usually a cone-shaped remnant of an extinct volcano, with at least 1,000 metres of height above the surrounding seafloor [2]. Seamounts are found in every ocean basin, and some of the long chains are thought to form as a plate moves over a hotspot, a long-lasting hot area deep below. As the plate moves on, the volcano is cut off from its supply of magma and a new one builds in its place [3]. USGS uses the Hawaiian Island-Emperor Seamounts chain as the classic example.
Other ridges are neither spreading zones nor volcano chains. Our map labels many long, narrow rises simply as ridges, so the label alone does not tell you how a ridge formed.
Where two plates collide, volcanoes can also form curved chains near trenches, called a volcanic island arc. The homepage has a short overview in its undersea mountain ranges section. This page explains what the range types are and how to tell them apart.
An example from our data
Our list has 112 named ridges and 420 named seamounts. They behave differently on the map.
The Mid-Atlantic Ridge is a spreading ridge, and the grid reads 1,477 m at its listed point. The Walvis Ridge is a long ridge in the South Atlantic that reads 892 m at its listed point, about 592 km from Namibia. The New England Seamount Chain, a line of seamounts, reads 1,409 m at its listed point.
A range has no single depth, so each of those numbers is the depth of one deepest grid cell near a listed point. Seamounts in our list use the depth of their summits. Peaks rarely reach the surface. The Lomonosov Ridge, in the Arctic, reads 155 m at its listed point. Look at the map for the long, pale lines.
The thing people get wrong
Because the ocean floor is out of sight, people picture it as one flat plain with the odd trench. It is rugged, and its mountain ranges are the longest and largest on the planet [1]. Another mistake is to think that every long undersea ridge is a plate boundary. Only the mid-ocean ridges mark spreading, and other ridges and seamount chains have different origins [3].
Undersea mountain range vs mid-ocean ridge
| Undersea mountain range | Mid-ocean ridge | |
|---|---|---|
| What it is | Any long chain of underwater mountains | One specific kind of range, along a spreading boundary |
| How it forms | Spreading, hotspots, old crust or other causes | Magma rising where plates pull apart |
| Example | New England Seamount Chain | Mid-Atlantic Ridge |
Frequently asked questions
What is the longest mountain range in the world?
The mid-ocean ridge system, nearly 65,000 kilometres long and mostly under water. The longest range on land is much shorter.
Are undersea mountains taller than Everest?
Measured from the seafloor, some are very tall, but the highest summit on land is higher above sea level. Most undersea peaks never reach the surface.
Sources
- What is the longest mountain range on earth?, NOAA Ocean Service.
- What is a seamount?, NOAA Ocean Service.
- Hotspots, USGS (This Dynamic Earth).
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).