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Marine Geology Phần 6

Vực thẳm ở dưới cùng của đại dương là được yên tĩnh và gần như hoàn toàn ở phần còn lại, với trầm tích từ từ rơi xuống và tích lũy với tốc độ khoảng 1 inch trong 20 thế kỷ. | MARINE GEOLOgy travel time over an extended period of 5 to 10 years could definitely indicate that the oceans are indeed warming. ABYSSAL STORMS The dark abyss at the bottom of the ocean was thought to be quiet and almost totally at rest with sediments slowly raining down and accumulating at a rate of about 1 inch in 20 centuries. Recent discoveries reveal signs that infrequent undersea storms often shift and rearrange the sedimentary material that has rested for long periods on the bottom. Occasionally the surging bottom currents scoop up the top layer of mud erasing animal tracks and creating ripple marks in the sediments much like those produced by wind and river currents. On the western side of the ocean basins undersea storms skirt the foot of the continental rise transporting huge loads of sediment and dramatically modifying the seafloor.The storms scour the ocean bottom in some areas and deposit large volumes of silt and clay in others. The energetic currents travel at about 1 mile per hour. However because of the considerably higher density of seawater they sweep the ocean floor just as effectively as a gale with winds up to 45 miles per hour erodes shallow areas near shore. The abyssal storms seem to follow certain well-traveled paths indicated by long furrows of sediment on the ocean floor Fig. 117 . The scouring of the seabed and deposition of thick layers of fine sediment results in much more complex marine geology than that developed simply from a constant rain of sediments.The periodic transport of sediment creates layered sequences that look similar to those created by strong windstorms in shallow seas with overlapping beds of sediment graded into different grain sizes. Sedimentary material deposited onto the ocean floor consists of detritus which is terrestrial sediment and decaying vegetation along with shells and skeletons of dead microscopic organisms that flourish in the sunlit waters of the top 300 feet of the ocean. The ocean depth influences