Crust of the Earth: A Symposium
An eclectic hypothesis regarding processes and sources of energy governing the transformation and deformation of the earth’s crust is offered for study.
Relatively light, sialic continental segments of the earth’s crust, essentially sedimentary in origin, have grown from small centers or lineaments to their present size, throughout geologic time.
The physico-chemical instability of an original simatic crust of the earth beneath an atmosphere and hydrosphere—an instability stemming from the sun’s radiation—results in weathering, physical and chemical sorting, sedimentation, loading and depression of the ocean floors, unloading and uplift of the continental segments with deep burial of sediments, their deformation, metamorphism, and finally ultrametamorphism with the formation of the diorite-granite suite of plutonic intrusives, largely derived from sediments, but receiving accessions of simatic invasions through geologic time.
Isostasy is a dominating link in this evolution.
Thus, of all the discontinuities within the gravitational field of the earth, the most important is regarded as that between the atmosphere and hydrosphere above and the lithosphere below.
The density distribution within the earth indicates gravitational adjustment during the early stages of the earth’s evolution when temperatures were higher and the subsurface was more mobile.
The earth’s figure, an oblate spheroid of revolution, fits the observed density distribution of an earth wherein there prevails essentially hydrostatic stress distribution from the center outward almost to the surface.
This stable figure of equilibrium has been maintained for billions of years and is being maintained today, against disturbing forces, in the region of the M discontinuity.
The imbalance between light sialic, essentially sedimentary continental segments of the crust buoyed up dynamically by the heavier simatic segments of the ocean basins results in stresses directed toward the ocean basins, in spreading of continental segments, and in their invasion by simatic magma.
Creep of heavy simatic material from beneath the ocean floors toward and beneath light, eroded, rising continental segments pari passu with depression of ocean floors or continental borders by geosynclinal loading results in stresses partly directed toward the continents. This directed stress is opposed by that residing in continental creep toward the ocean basins. Thus a stress couple initiates thrust faulting, gravitational sliding, extended deformation of geosynclinal depressions, and also invites volcanism.
Thus many volcanic island arcs are linked in origin with geosynclinal depressions and illustrate progressive capture of ocean basins by continental segments, as the geosynclinals are folded, metamorphosed, and progressively uplifted. The intercepting, fringing Asiatic arcs are illustrations of this world-wide process.
Volcanism is linked to the potential energy of the earth’s residual heat, to gravitation, to the effects of the temperature-pressure gradient on melting of rocks, and to imbalance between continental and oceanic segments.
Volcanism on the ocean floors implies subsidence of these floors.
The concentration of useful and precious metals is linked to the evolution of continental segments. Likewise the concentration of radioactive minerals in sedimentary rocks and sialic intrusives.
The processes of continental evolution outlined imply continuous deformation of the earth’s surface, in time, but not in place.