One of my flaws as a geologist and rock collector is failing to label and clearly identify all of my samples. Today’s deskcrop is a great example of a rock with a hypocrystalline (partly glass, partly crystalline) and porphyritic (two distinct grain sizes – plagioclase phenocrysts in a glassy matrix) texture. I’m pretty sure this sample comes from Mount Lassen, which should make it dacitic in composition (despite its very dark color) – compare it to yesterday’s porphyritic hornblende dacite to see just how different rocks named “porphyritic dacite” can look! – but I’m just not certain because I never labeled it and it’s been separated from the rocks I originally collected it with. I have had the opportunity to hike to the summit of Lassen and I recall rocks there that looked very much like this, but whether this sample originates there or somewhere else in the Cascades, I cannot say for sure. In any case, it makes a useful teaching sample.
Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!