Oceanic crust is the thin, dense, basalt-rich rock that floors the oceans, while continental crust is the thicker, lighter rock that forms the continents. Oceanic crust is created at ridges and destroyed at subduction zones, so it is much younger.
Key takeaways
- Oceanic crust is generally less than 10 km thick and denser than continental crust.
- It is made at mid-ocean ridges and recycled at subduction zones, so it is seldom older than about 200 million years.
- Continental crust is lighter, so it floats higher and does not usually sink at subduction zones.
- About 71% of the surface in our grid is below sea level, mostly over oceanic crust.
How the two kinds of crust differ
The Earth's outer shell comes in two kinds. Oceanic crust lies under the oceans. Continental crust makes up the continents, including their flooded edges [1].
Thickness. Oceanic crust is generally less than 10 km thick. Continental crust is thicker [1].
Density. Oceanic crust is mainly dark, iron- and magnesium-rich volcanic rock, and its upper layer is basalt. Its mean density is about 3.0 g/cm3. Continental crust is about 2.7 g/cm3 [1]. The difference sounds small, but it decides where each kind sits: lighter continental crust rides higher, and denser oceanic crust sits lower and holds the ocean water.
Age. Oceanic crust is continuously created at mid-ocean ridges as plates pull apart and magma rises [1][2]. At the other end, it sinks back into the Earth at subduction zones. Because of this, it is seldom older than about 200 million years [1]. Continental crust is not recycled so readily, since it generally does not subduct. When two continents collide, it crumples upward, as in the Himalayas [2].
At a meeting point. Where oceanic crust meets continental crust, the oceanic plate always goes under, because continental crust is less dense [1]. That is how deep ocean trenches form [2].
The boundary between the two is usually not the coast. The continental shelf and slope are still continental crust, and the change to oceanic crust lies further out, near the foot of the margin. Our map does not draw crust type, so you will not see this boundary on it. You can see its effects, though, in the sharp change in depth between shelf and deep plain.
An example from our data
The crust type sets the depth of the ground. Oceanic crust makes up most of the seabed, and about 71% of the surface in our grid lies below sea level, an area of 362 million km². The land is 147.9 million km².
The Mid-Atlantic Ridge is a place where new oceanic crust is being made, and the ridge crest is much shallower than the plains beside it. Our deepest grid cell near it is 1,477 m. At the other end, the Mariana Trench is where old oceanic crust is sinking back into the Earth, and has the deepest point on the map at 10,927 m.
The Atlantic Ocean has a mean depth of 3,883 m, and only 5.1% of it is shallower than 200 m, which is mostly flooded continental edge. The rest is mainly oceanic crust.
The thing people get wrong
The common idea is that the continents sit on top of the ocean floor as separate lumps. In fact the two kinds of crust are part of the same rocky shell, and the plates that carry them move together. A plate can carry both: the South American Plate has continent on one side and ocean floor on the other.
Another mix-up is to say that the ocean floor is "old". It is the reverse. The oldest oceanic crust is about 180 to 200 million years old [1], while continental rocks can be billions of years old. The ocean floor is constantly renewed.
Frequently asked questions
Which is denser, oceanic or continental crust?
Oceanic crust. It has a mean density of about 3.0 g/cm3 against about 2.7 g/cm3 for continental crust.
Why does oceanic crust sink under continental crust?
Because it is denser. Where the two meet, the oceanic plate always goes under.
Sources
- Oceanic crust, Wikipedia.
- Understanding plate motions, 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).