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Observations of heat flow and gravity suggest that continental structure extends to depths of the order of 500 km. The preliminary studies of surface waves tentatively confirm the existence of regional differences between continental and oceanic mantle. The distribution of earthquake foci along continental borders and the concentration of deep-focus earthquakes at the borders similarly imply differences in thermal structures extending to depths of the order of a few hundred kilometers. The deep structure of continents places heavy restrictions on any theory of continental drift. A relative motion of the continents must involve the mantle to depths of several hundred kilometers; it is no longer possible to imagine thin continental blocks "sailing" over a fluid mantle.
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