True/false- a porphyritic igneous rock consist of xenoliths in a fine-grained groundmass?

False. A porphyritic rock consists of a fine grained groundmass (this is the main content of the rock and may be so fine grained that individual crystals are impossible to distinguish with the naked eye) and larger crystals(called phenocrysts). It is the presence of the phenocrysts that make a rock porphyrytic. The two different sizes of crystals( the groundmass and the phenocrysts) tell you that cooling of this volcanic rock occured in two stages. Typically the phenocrysts start to form in the molten rock deep below the surface where higher temperatures alow the formation of larger crystals, then the molten lava containing the phenocrysts is erupted at the surface and cools rapidly giving the rest of the rock( the groundmass) its fine grained texture. Xenoliths on the other hand are bits of another rock (not individual crystals) that become incorporated into magma, often as it forces its way to the surface (a dyke) or between two sedimentary layers (a sill).


Hope this helps.True/false- a porphyritic igneous rock consist of xenoliths in a fine-grained groundmass?
Xenoliths are foreign bodies captured by the magma (bits of wall rock, basically). Their presence is unrelated to the term porphyry.True/false- a porphyritic igneous rock consist of xenoliths in a fine-grained groundmass?
Yup, it is broken and fractured stuff in a solidified mass.


true.