Elastic or plastic buckling of the Earth’s crust

In a very simplified form one could say that with the exception of the Undation theory of van Bemmelen (1933a—d, 1948, 1952) and suggestions by Glangeaud (1947, 1948) the orogenic forces are generally sought in a lateral compression either by contraction due to cooling (Jeffreys, 1952) or under the influence of convection currents (Vening Meinesz, 1934, 1950; Griggs, 1939; Hess, 1948, etc.) below the crust. In the original conception of Vening Meinesz the buckling started as an elastic deformation but since Bijlaard (1938) showed that plastic buckling oblique to the main stress direction was more probable, Vening Meinesz changed over to this view. The difficulty in the elastic buckling hypothesis was the enormous stress (40—50.000 kg/cm²) needed to buckle a crust of some 30 km thickness, a force which long before the elastic waves were formed would have crushed the material. With Smulochowski (1909) Vening Meinesz considered the possibility that a layered crust (some 15 layers would be needed) could be elastically deformed by a much smaller stress (3000 kg/cm²), but there was no reason at all to expect such discontinuity surfaces in the crust, and the idea had to be abandoned. The buckling, whether plastic or elastic, was proposed in order to explain the negative anomaly zones, found by Venino Meinesz to be runningparallel and next to the great Indonesian island arcs. The depth to which the buckle of sialic crust penetrates into the sima, causing the mass defiency measured at the surface, depends in this theory on the total compression and on the age of the phenomenon, because fusion at the bottom of the buckle would diminish its size gradually. Therefore the root below the Alps had its size decreased since the major paroxysms in post Eocene and Miocene times.