. Bulletin. Natural history; Science. marked tectonic differences between the Pacific and Atlantic margins. The San Andreas fault of California is also a plate boundary, a transform fault connecting ridges in the Gulf of California (a new ocean, like the Red Sea and Gulf of Aden) with ridges off the Oregon coast. The plate to the west of the fault is sliding laterally northwestward relative to the American plate to the east and explains the earthquakes of the area. Two major concerns of earth science can be neatly and persuasively accommodated within the framework of plate tectonics. They are
. Bulletin. Natural history; Science. marked tectonic differences between the Pacific and Atlantic margins. The San Andreas fault of California is also a plate boundary, a transform fault connecting ridges in the Gulf of California (a new ocean, like the Red Sea and Gulf of Aden) with ridges off the Oregon coast. The plate to the west of the fault is sliding laterally northwestward relative to the American plate to the east and explains the earthquakes of the area. Two major concerns of earth science can be neatly and persuasively accommodated within the framework of plate tectonics. They are the theory of continental drift and the explanation of erogenic belts and orogenesis. The continental drift theory received new impetus during the early to mid-1950s from the findings of paleomagnetic directions in continental rocks. These are the directions along which the rocks were magnetized in the Earth's magnetic field, from which can be determined the apparent pole position at the time the rocks were formed. Study of rocks of different ages revealed that the position of the pole has changed throughout a large segment of geological time and, further, that this polar wandering path for the North American continent was systematically displaced from that of Europe except for the last few tens of millions of years, when there is an approximate convergence. The displaced curves were explained by continental drift. Since then many additional data have been added, so the continental drift theory is now reinstated and respectable. VOLCANIC CORDILLERA TRENCH 4 MOHO CONTINENTAL LITHOSPHERE I. Schematic vertical section of lithosptiere depicting one type of mountain building at plate boundaries: the oceanic tithosphere underthrusts a continental margin. A second variation of mountain building at plate boundaries: an oceanic plate underthrusts another oceanic plate, creating an island Please note that these images are extracted from scanned page images that may have been digita
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