. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 430 FISHER AND HESS [chap. 17 account for the mass distribution, the seismic velocities, and the forces acting to keep trendies far out of isostatic equihbrium. Such a hypothesis is neces- sarily lii. It differs from previous proposals in that the mantle and "crust" Consolidated sediments ond volcanic debris 1000 + kg/cm i TI 1 111 I I r. deserpentinization + 500°C Partial fusion mogmo generation Fig. 9. Supposed structure, with typical seismic velocities (l'^ in km/sec), for a hj])0- thetical trench-island arc a
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 430 FISHER AND HESS [chap. 17 account for the mass distribution, the seismic velocities, and the forces acting to keep trendies far out of isostatic equihbrium. Such a hypothesis is neces- sarily lii. It differs from previous proposals in that the mantle and "crust" Consolidated sediments ond volcanic debris 1000 + kg/cm i TI 1 111 I I r. deserpentinization + 500°C Partial fusion mogmo generation Fig. 9. Supposed structure, with typical seismic velocities (l'^ in km/sec), for a hj])0- thetical trench-island arc association. are moving together as a unit rather than as in Griggs' (1939) experiment where the weaker mantle material was moving at a higher velocity and acting on the crust by viscous drag. It is postulated that the "crust" is serpentinized peridotite (70% serpentinized, Vp = km/sec, density g/cm^) rather than basalt. The crustal layer goes down with the descending limb of the convection cell until it reaches a temperature in the neighborhood of 500°C where a deseq)entinizing reaction takes place, releasing water. Similarly, the original supracrustal volcanics and sediments descend until melting or partial melting occurs. Fluids, magma or water, rise, migrating toward the island arc or concave side of the structure. The reason for migration upwards on the con- cave rather than the convex side of the arc is given in Fig. 10. On this side there will be a tendency for open fractures to form at right angles to the direc- tion of compression (at right angles to the trench axis). Strong deformation. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hill, M. N. (Maurice Neville), 1919-. New York : Interscience Pub.
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