What is the difference between phaneritic and porphyritic rocks?
- Valaya, Tucson, AZ
Phaneritic: any coarse-grained igneous rock, often intrusive, usually formed as a result of a longer cooling history (ex. granite, gabbro).
Porphyritic: an igneous rock with one mineral (called the phenocryst) exhibiting a grain size larger than the remainder of the minerals (called the groundmass). Porphyritic textures are most common in volcanic rocks where one of the minerals will begin to crystallize prior to eruption (therefore producing larger grains), with the rest cooling more rapidly after eruption. I compare them to chocolate-chip cookies in class. Examples include hornblende andesite porphyry, rhyo-dacite porphyry, and many others.
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