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How is ocean crust made?

The crust beneath oceans and continents is fundamentally different. Continental crust is made of light-colored rocks called andresite and granite. Ocean crust is composed of dark-colored rocks called basalt and gabbro. Ocean crust originates as a "melt" that forms in submicroscopic pores in rocks in Earth's hot mantle and rises to the surface. Scientists have pieced together clues to discover: 1) how melt formed over hundreds of kilometers in the mantle is focused into a five-kilometer volcanic zone beneath mid-ocean ridges, and 2) how oceanic crust is formed with a relatively uniform, three-tiered structure consisting of gabbro, sheeted dikes, and lava flows. (Illustration by E. Paul Oberlander)

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