The surface of the earth lies at two levels, the basalt-floored ocean basins and the sial-capped continental platforms. These great segments of the earth's crust are in isostatic equilibrium. Adjustment is made to a shrinking interior by subsidence. First to drop are the heavier ocean basins. This condition of disequilibrium is terminated by subsidence of the continental platforms to approximately the same relative levels as before. The subsidence of the ocean basins may appear, in the uppermost rocks of the crust, as a great downfaulted block (Pacific type) or as a downwarp (Atlantic type). Both the continental and the ocean-basin segments are subject to fragmentation and local subsidence. Examples are basins, geosynclines, grabens, and foundered continental border zones.
The downward movement of crustal segments caused them to crowd together, and the resulting compressional forces were expressed in folds and thrust faults, arcuate mountain systems and island arcs, and geanticlines and swells. Volcanoes were a possible by-product anywhere, but batholithic intrusions were confined to the sial-capped continents.
Possible causes for a shrinking globe include downward solidification of a liquid core, continued cooling of solid rock, and increased density due to pressure conversions. If downward solidification has taken place, the freezing of the crust to its present thickness of 1820 miles would have been accompanied by a reduction of radius of 140 miles and of circumference of 880 miles, using 11.5, the expansion percentage when basalt liquefies, as the contraction factor.
The submarine canyons, some of which extend to depths of 15,000 feet, can be explained as due to sub-aerial erosion following a subsidence of the ocean basins through a vertical distance of 20,000 to 30,000 feet, which lowered sea level the required distance. Subsidence of continental areas, such as the Gulf Coast of North America, to even greater depths is known to have taken place. The subsidence of the continental platforms restored the isostatic balance and brought an end to the canyon cutting and to other topographic changes, such as the truncation of volcanic cones and the cutting of terraces, that were taking place on the exposed sea floor.
Differential subsidence also explains the glacial climates. A lowering of sea level of 3000 feet would be adequate; this could be brought about by a subsidence of the deeper parts of the ocean basins of 5000 feet. During the Pleistocene there were three subsidences of at least 5000 feet and one of perhaps 25,000 feet.
The lagging behind, in time of subsidence, of strategically placed crustal blocks would create temporary land bridges. Widespread inundation of continental areas took place due to (1) rotation of the platforms during subsidence; (2) drag of the marginal belts due to ocean-basin subsidence; and (3) downwarp (basins, geosynclines) due to fragmentation and differential subsidence of the continental segments.
The normal, isostatically balanced, condition is as today, with the continental platforms largely emergent. The sedimentary sections with their contained fossils available to us by being above present sea level can only be considered brief, irregularly spaced, and totally inadequate samples of the sedimentation and evolutionary history of the earth.