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March 16, 2007, Newsletter Issue #131: Texture - Porphritic
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Tip of the Week
If an igneous rock cools very slowly, it has individual mineral grains that you can see with the unaided eye. If an igneous rock cools quickly, it has very small mineral grains that you canīt see without a hand lens or microscope.
But what if an igneous rock seems to be pretty much all one color (say, light gray) but is scattered throughout with mineral grains, too?
Chances are that rock is a porphyry, that is, it has porphritic texture. (Remember that texture refers to the size, shape, and arrangement of mineral grains.)
A porphry has undergone two stages of cooling - first, it began to cool slowly, and individual minerals began to form (theyīll be called "phenocrysts" in the porphyritic rock thatīs going to result). Then something happened - perhaps pressure built up in the magma chamber, and the volcano blew. So the rest of the magma cools quickly into a fine-grained "groundmass".
Result: a fine-grained groundmass that might look like a solid gray or whitish rock, with scattered bits of mineral grains (the phenocrysts) throughout.
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