. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography pen if, with the changing patterns of plate motions, a subduction zone (a trench) were created along a former trailing edge, forming a new plate boundary. The Atlantic would be trans- formed into a closing ocean with its geosynclinal prisms riding toward the trench. The continental margin and the trench would eventually collide, collaps- ing the eugeocline into a contorted mountainous foldbelt and also folding the miogeocline to a much lesser extent
. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography pen if, with the changing patterns of plate motions, a subduction zone (a trench) were created along a former trailing edge, forming a new plate boundary. The Atlantic would be trans- formed into a closing ocean with its geosynclinal prisms riding toward the trench. The continental margin and the trench would eventually collide, collaps- ing the eugeocline into a contorted mountainous foldbelt and also folding the miogeocline to a much lesser extent. Before that happened the continental margin would encounter and incorporate an island arc, similar to the island arcs found along the perimeter of the west- em Pacific. These arcs are created by tectonic and magmatic activity triggered by the plunging crustal plate. It is also quite possible that the Atlantic Ocean would close entirely, causing North Afri- ca to collide with eastern North Amer- ica. The collision of India with the un- derbelly of Asia, throwing up the Hima- laya rampart, would be a present-day analogy. One can imagine many possible scenarios, depending on the geometry of plate boundaries and other variables. The creation of a eugeoclinal foldbelt is of course considerably more than simply the accordion-like collapse of a continental-rise prism. The foldbelt is sheared into thrust faults and the land- ward edge of the eugeocline is common- ly thrust onto the adjacent miogeocline. The descending crustal plate is not en- tirely consumed within the earth's hot mande, with the result that low-density magmas buoyantly rise and invade the eugeocline. This leads to intrusions of granodiorite (a granite-quartz rock) and the growth of volcanic mountains con- sisting of andesite (the rock characteris- tic of the Andes). This lava is highly ex- plosive because it is charged with water sweated out of the descending plate. Magma is not generated from the plung- ing lithosp
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