engendered tsunamis of large amplitude that propagated across the Pacific Ocean basin and caused damage and death thousands of kilometers from their source. Along the Oregon-California coast, 16 people were killed by the Alaska tsunami. In Crescent City, California, a series of large tsunamis inundated the harbor, beginning at four and a half hours, with the third and fourth wave causing the most damage. After the first two had struck, seven people returned to a seaside tavern to recover their valuables. Since the ocean seemed to have returned to normal, they remained to have a drink and were caught by the third wave, which killed five of them.2

1  

National Research Council, The Great Alaska Earthquake of 1964, National Academy Press, Washington, D.C., 15 volumes, 1972-1973.

2  

B. Bolt, Earthquakes and Geological Discovery, W.H. Freeman, New York, p. 155, 1963.

of the North American plate overriding an extension of the East Pacific Rise along a subduction zone paralleling the West Coast. Her synthesis, which accounts for seemingly disparate events (e.g., andesitic volcanism in northern California, strike-slip faulting along the San Andreas, compressional tectonics in the Transverse Ranges, rifting in the Gulf of California) was grounded in the kinematical principles of plate tectonics (73), and her paper did much to convince geologists that the new theory was a useful framework for understanding the complexities of continental tectonics.

Convergent plate boundaries in the oceans were observed to be broader than the other boundary types, with the zone of geologic activity on the surface encompassing the trench itself, the deformed sediments and basement rocks of the forearc sequence, the volcanic arc that overlies the subducting slab, and sometimes an extending back-arc basin (74). Nevertheless, the few-hundred-kilometer widths of the ocean-ocean convergence zones did not compare with the extensive orogenic terrains that mark major continental collisions. The controlling factors were recognized to be the density and strength of the silica-rich continental crust, which are significantly lower than those of the more iron- and magnesium-rich oceanic crust and upper mantle (75). When caught between two converging plates, the weak, buoyant continental crust resists subduction and piles up into arcuate mountain belts and thickened plateaus that erode into distinctive sequences of sedimentary rock. This distributed deformation also causes metamorphism and melting of the crust, generating siliceous magmas that intrude the crust’s upper layers to form large granitic batholiths. In some instances, the redistribution of buoyancy-related stresses can lead to a reversal in the direction of subduction.



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