. Bulletin. Natural history; Science. These now better understood processes are important components of any conceptual model of global dynamics. But other necessary components involve interpretations of the morphological, structural, and compositional data on the crust and sub-crust. The model, then, in turn, enables us to look again at the accumulated data on the continental crust, particularly that exposed to our view on the land surface, and reinterpret them and attempt to develop a more rational view of the evolution of the large-scale features of the Earth's crust through geological time.
. Bulletin. Natural history; Science. These now better understood processes are important components of any conceptual model of global dynamics. But other necessary components involve interpretations of the morphological, structural, and compositional data on the crust and sub-crust. The model, then, in turn, enables us to look again at the accumulated data on the continental crust, particularly that exposed to our view on the land surface, and reinterpret them and attempt to develop a more rational view of the evolution of the large-scale features of the Earth's crust through geological time. The boundaries of the plates that constitute the lithosphere are of three fundamental types, fvlid-oceanic ridges form an undersea mountain system 35,000 miles long with a central rift along much of its length. The rift marks one type of boundary between plates. A second type of boundary is comprised of deep sea trenches with associated arcs of volcanic islands (such as the Aleutians) or cordilleran mountain ranges and volcanoes (the Peru-Chile coast, for example). The full extent of these oceanic features was revealed by the late 1950s to early 1960s. Also revealed were magnetic anomalies of the sea floor and major fracture features which offset the mid-ocean ridges. These fracture features, called transform faults, comprise the third boundary type. Recognition of these global features, added to many other tectonic data and theories, undoubtedly contributed to the concept of sea-floor-spreading, enunciated in 1962. As part of this concept, the mid-ocean ridges were interpreted as sites of upwelling of molten rock from the mantle, thus producing new sea floor material, which then spreads away from the ridges and ultimately plunges downward into the mantle at the site of the trenches. Subsequent additional information and interpretations have served to emphasize the plausibility of the theory that the sea floor is spreading. For one thing, magnetic anomalies of large areas of t
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