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Rhyolite

A fine-grained extrusive volcanic rock, similar to granite in composition and usually exhibiting flow lines.

A very light-colored, aphanitic (not visibly crystalline), volcanic rock that is rich in silica and broadly equivalent to granite in composition. Migration of rhyolitic magma through the Earth's crust, which causes much of the Earth's explosive and hazardous volcanic activity, represents a major process of chemical fractionation by which continental crust grows and evolves.

Igneous rock that is the volcanic equivalent of granite, whose chemical composition is similar. Rhyolites are known from all parts of the Earth and from all geologic ages; they are found mostly on the continents or their immediate margins, but small quantities have been described from remote islands.

Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite, giving it a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende.

It is composed of the light-colored silicates and is usually buff to pink in color. Occasionally it is light gray. In texture it is classified as aphanitic. It frequently contains voids and glassy fragments, evidence of having formed in a surface environment with rapid cooling. Rhyolite is much less common than granite, but there are large lava flows and deposits of rhyolite in Yellowstone Park, Wyoming, USA.

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 called rhyolite tuff (composed of pyroclasts and fragments less than two millimeters across). Also formed in a similar manner is the rock called rhyolite breccia (composed of pyroclasts and fragments greater than two millimeters across).

Material Notes:
Rhyolite is an igneous rock with a fine texture and grain size < 1 mm (extrusive). It is acidic, with a total silica content greater than 65%. Felsite is generally lighter in color than andesite.

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