| Abstract: | A type of continental-oceanic transition zone, referred to as the Columbian transition zone, is distinguished from two other commonly known types of these zones. The subsidence of the Earth's crust, typical of all transition zones, is shown to be connected (by geophysical properties) to the transformation of continental crust into intermediate crust and later into oceanic. The most likely mechanisms of such changes are the basification of continental crust, its foundering, block by block, into the heated upper mantle, and its substitution by new oceanic crust. The evolution of transition zones of the Pacific type is largely influenced by deep faults, which reach down to the level of undepleted mantle. From this level, the volatile products rise to the surface which results in the formation of calc-alkali magmas on island arcs. The Benioff zones are deep faults, whose inclinations are dependent on the density contrasts in the upper mantle on either side of the Benioff zones. The denser mantle flows beneath the mantle of lower density. This phenomenon is depicted by plate tectonics as subduction.On the whole, the evolution of transition zones gives rise to the growth of the oceans at the expense of the continents, though oceanic crust becomes thicker by addition of volcanogenic layers composed of andesite, in the transition zones (type two) of the Pacific type at island arcs. |
