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oceanic ridge system (ridge)rift valley

ssss1 Map of the ocean floor showing the distribution of the oceanic ridge system.

Source: World Ocean Floor Manuscript Map; drawn by Berann, H.C., US Library of Congress, public domain after Heezan, Bruce C. and Tharpe, Marie.

One of the most significant discoveries of the twentieth century (Dietz 1961; Hess 1962) was that oceanic crust forms along the axis of the ridge system, then spreads away from it in both directions, causing ocean basins to grow through time. The details of this process are illustrated by ssss1. As the lithosphere is thinned, the asthenosphere rises toward the surface, generating basaltic‐gabbroic melts. Melts that crystallize in magma bodies well below the surface form basic plutonic rocks such as gabbro that become layer 3 in oceanic crust. Melts intruded into near vertical fractures above the chamber form the gabbroic‐basaltic parallel “sheeted” dikes that become layer 2b. Lavas that flow onto the ocean floor commonly form basaltic pillow and sheet lavas that become layer 2a. The marine sediments of layer 1 are deposited atop the basalts as they spread away from the ridge axis. In this way layers 1, 2, and 3 of the oceanic crust are formed. The underlying mantle consists of ultramafic rocks (layer 4). Layered ultramafic rocks form by differentiation near the base of the basaltic‐gabbroic magma bodies, whereas the remainder of layer 4 represents the unmelted, refractory residue that accumulates below the magma bodies.