lacolitos

Laccoliths, or lacólitos in Spanish, are a class of igneous intrusions that form concordant with preexisting sedimentary layers. They are plutonic bodies that intrude between strata and push the overlying rocks upward, creating a dome-like, lens- or mushroom-shaped structure at the surface. This doming distinguishes laccoliths from sill intrusions, which remain relatively flat and do not uplift the surrounding rocks, and from dikes, which cut across strata.

Formation and characteristics: Laccoliths develop when silica-rich magma is injected between sedimentary layers and accumulates laterally. The pressure from the accumulating magma deforms and uplifts the overlying sheet of rock, producing a characteristic mushroom-like cross-section. The intrusion is usually thicker than a typical sill and often composed of felsic to intermediate rocks, such as granodiorite, diorite, or rhyolitic compositions. Contact with the surrounding rocks is typically sharp at the intrusion’s top and bottom, with a relatively flat base and a domed roof.

Geologic significance: Laccoliths provide insights into shallow-level magmatic processes and the interaction between intruding magma and surrounding sedimentary rocks. Erosion can expose these features as isolated highs, with the laccolith’s core often preserved beneath the uplifted cap of sedimentary strata.

Examples: The Henry Mountains in Utah are among the classic and well-studied laccoliths. The San Juan Mountains of southwestern Colorado contain extensive laccolithic complexes, illustrating the regional significance of this intrusion style.

See also: sill, dike, stock, pluton.