FIGURE 2.1 Schematic cross-section of a mid-oceanic ridge spreading center (a divergent plate boundary). Curvilinear mantle flow lines (arrows) show the circulation paths followed by rising asthenosphere and its cooling and conversion to lithosphere. Basaltic magma is shown as black coalescing blobs. Layers 1, 2, and 3 are deep-sea sediments, basaltic lava flows, and intrusive equivalents, respectively. M marks the Mohorovicic Discontinuity (the crust-mantle boundary).

SOURCE: Ernst (1990).

divergent plate boundaries—are spreading centers where the cooling lithospheric plates that overlie the ductily flowing mantle currents are transported at right angles away from the ridge (see Figure 2.1).

As it moves away from the ridge axis, the cooling oceanic lithosphere gradually thickens at the expense of the upper part of the asthenosphere. Heat is continuously lost, so the lithosphere-asthenosphere boundary (solid, rigid mantle above; incipiently molten, ductile mantle below), which is very close to the sea bottom beneath the oceanic ridge, descends to greater water depths away from the spreading center because its overall density increases. Unlike light continental lithosphere floating on a denser mantle, the oceanic lithosphere has a slightly greater density than the asthenosphere below, and so the oceanic plate will sink back into the deep mantle where geometrically possible.