Rhyolite is a fine-grained or glassy igneous rock that commonly is
extrusive or volcanic. It has a pink, red, tan, white, gray, purple, or
black color. This rock, like granite, is made up chiefly of feldspar and
a silica mineral, such as quartz, but other minerals may be present.
Both rhyolite and granite form from the same kind of molten rock
material. Nevertheless, even though their compositions are the same,
these two rocks do not look alike. Their textures differ because granite
forms slowly and rhyolite forms quickly.
Much of the Texas rhyolite formed from hot, molten lava. This lava
flowed out onto the surface either through volcanic cones or cracks in
the ground. Some of the lava cooled and hardened too quickly for mineral
grains to develop. This rapidly cooled lava formed a rhyolite rock that
is made up, at least partly, of glass. In many of the rhyolites,
crystalline mineral grains were able to form, but these grains are
extremely small, and you may not be able to distinguish them even with a
magnifying glass. Some rhyolite, because it hardened from moving,
flowing lava, has streaks and bands of different colors and textures.
This rhyolite has _flow structure_.
One variety of rhyolite has easily seen crystals and grains of minerals,
such as feldspar, quartz, and mica, scattered through a mass of the tiny
crystalline grains (in much the same way that raisins are scattered
through a cake). The easily seen crystals and grains are called
_phenocrysts_, and the rock itself is called a _rhyolite porphyry_.
Many rhyolites and rhyolite porphyries occur in the Tertiary igneous
rocks of the Trans-Pecos country of west Texas. Just a few of these
localities include the Barrilla Mountains of Jeff Davis and Reeves
counties, the Chisos Mountains of Brewster County, the Chinati Mountains
of Presidio County, and the Davis Mountains of Jeff Davis County.
Rock Crystal. _See_ Quartz.
Rock Gypsum. _See_ Gypsum.
Rock Salt. _See_ Halite.
Rose Quartz. _See_ Quartz.
Salt. _See_ Halite.