rhyolite porphyry - gray rhyolite porphyry with large orthoclase phenocrysts. Textbook example of two-stage cooling. Student unit of 5 specimens
Rhyolite is a silica rich extrusive igneous rock with a composition similar to that of granite.
Porphyries exhibit two stages of cooling - slow cooling deep underground and then more rapid cooling on or near the surface. The large white crystals are orthoclase, a potassium feldspar that forms crystals at high temperatures. The smaller surrounding crystal matrix is made up of crystals of quartz, sanidine and biotite. The biotite crystals are six-sided and easily seen under a hand lens.
An earth science student should recognize this as a porphyry, with large phenocrysts surrounded by very much smaller crystals and that the two sizes of crystals, large surrounded by small, shows that this rock cooled in two distinct stages. The student should be able to tell you that the large crystals formed deep underground, where cooling was slow. The magma then was injected to the near-surface or extruded as lava. Cooling was then very fast, forming the surrounding mass of smaller crystals that are almost individually invisible to the naked eye. The student should also identify the phenocrysts as a feldspar.
Phenocrysts (pheno = appear in Greek) are the typical characteristic of a porphyry. Granite can also be porphyritic when feldspar phenocrysts formed in a deep magma that was later brought closer to the surface where it finally cooled and crystallized.
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 nearby geothermal plant inside the China Lake Naval Weapons Test Station generating enough electricity that the U.S. Navy can sell 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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