28 June 2012

Thunderbird rhyolite

Posted by Callan Bentley

Today, we return to Texas, and the Franklin Mountains. Just up the hill from the Permian fusilinids, we pass through the crest of the range (here’s a view looking back to the west) and encounter different rocks…

Up there, you’ll find a variety of lithologies, but the rocks that caught my eye were a distinctive pair of rhyolites: one red, and one black:

These are rhyolites of the Thunderbird complex, which is genetically related to the Red Bluff Granite featured here previously. While the Red Bluff Granite cooled at depth, these rhyolites erupted on the surface. Their porphyritic texture makes them easy to spot as you hike up the trail south of the Woodrow Bean Transmountain Road near the crest of the ridge.

Have you ever seen a black rhyolite before? This was a first for me. The pink blotches, of course, are potassium feldspar.

One more shot, showing the two lithologies adjacent to one another for comparison purposes…

Like all my other west Texas photos, these shots come from an excellent field trip to the Franklins and other local areas led by Josh Villalobos of El Paso Community College.