Oceanic crust

Oceanic crust is the uppermost layer of the tectonic plates that lie beneath the world's oceans.

It sits directly above the rigid uppermost layer of the Earth's mantle. When the crust and this mantle layer are combined, they form the oceanic lithosphere. This layer is much thinner than the continental crust found under land. While continental crust is often thick, oceanic crust is generally less than 10 kilometers thick. However, it is much denser than the crust on land. Oceanic crust has a mean density of about 3.0 grams per cubic centimeter. In contrast, continental crust has a density of about 2.7 grams per cubic centimeter.

The formation of this crust is a continuous process driven by plate tectonics. Most of this activity happens at mid-ocean ridges, where tectonic plates move away from each other. As these plates diverge, magma rises from the mantle into the spreading center. This center is often a partly solidified crystal mush made from earlier magma injections. Some of this magma forms magma lenses, which then feed sheeted dikes. When the lava reaches the surface, it cools into pillow lavas.

These lavas are often chemically modified by contact with seawater as they cool. Most magma, however, crystallizes at depth within the lower part of the crust. This deep magma can mix and react with the pre-existing rocks and crystal mush.

Geologists have identified three distinct layers that make up the oceanic crust. The first layer is a thin layer of sediment, averaging only 0.4 km thick. Near continental margins, this sediment is terrigenous, which means it comes from the land. In the deep sea, these sediments are often made of tiny shells from marine organisms. These shells can be calcareous or siliceous. The second layer is about 2 km thick and is split into two parts. Layer 2A is a 0.5 km thick volcanic layer of glassy or finely crystalline basalt. Layer 2B is a 1.5 km thick layer composed of diabase dikes.

The third layer is the largest, making up over two-thirds of the total volume. It is about 5 km thick and forms from the slow cooling of magma deep beneath the surface. This layer consists of coarse-grained gabbro and ultramafic cumulates. Because it cools so slowly, the crystals have time to grow larger. Scientists have not yet drilled a complete section through the entire crust. To understand its composition, they use several different methods. They study ophiolites, which are sections of oceanic crust pushed onto continents. They also use submersibles, dredging, and drilling to collect samples from the seafloor.

The thickness of the crust can change depending on how fast the plates move. Most oceanic crust stays around 7 kilometers thick, with a variation of plus or minus 1 km. At very slow spreading ridges, such as the Gakkel Ridge in the Arctic Ocean, the crust is thinner. These ridges produce crust only 4 to 5 km thick because the mantle cools quickly. Conversely, crust can be much thicker above mantle plumes. Iceland is a notable example, with a crust thickness of about 20 km. This happens because the hotter mantle creates more melt at greater depths.

Oceanic crust is part of a massive recycling system known as the Wilson Cycle. This cycle involves the repeated creation and destruction of oceanic crust during the formation of super-continents. New crust is created at ridges, while old crust is consumed at convergent boundaries. At these boundaries, one plate subducts, or sinks, beneath another plate. If an oceanic plate meets a continental plate, the oceanic plate always subducts because it is denser. Because of this constant recycling, most oceanic crust is rarely more than 200 million years old.

The oldest large-scale crust is found in the west Pacific and northwest Atlantic. Some remnants in the eastern Mediterranean may be as old as 340 million years from the ancient Tethys Ocean.

A fascinating feature of the crust is its magnetic pattern. As magma cools at mid-ocean ridges, its magnetic polarity aligns with the Earth's magnetic poles. This magnetic signature is then frozen into the basaltic rock. As new magma pushes older rock away from the ridge, it creates a symmetrical pattern of magnetic lines. These lines appear as alternating positive and negative sections parallel to the ridge. This pattern provides a record of how the ocean floor has spread over time.