A laccolith is a dome-like intrusion of igneous rock, usually arching the overlying strata and with an approximately flat base. Laccoliths are generally less than 16 kilometers (10 miles) in diameter with thickness of 30–900 meters (100–3,000 feet). They are formed of magma; they are smaller than batholiths.

Phacoliths are similar but lens-shaped, with a concave side facing downward and a convex side facing upward. See also dike and sill.