Laccolith

A laccolith is a sheet intrusion that has been injected between two layers of sedimentary rock. It is a small area that broke away from the main area of magma but did not have the pressure to break through the surface. In other words, a laccolith is a small magma chamber which has formed near the surface. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.

Laccoliths are formed by relatively viscous magmas at relatively shallow depths, such as those that crystallize to diorite, granodiorite, and granite. Cooling underground takes place slowly, giving time for larger crystals to form in the cooling magma. The surface rock above laccoliths often erodes away completely, leaving the core mound of igneous rock. The term was first applied as laccolite by Grove Karl Gilbert after his study of intrusions of diorite in the Henry Mountains of Utah in about 1875.

At places, such as in the Henry Mountains and other isolated mountain ranges of the Colorado Plateau, some intrusions demonstrably have shapes of laccoliths. The small Barber Hill syenite-stock laccolith in Charlotte, Vermont USA, has several volcanic trachyte dikes associated with it. Molybdenite is also visible in outcrops on this exposed laccolith.