Mid-Ocean Ridge

Describe the sediments you would expect on the ocean floor near a mid ocean ridge, from the ridge crest to the deep ocean floor 4000m below sea level on either side of the ridge.

How active divergent margins evolve determines the sedimentology of mid-ocean ridges. Hot mantle upwelling causes the continental crust to start rifting. Upwarping of domed uplifts sheds coarse, immature alluvial and fluvial deposits. Volcanic activity related to the upwelling produces volcaniclastic material. As the rift extends, the uplift and extension breaks the crust. The downdrop of the central crustal block forms a fault graben, that widens during continued spreading, with additional downdropped normal blocks on the flanks. The graben forms a basin, accumulating more coarse, immature alluvial and fluvial sediments. The center of graben forms lakes, producing lacustrine or evaporite deposits, while the fault edges contain volcanic vents, producing ashfall and more volcaniclastic deposits.

Eventually, the rift spreads enough to be invaded by ocean water, although the crust is still continental crust. This proto-oceanic gulf buries the fault graben with shallow marine sediments: shale, limestone, and pelagic ooze. If the gulf is narrow, the restricted circulation can lead to the formation of more evaporite deposits. Continued volcanic activity produces alkalic deposits.

Finally, true seafloor spreading starts, with ocean crust forming at a spreading mid-ocean ridge. The crust is thickest at the mid-ocean ridge, where the crust is young and hot, growing more dense and thin as it cools, moving away from the ridge. The continental margins continue to spread and separate, sinking down the ridge flanks. Coating the mid-ocean ridges are calcareous ooze at the crest, red clay along the flanks. In areas of equatorial upwelling, the coating integrates siliceous ooze. The seafloor is covered in pelagic ooze and shales, transitioning to proto-oceanic gulf sediments at the base of the continental shelf, with turbidites and delta sediments building the continental embankment.

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