[理学]海洋地质前沿.ppt

Deep-sea trenches off continental margins and volcanic inland arcs were initially though to have created by the tremendous weight of sediments washed off the continents and pulled down into the mantle by a dense underlying material. The downward pull on the sediments formed vast bulges in the ocean floor called geosynclines. However gravity surveys conducted over the trenches indicated that the pull of gravity was much too weak to account for the sagging of the seafloor. The trenches were also found to be sites of almost continuous earthquake activity deep in the bowels of the earth. The deep-seated earthquake acted like beacons marking the boundaries of a large slab of crust descending into the mantle. The unusual activity of the trenches suggested that they were sites where old oceanic crust subducted into the Earth’s interior. Perhaps here at last was the engine that drove the continents around the surface of the Earth. 2.3 GEOLOGIC OBSERVATIONS Observations of these and other fascinating geologic features on the ocean floor led to the seafloor spreading theory. The hypothesis described the creation and destruction of the ocean floor at specific regions around the world. The seafloor spreading theory resolved many problems connected with the mysterious characteristics on the seafloor, including the midocean ridges, the relatively young ages of rocks in the oceanic crust, and the formation of island arcs. But more importantly, here at last was the long-sought mechanism for continental drift. The continents do not plow through the ocean crust like ice breakers slicing through frozen seas, as previously thought, but instead tide above mantle like ships caught in mobile ice floes. Exploration of the ocean floor bought a new understanding of the forces that shaped the planet. After overwhelming geological and geophysical evidence was collected from the floor of the ocean in support of the theory of continental drift, geologists finally abandoned t