Composition and evolution of the oceanic crust
Abstract Laboratory measurements of compressional wave velocities in rocks are consistent with an oceanic crust composed of an assemblage of hornblende and plagioclase (layer 3) overlain by tholeiitic basalt (layer 2). Partial melting of mantle peridotite under mid-oceanic ridges marks the initial stages in the development of the oceanic crust. Tholeiitic magma, which escapes to the ocean floor, forms layer 2. Layer 3, composed of amphibolite and hornblende gabbro, originates beneath the ocean floor by crystallization of tholeiitic magma under hydrous conditions and subsequent metamorphism in the vicinity of ridge crests. Once formed, the oceanic crust is transported laterally by horizontally spreading upper mantle. Disposal of the oceanic crust is accomplished by downward movement in the vicinity of descending limbs of convection cells. Partial melting of the oceanic crust in regions of downward convection results in the formation of the calc-alkaline suite of rocks and eclogite.