Is porphyritic texture always indicative of a 2 stage cooling process?

Question

Is porphyritic texture always indicative of a 2 stage cooling process? Can't the phenocrysts and groundmass be formed at the same time depending upon the chemical constituents forming the rock? The groundmass being made up of constituents least in proportion and the phenocrysts being made up of the constituent in greatest proportion?

Answer 1

Yes, in most cases, a porphyritic texture will always indicate that the rock went through at least two stages of cooling. Phenocrysts are crystals much larger in size relative to the surrounding matrix and require a longer residence time in the magma chamber to grow (thus, having a slower cooling rate). During eruption, the remaining liquid phase in the magma chamber is rapidly cooled to produce the groundmass (usually composed of microphenocrysts). The proportion of phenocrysts to groundmass can vary from rock to rock, but volcanic rocks with a porphyritic texture typically have a greater abundance of groundmass.