MAJOR CONCEPTS
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 shallow earthquakes are concentrated along the rift zone at the ridge crest.
3. The ridge’s characteristics depend upon the spreading rate. As oceanic lithosphere moves away from the ridge, it cools, becomes thicker and denser, and subsides
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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. Seawater is heated as it circulates through the hot crust and causes extensive metamorphism. Locally, the hydrothermal fluids produce hot springs on the seafloor.
7. Continental rifting occurs where divergent plate margins develop within continents. The East African Rift, the Red Sea, and the Atlantic Ocean illustrate the progression from continental rifting to seafloor formation.
8. 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 beneath a thick layer of shallow-marine sediments
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