Divergent Plate Margins: Mid-Oceanic Ridges

Mid-Ocean Ridges:

• sites of sea floor spreading (i.e., divergent boundaries)• characterized by elevated position (one to three kilometers above the ocean floor)• faulting (transform boundaries) and numerous volcanic structures• extensive system covering ~65,000 km of ocean floor• active rift zone is the site of magma upwelling from the underlying asthenosphere• frequent but relatively weak earthquakes

Abyssal Plains:

• incredibly broad, flat, and featureless areas of the deep ocean floor• comprised of thick accumulations of sediment• sediments consist primarily of distal turbidites and deep-sea clays• typically depths range from 3,500 to 5,500 meters below sea level

Seamounts:

• isolated (mid-plate) volcanic peaks• many associated with 'hot spots' resulting from rising plumes of mantle material• best example is the mid-Pacific volcanic system • extends from Hawaiian Islands to Midway, northward to the Aleutian Trench• volcanoes in the chain increase in age with increasing distance from Hawaii• successive volcanic mountains form as plates move across

stationary mantle plume• hot spots therefore track plate motion• seamounts also form near oceanic ridges• often eroded to near sea level during existence as islands• islands 'sink' as moving plate carries the volcano away from ridges or hot spots• submerged, flat-topped seamounts known as guyots

Atolls:

coral reef complexes extending to depths of thousands of metersproblematic because corals require warm, shallow, sunlit water explained as coral reefs forming on the flanks of sinking volcanic islandsas islands slowly sink, corals continue to build reef complex upward