Rhyolite is cryptocrystalline, which means that the size of the crystals is too small to be seen even under polarized microscope. It has a SiO2 content of 65 % or higher. The counterpart of rhyolite formed deeply underneath the surface is called granite. The rhyolite in Baraboo area has two different colors: red and black. Whitish veins (vertical direction) can be seen in the red rhyolite.
Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lavas. They can also occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra or of ignimbrites.
During the second millennium BC, rhyolite was quarried extensively in what is now eastern Pennsylvania in the United States. Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site in Adams County, where as many as fifty small quarry pits are known.
Types of rhyolite rock
Rhyolite Rock (video)