ABSTRACT
Magma is the most important concept in igneous petrology. The origin, evolution and movement of magma has a crucial influence on planetary development, Magmas are generated by diverse processes and at different times during the evolution of a planet one of these processes is likely to become dominant. Decompression melting typically occurs during the ascent of diapirs and mantle plumes, the up-arching of the lithosphere, or when faulting or fracturing causes a decrease in pressure. Geochemical, geophysical and petrologic models all require some volcanoes to have high-level magma chambers. Field data show that the country rocks may either flow plastically, or pseudoplastically, away from a magma, or they may deform by fracturing. The volatiles reduce the melting temperature of the upwelling mantle materials and as this magma moves upwards its volume is augmented by decompression melting. One might ask how magmas are able to break out of magma chambers established at their level of equivalent density.
