dacites

Dacite is a volcanic rock of intermediate composition, typically light in color, and intermediate in texture between andesite and rhyolite. It is an extrusive igneous rock, meaning it is formed from lava that has erupted onto the Earth's surface. Dacite is characterized by its mineral content, which usually includes plagioclase feldspar, quartz, and biotite or hornblende. Amphibole is also a common mineral found in dacite. The texture of dacite can range from aphanitic (fine-grained) to porphyritic, where larger crystals are embedded in a fine-grained groundmass.

The chemical composition of dacite falls between that of andesite and rhyolite, with a silica content generally between 63% and 68.5%. This intermediate composition results in a magma that is more viscous than basaltic or andesitic magmas, leading to more explosive volcanic eruptions. Dacite is commonly found in volcanic arcs, particularly those associated with subduction zones, such as the Andes Mountains, the Cascades, and parts of Japan. It is often associated with stratovolcanoes and can form lava flows, domes, and pyroclastic deposits. Examples of dacite formations can be found at Mount St. Helens in Washington State and in the Santorini caldera in Greece. Its occurrence indicates geological settings where intermediate magmas are generated and erupted.