Figure 29. Turner Falls in the Arbuckle Mountains.
Site 6. Colbert Rhyolite.
Rhyolite is an extrusive igneous rock
with a mineral composition similar to granite (feldspar, quartz and minor mafic
minerals). Colbert Rhyolite forms a peak overlooking Turner Falls in the
Arbuckle Mountains (the high ground to the upper left in the photo below).
Figure 29. Turner Falls in the Arbuckle Mountains.
Outcrops of the rhyolite are fine-grained and, unlike
sedimentary rocks, have no obvious layering. A typical outcrop is shown below.
Figure 30. Outcrop of Colbert Rhyolite (small boy for scale).
Like most extrusive igneous rocks, the individual mineral
crystals are microscopic, forming a fine-grained groundmass. In the
Colbert Rhyolite, however, there are common feldspar phenocrysts (larger
crystals), which are large enough to be seen with the naked eye.
Figure 31. Close up of Colbert Rhyolite showing feldspar phenocrysts (nickel for scale).
Site
7. Tishomingo Granite.