rhyolite

Imagine a volcano erupting with thick, sticky lava that cools very quickly. The rock that forms from that lava is often rhyolite. It is a type of volcanic rock, usually light-colored, often pink, pale gray, or tan. Because it cools fast, it usually does not have large crystals, so it often looks fine-grained and can sometimes look glassy. You can think of it as the volcanic cousin of granite: they are made of much the same material, but one forms at the surface and the other deep underground. Rhyolite often appears in places with old lava flows, volcanic domes, and steep rocky cliffs.

Rhyolite is an extrusive igneous rock, which means it forms from lava at or near the Earth's surface. Unlike granite, which cools slowly underground, rhyolite cools quickly after eruption. Because of that rapid cooling, large crystals usually do not have time to grow, so the rock is typically fine-grained and can sometimes be partly glassy. The name comes from a Greek word meaning "stream," a reference to the flow patterns that can appear in the rock.

In composition, rhyolite is very similar to granite. Both are rich in silica and usually contain minerals such as quartz and feldspar. Rhyolite often forms from thick, silica-rich lava, and it is common in volcanic regions. It can be found in places such as Yellowstone and other areas with a long history of volcanic activity.

Rhyolite has also been useful to people. Some forms break with smooth, curved surfaces, which can create sharp edges, so certain pieces were used for tools. Today, polished specimens may be used as decorative stone or in jewelry. Some samples contain larger crystals set in a finer background, giving them a porphyritic appearance.

So, if you see a light-colored, fine-grained rock in a volcanic area, it may be rhyolite: a fast-cooled surface rock that shares much of its makeup with granite but tells a more explosive story.