The thickness of the continents

Abstract

The continents are distinguished from the oceanic basins not only by a difference of elevation, but by a difference of composition and structure. The evidence is partly from seismology, partly from observations of gravity, and partly from measurements of thermal gradients at the surface. The information about the Eurasian continent is at present more closely co-ordinated than for the rest of the world. Below the continents and the ocean floors there seems to be ultrabasic rock (in which the velocity of sound is 8.1 km per sec.) extending to a great depth. The Atlantic floor, where explored seismically, consists probably of 1 km of sediments resting on some 6 km of basic rock. The continent of Eurasia is not of uniform thickness; below the comparatively thin layer of sedimentary rocks there appears to be 24 km of granitic rock underlain by 8 km of basic rock, in which the respective velocities of sound are 6.3 and 5.4 km per sec. It is, however, by no means established that the basic layer exists in all parts of the continent; but the average total thickness of Eurasia is about 33 km. The evidence for these statements is mainly seismological, with some general support from thermal and gravity data. The North American continent seems to be somewhat thicker, perhaps 36–40 km. The estimated thickness in the Transvaal is 38 or 34 km according as the existence of a basic layer below the granite is, or is not, postulated. A little evidence about the layering of Europe is forthcoming from the transmission of direct current through the ground. Many more studies, preferably using seismic, gravimetric, and thermal techniques closely co-ordinated, are needed before any considerable advance may be expected.