A cut block of trachyandesite lava from a volcano in Auvergne, France, used as building stone, forming part of the wall of Clermont-Ferrand cathedral, France.
Trachyandesite is an extrusive igneous rock. It has little or no free quartz, but is dominated by alkali feldspar and sodic plagioclase along with one or more of the following mafic minerals: amphibole, biotite or pyroxene. Small amounts of nepheline may be present and apatite is a common accessory mineral.
Trachyandesitic magma can produce explosive Plinian eruptions, such as happened at Tambora in 1815.