Rhyolite

Last modified by Roelof Zwaan on 2021/09/30 15:18

Rhyolite is a fine-grained extrusive igneous rock. Rhyolite consist of quartz, plagioclase and sanidine which gives its pink or gray color. Grains can't be spotted with the naked eye due to the fact that the lava cooled very rapidly and minerals had no time to develop. Rhyolite forms from granitic magma in continental or continent-margin volcanic eruptions. Rhyolite doesn't form at ocean ridges in contrary to other extrusive igneous rocks.

Characteristics

When granitic magma erupts it produces certain rock types where gas can be present in the rock. In rhyolite this produces vugs which contain crystals, opals or glassy material.

Rhyolite can have a various amount of minerals present where plagioclase and quartz and sanidine are abundant. There may also be some hornblende or biotite present. A more detailed description can be found in the picture on the right. Rhyolite is an intrusive rock which means phenocrysts can be present. This are large crystals suspended in a matrix of fine crystals. These are present due to mixing of other rocks into the cooling parent magma.

Environment

Eruptions of granitic magma can produce rhyolite, pumice, obsidian, or tuff. These rocks have similar compositions but different cooling conditions. Explosive eruptions produce tuff or pumice. Effusive eruptions produce rhyolite or obsidian if the lava cools rapidly. These different rock types can all be found in the products of a single eruption.

Granitic magmas are rich in silica and often contain up to several percent gas by weight. As these magmas cool, the silica starts to connect into complex molecules. This gives the magma a high viscosity and causes it to move very sluggishly.

When this sluggish magma cools it can also form unstable lava domes. This domes can be very dangerous due to the fact that they can break at any moment and that results in a large explosion.