Compressional mountains of the Appalachian type reveal a pattern and a cycle of events so consistent as to suggest strongly a common genesis.
The cycle starts with the upwarping of a relatively large area which, if above sea level, is accompanied by a downwarp around its margins—a geosyncline—which serves as a catchment basin for sediments. After a considerable time, crustal movement off the upwarp causes thrusting and crumpling at its margins, overriding and folding any sedimentary rocks along the side of the geosyncline nearest the upwarp. After halts enduring for perhaps a geologic period, thrusting is repeated one or more times from the same direction. Compelling evidence indicates the presence of magma beneath the upwarped area.
Such a succession of events occurring in a definite order cannot be fortuitous. It demands a common cause for mountain systems of the compressional type.
A mechanism which seems best to explain the events and sequences of the cycle is as follows: Atomic heating expands the crust and subcrust and melts a portion of the crust within a limited area, causing a domed regional uplift on a foundation of molten material having no permanent strength. Erosion of the uplifted area causes isostatic transfer, initiating an adjacent downwarp whose sinking is accentuated as it is filled with sediment. The crust creeps slowly down the slopes of the dome, and eventually thrust-faults toward the downwarp and folds its sedimentary rocks. Repeated movements occur, but finally crustal sliding off the dome causes tension and block-faulting in its central parts and copious emission of lavas and escape of heat. After this final orogenic spasm, the lateral creep of the crust ceases and the upwarped area subsides as the magma beneath it cools and congeals.