rhyolite porphyry - student specimens of a pink rhyolite with large feldspar phenocrysts - UNIT OF 5 SPECIMENS
The large feldspar phenocrysts should tell students that this cooled for some time at depth, allowing the large white feldspar crystals to form at high temperatures. The magma was then brought to the surface where rapid cooling froze the crystals of the remaining minerals before they had time to grow. Two stages of cooling are typical of porphyries. Phenocrysts, the larger crystals that formed at depth, are named from the Greek word phainos for appear.
Rhyolite is the volcanic surface-cooled equivalent of plutonic granitic rocks which entirely cool at depth, and is rich in silica. This makes rhyolite flows viscous - they do not flow over wide areas as does silica-poor basalt. Rhyolite varies widely in appearance, and is often stained by impurities.
Because of the rapid cooling, the crystalline nature of rhyolite is often hard to detect. Tuff, welded volcanic ash with the same composition, can look similar, though tuffs often have angular clasts (broken fragments) of other igneous rocks that were torn from the walls of the vent welded into the tuff. This particular rhyolite porphyry has very large feldspar crystals, and smaller ones of black biotite mica (six sided) and hornblende (black, rectangular) as well as glassy sanidine - a feldspar. Huge blocks of this porphyry are buried in a pyroclastic flow that is mostly pumice.
Potassium-argon ages from nearby rhyolites in ash flows in the Coso Formation yield ages of around 3.4 million years. These pumice flows represent a single eruption or a series of eruptions in the Coso Range just east of Haiwee Reservoir, the water from which flows to perpetually thirsty Los Angeles. The area is geologically active, with a geothermal plant not far from where these specimens were collected, but inside the China Lake Naval Weapons Test Station, generating enough electricity that the U.S. Navy sells the excess to the commercial grid.
If you are a science or earth science teacher purchasing this as a teaching specimen for your class, consider having your students compare this with a rhyolite porphyry that cooled for a shorter time at depth. We have examples with very small feldspar phenocrysts that spent little time cooling at depth.
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