The Extrageosynclinal Type of Magmatic Development

Abstract

Judging from the present, magmas do not appear under conditions of tectonic calm. Any appearance of modern magmatism or magmatism of the recent geologic past has been associated with zones that are to some extent tectonically active. We cannot determine the conditions of massive outpourings of trap on platforms or the penetration of large masses of the same basaltic magma into the body of a platform. However, it is hard to imagine that such vigorous phenomena might appear under calm conditions. Outpourings of plateau basalts relatively near us in time clearly took place against a background of rather active tectonics: movement along steep-angle faults, formation of deep fractures, and the like. At least the eruptions of Iceland and Ethiopia have been of this character. Outside platforms, within zones of culminated folding, there is no doubt about such conditions, but here the volume of erupted lava has generally been less than on platforms.