Trachyte

An extrusive igneous rock composed mostly of alkali feldspar, formed when lava rich in silica and alkali metals cools quickly at or near the surface. It is typically light colored and fine grained, though it can appear black where it consists mostly of volcanic glass, and it often shows a porphyritic texture with larger feldspar crystals set in a finer groundmass. Trachyte represents the silica rich product of fractional crystallization of alkaline basaltic magma, in which calcium, magnesium and iron are progressively removed as the magma cools underground, leaving a composition dominated by alkali feldspar. Its name comes from the rough, irregular surface texture caused by minute vesicles that make broken specimens feel coarse to the touch, and it is considered the volcanic equivalent of syenite, its slow cooled intrusive counterpart.