A mid-ocean ridge is a long chain of undersea volcanic mountains that runs along the boundary where two tectonic plates pull apart. New ocean floor forms there, and the whole system winds around the globe like the seam of a baseball.
Key takeaways
- The mid-ocean ridge is the longest mountain range on Earth, nearly 65,000 kilometres long, with more than 90 per cent of it under water [1].
- Our named-feature list includes 112 ridges, among them the Mid-Atlantic Ridge and the East Pacific Rise.
- Slow-spreading ridges have a deep rift valley down the middle, while fast-spreading ones have a smooth, broad crest [1].
- A ridge has no single depth, so each page on the map shows the depth at one listed point, such as 1,477 m for the Mid-Atlantic Ridge.
How a mid-ocean ridge works
A mid-ocean ridge sits on a divergent plate boundary, a line where two tectonic plates are moving apart [2]. As the plates separate, molten rock rises from below to fill the gap, cools and hardens as basalt, and becomes new ocean crust [1]. The plates carry that crust away on either side, and more magma rises behind it. The ridge is the raised, volcanic crest where all this happens.
The scale is huge. NOAA describes the mid-ocean ridge system as a continuous range of underwater volcanoes that stretches nearly 65,000 kilometres, with an average water depth to the top of the ridge of 2,500 metres [1]. The Atlantic section is the best known. It is slowly spreading, at around 2 to 5 centimetres a year [1], and Iceland sits right on top of it, which is why Iceland is slowly splitting in two [2].
How fast the plates part decides what the ridge looks like [1]:
- Slow spreading (the Mid-Atlantic Ridge) makes steep, irregular mountains with a rift valley down the middle, about as deep and wide as the Grand Canyon.
- Fast spreading (the East Pacific Rise, at about 6 to 16 centimetres a year) makes a broad, smooth, gently sloping crest with only a narrow crack along the top.
Ridges are not perfectly straight. They are cut into segments and shifted sideways by transform faults, and the scars those faults leave in the older seafloor are fracture zones. Our homepage section on undersea mountain ranges gives the overview. This page covers what a ridge is and why it is there.
An example from our data
Our list has 112 named ridges, but only some are mid-ocean ridges. The rest are other long mountain chains. Three that sit on spreading boundaries show the range of settings.
The Mid-Atlantic Ridge runs down the middle of the Atlantic. At its listed point the GEBCO grid reads 1,477 m. The East Pacific Rise is the fast-spreading example, and its listed point reads 3,140 m, about 940 km from Chile. The Southwest Indian Ridge reads 4,184 m at its listed point.
Those three numbers do not rank the ridges. A ridge is a long chain, so a page shows the depth of one deepest cell near a listed point, not the depth of the whole thing. The best way to see the pattern is on the map: look for the long, pale lines that run down the middle of an ocean.
The thing people get wrong
Many people think the ocean floor is flat and the same age everywhere. It is neither. The floor is youngest at the ridge and gets older with distance from it [4]. That is why a ridge stands high: new crust is hot and buoyant, and it sinks as it cools.
A second mistake is to treat the ridge as a single wall or crack. It is a broad belt of mountains and valleys, in places more than 800 kilometres across [4], made of thousands of separate volcanoes and ridge segments [3].
Mid-ocean ridge vs undersea mountain range
| Mid-ocean ridge | Undersea mountain range | |
|---|---|---|
| What it is | A spreading boundary where new seafloor is made | Any long chain of mountains under the sea |
| Where | Along the line where two plates part | Anywhere, including away from plate boundaries |
| Example on our map | Mid-Atlantic Ridge | Walvis Ridge |
Frequently asked questions
Is the mid-ocean ridge one mountain range?
It is one connected system, but it is made of thousands of separate volcanoes and ridge segments, and each ocean's section has its own name, such as the Mid-Atlantic Ridge.
Why is the ocean floor shallower along a ridge?
Because the crust there is newly made, hot and buoyant. It cools, sinks and gets deeper as it moves away from the ridge.
Can you walk on a mid-ocean ridge?
Almost never. More than 90 per cent of the system is under water. Iceland is the best-known place where a spreading ridge rises above the sea.
Sources
- What is a mid-ocean ridge?, NOAA Ocean Exploration.
- Understanding plate motions, USGS (This Dynamic Earth).
- What is the longest mountain range on earth?, NOAA Ocean Service.
- Developing the theory, 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).