- A batholith (from Ancient Greek bathos ‘depth’ and lithos ‘rock’) is a large mass of intrusive igneous rock (also called plutonic rock), larger than 100 km2 (40 sq mi) in area, that forms from cooled magma deep in the Earth’s crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such as granite, quartz monzonite, or diorite (see also granite dome).