laccoliths

Laccoliths are a type of concordant igneous intrusion formed when magma, typically viscous felsic or intermediate, intrudes between layered sedimentary rocks. Rather than cutting across the layers as a dike would, the intrusion spreads laterally, lifting the overlying strata to form a dome or cap. The resulting body is often lens- or mushroom-shaped, with a relatively flat floor and a convex roof. The term laccolith denotes this domed intrusion and was coined in the 19th century to describe such structures.

As magma pools and crystallizes, the pressure inflates the overlying layers, creating a characteristic dome. Emplacement occurs at shallow depths, typically a few kilometers or less; thickness can range from tens to a few hundred meters, with lengths of several kilometers.

Laccoliths commonly derive from silica-rich magmas with high viscosity such as rhyodacite, rhyolite, or andesite—though compositions vary. Cooling yields porphyritic textures, and intrusions can cause contact metamorphism and hydrothermal activity in the surrounding rocks.

Henry Mountains in southern Utah are among the best-known laccoliths; the La Sal Mountains near Moab, Utah, also represent laccolithic intrusions. Similar structures occur in the western United States and elsewhere.

They are studied to understand igneous intrusion processes, magma migration, and crustal tectonics. Erosion exposes laccoliths, aiding mapping and dating and revealing their contact relationships with surrounding sedimentary rocks.