1. Divergent plate boundaries are zones where lithospheric plates move apart from one another. They are characterized by tensional stresses that typically produce long rift zones, normal faults, and basaltic volcanism.

2. An oceanic ridge marks divergent plate boundaries in the ocean basins. It is a broad fractured swell with a total length of about 70,000 km. Basaltic volcanism and earthquakes are concentrated along the rift zone at the ridge crest.

3. The ridge is broken into segments and its characteristics depend upon the rate of spreading.

4. Oceanic crust is generated at divergent plate boundaries and is composed of four major layers: (a) deep marine sediment, (b) pillow basalts, (c) sheeted dikes, and (d) gabbro. Below the crust lies a zone of sheared peridotite in the upper mantle.

5. At divergent plate boundaries, basaltic magmatism results from decompression melting of the mantle. The magma then collects into elongate chambers beneath the ridge and some is intruded as dikes or extruded along the rift zone.

6. As the oceanic lithosphere moves away from the ridge it cools, becomes thicker and more dense, and subsides.

7. Seawater is heated as it circulates through the hot crust and causes extensive hydrothermal alteration and large volumes of metamorphosed basalt. Locally, the hydrothermal fluid vents to produce hot springs on the seafloor.

8. Continental rifting occurs where divergent plate margins develop in continents. The East African rift, the Red Sea, and the Atlantic Ocean show various steps in a progression from continental rifting to seafloor formation.

9. Continental rifting creates new continental margins marked by normal faults and volcanic rocks interlayered with thick sequences of continental sedimentary rocks. As the continental margin subsides, it is gradually buried by a thick layer of shallow marine sediments.

Copyright © 1995-2010, Pearson Education, Inc., publishing as Pearson Prentice Hall Legal and Privacy Terms