A laccolith is an igneous intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. 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 tend to form at relatively shallow depths and are typically formed by relatively viscous magmas, 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.

It is often difficult to reconstruct shapes of intrusions. For instance, Devils Tower in Wyoming was proposed to be the remnants of an ancient laccolith. The rock would have had to cool very slowly so as to form the slender pencil-shaped columns of phonolite porphyry seen today. However, erosion has stripped away the overlying and surrounding rock, and so it is impossible to reconstruct the original shape of the igneous intrusion; that rock may not be the remnant of a laccolith. At other localities, 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.

See also

• Batholith
• Dike
• Sill

References

• Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., pp. 13-15, Freeman, ISBN 0-7167-2438-3
• Jules D. Friedman and Curtis Huffman, Jr., coordinators, Laccolith Complexes of Southeastern Utah: Time of Emplacement and Tectonic Setting -- Workshop Proceedings, United States Geological Survey Bulletin 2158, 1998. http://pubs.usgs.gov/bul/b2158/B2158.pdf