Rhyolite is an extrusive or volcanic rock which is generally fine-grained and possesses a felsic composition. Rhyolite is ususally light gray to pink in color, and it can form in two different ways. The most spectacular way to form rhyolite is to have a very explosive eruption which hurls hot lava and fragments of the volcano into the atmosphere. Gravity eventually brings these pyroclasts back down to Earth's surface where they are deposited as layers. If the pyroclasts are hot enough, they might weld together to form a rock shown in picture 1 called rhyolite tuff (composed of pyroclasts and fragments less than two millimeters across). Also formed in a similar manner and shown in picture 1 is the rock called rhyolite breccia (composed of pyroclasts and fragments greater than two millimeters across).
Picture 2 shows an outcrop of rhyolite in New Mexico. Native Americans carved dwellings into this relatively soft rock about 1000 years ago in what is now called Bandolier National Monument.
Picture 3 shows a closeup of the Bandolier rhyolite which is both tuff and breccia.
Rhyolite can also form due to the eruption of cool and thick (high-viscosity) lava as a slow, toothpaste-like flow. Such eruptions can result in rocks having frothy or glassy textures as shown in picture 4.
| 1
| 2
| 3
| 4
|
| rhyolite tuff and breccia | Bandolier rhyolite | rhyolite outcrop closeup | rhyolitic flow |
CHOOSE YOUR NEXT IGTNEOUS ROCK CARD