Crust of the Earth: A Symposium
Isostasy implies that the differences in surface elevation of continents and ocean basins must reflect differences in density that in turn imply gross lithologic contrasts between these crustal segments. Some petrologists infer that tholeiites and magmas more siliceous are wholly continental. Oceanic rocks, collected from islands, may not fairly represent the oceanic crust, but at any rate they do not differ sharply from continental rocks. All major magma types occur; the Hawaiian magmas are saturated, for instance. Quartzose rocks seem confined to islands that rise from high ridges, but such rocks are sufficiently abundant to suggest that most high ridges are partly sialic. Probably the deeper oceanic basins are essentially free from sial.
The secular loss of sialic material to the pelagic areas requires that the continents should be either lower or smaller than in the past, unless there has been concurrent addition to the sial from the mantle. A rough computation, based on Arrhenius’ studies of deep-sea cores, suggests a loss of 1.5 km of material from the continents during 2 billion years. The fact that the continents seem as large or larger than in early geologic history suggests addition of sial during geologic time.
If the continents have grown areally by accretion of shelf geosynclines to a central nucleus, it seems that pelagic sediments and simatic geosynclinal floors should be common. Perhaps, though, their absence is not a conclusive argument against such growth: metasomatism may have changed the original composition of a mafic rock during orogeny, and furthermore many of the Pacific border mountains show no geosynclinal floors older than Paleozoic. A former simatic basement may have been folded downward during mountain making and be now overlain by sial crowded over it. Thus we may have former oceanic segments changed to continental. There are also many areas where stratigraphic and other evidence compels us to assume that former continental segments have been depressed at least as much as 3000 meters.
These changes in level raise isostatic problems as great as those of plateau uplift, but of the same kind. It is suggested that thinning or thickening of sial by subcrustal erosion or deposition is responsible for both.
The contrasts between continents and ocean basins invite attention to the visible processes now operating to modify them. These processes, though powerful, do not seem to account for the diversities.
The shore line—junction of the realms of denudation and deposition—is critical in dynamical geology. Sediment is now being carried across this boundary at a rate great enough, if continued, to erase all the topography above sea level in less than 10 million years, if compensating uplift did not occur. An analysis indicates that sub-crustal flow induced by isostatic response to unloading may influence both coastal structures and differentiation of sial, but such flow does not in any way explain the contrast between Pacific and Atlantic structures nor can it be the governing factor in orogeny. These must result from other movements deep within the mantle, perhaps piloted by the shallow movements.