A fracture zone is a long, narrow line of steep valleys and ridges across the ocean floor, hundreds or even thousands of kilometres long. It marks where an offset in a mid-ocean ridge has left crust of different ages side by side.
Key takeaways
- Oceanic fracture zones are ocean-floor valleys that offset spreading ridges, some hundreds to thousands of kilometres long and up to 8 kilometres deep [1].
- Only the section between two ridge segments is an active plate boundary. The rest of the fracture zone is usually seismically inactive [2].
- Our map lists 29 named fracture zones, such as the Charlie-Gibbs Fracture Zone, read at 4,013 m at its listed point.
- A fracture zone separates ocean crust of different ages, which sit at different heights because younger crust is more buoyant [2].
How a fracture zone forms
Mid-ocean ridges are divergent boundaries where new crust is made. They are not straight. The ridge is broken into segments that are offset from each other, and between each pair of segments a transform fault takes up the sideways shift [2]. Plates on either side of the active fault slide past each other in opposite directions.
A fracture zone is what is left as that offset is carried away from the ridge. Beyond the ends of the transform fault, the crust on both sides belongs to the same plate and moves in the same direction, so the line stops being a plate boundary. It remains as a long line across the seafloor, where crust formed at the two different ridge segments sits side by side [2].
That is why fracture zones show up so clearly on a bathymetric map. The crust on one side is younger than on the other, and younger crust sits higher because it is warmer and more buoyant. The fracture zone is therefore marked by a step in height as well as by a trough [2]. USGS describes oceanic fracture zones as ocean-floor valleys that horizontally offset spreading ridges, some of them hundreds to thousands of kilometres long and as much as 8 kilometres deep [1].
Fracture zones are usually defined with bathymetric, gravity and magnetic surveys [2]. The homepage explains how the seabed is mapped.
A fracture zone is also not always the same as a transform fault, even though the names are often swapped. Strictly, the transform fault is the active part and the fracture zone is the extension beyond it [2].
An example from our data
Our list has 29 named fracture zones. The Charlie-Gibbs Fracture Zone crosses the Mid-Atlantic Ridge in the North Atlantic, and the grid reads 4,013 m at its listed point. Its nearest country is Portugal, 1,420 km away.
The Hero Fracture Zone is listed at 4,561 m, and the Heirtzler Fracture Zone at 2,946 m. Each depth is the deepest grid cell near the listed point, not the depth along the whole zone, so they should not be read as a ranking.
The best way to see them on the map is to zoom out over the Atlantic and look for thin lines that run at right angles across the Mid-Atlantic Ridge.
The thing people get wrong
Fracture zones are often described as places where the ocean floor is cracking and where earthquakes happen. The active transform section does produce shallow earthquakes. The fracture zone beyond it is usually seismically inactive, because both sides are moving together as one plate [2]. It is more like a healed scar than an open wound.
Fracture zone vs transform fault
| Fracture zone | Transform fault | |
|---|---|---|
| Length | Hundreds or thousands of kilometres [1] | Only between two ridge segments |
| Plate boundary? | No, apart from its active part | Yes |
| Earthquakes | Usually few | Frequent and shallow |
Frequently asked questions
Is a fracture zone the same as a fault?
Not quite. The active part between two ridge segments is a transform fault. The fracture zone is the longer scar that continues past it, where the plates on both sides move together.
Are fracture zones dangerous?
Mostly not. The part beyond the ridge segments is generally quiet, and the active transform section is far out at sea.
Sources
- Understanding plate motions, USGS (This Dynamic Earth).
- Fracture zone, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).