Oceanic crust

Oceanic crust is the relatively thin, dense outer layer of Earth beneath ocean basins. Made chiefly of basaltic rocks, it forms at mid-ocean ridges and is recycled into the mantle at subduction zones.

Basalt: A dark volcanic rock rich in iron and magnesium, formed when basaltic lava cools. Basalt makes up most of the crust’s upper volcanic layer.
Mid-ocean ridge: An underwater mountain chain where tectonic plates separate and new oceanic crust forms. Rising magma at ridges supplies most newly formed oceanic crust.
Mohole project: A 1960s scientific effort to drill through oceanic crust and reach the mantle. The project tested whether drilling could directly sample the crust–mantle boundary.
Continental crust: The relatively thick, buoyant crust underlying continents and continental shelves. Its thickness, composition, and long survival contrast with oceanic crust.
Gabbro: A coarse-grained igneous rock with composition similar to basalt, crystallized slowly underground. Gabbro forms much of the lower crust beneath the basaltic lavas.
Seafloor spreading: The process by which new oceanic lithosphere forms at ridges and moves outward on both sides. Spreading carries young crust away from its birthplace.
Hess Deep: A deep rift in the seafloor near the Galápagos triple junction exposing young oceanic lithosphere. Its walls reveal crust formed near a fast-spreading ridge.
Earth's mantle: The thick rocky layer between Earth’s crust and its metallic core. Mantle rock is the source of oceanic-crust magma and its destination after subduction.
Pillow lava: Rounded masses of lava formed when underwater eruptions cool rapidly against seawater. Pillow structures are common where new crust erupts beneath the ocean.
Partial melting: Melting in which some minerals in a rock liquefy while others remain solid. Partial melting of mantle rock generates magma that builds oceanic crust.

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