laccolithit

Note: Laccolithit is not a standard term in geology; the familiar term is laccolith. The following describes laccoliths.

A laccolith is a tabular, concordant igneous intrusion that forms between layers of sedimentary rock. As magma is injected between strata, it accumulates and inflates the surrounding layers, creating a distinctive dome or lens-shaped bulge in the overlying rocks. This uplift gives laccoliths their characteristic mushroom-like cross-section, with a relatively flat or gently curved floor and a domed roof.

Formation and structure: Laccoliths originate when viscous magma intrudes between sedimentary layers and compresses the surrounding rocks. The intrusion expands laterally, pushing the overlying strata upward rather than cutting across them. The contact with the host rocks is typically sharp at the margins and baked zones or contact metamorphism can occur around the intrusion. The magma usually crystallizes into coarse-grained rocks as it cools slowly within the crust.

Composition and texture: Laccoliths are commonly composed of felsic to intermediate rocks, such as granodiorite, granite, diorite, or rhyolite, reflecting relatively silica-rich magmas. The interior may exhibit varying grain sizes, with coarser textures near the center due to slower cooling, and fresher margins where the heat is more limited.

Size and occurrence: Laccoliths are smaller than large batholiths and can extend over kilometers laterally while being tens to hundreds of meters thick. They occur in tectonic settings where magma can exploit available pathways between sedimentary layers, such as continental margins and rift zones.

Significance: Laccoliths record episodes of magma intrusion and crustal deformation, offering insights into magma supply, crustal structure, and the geothermal history of the region. A well-known example is the laccolith family exposed in the Henry Mountains of Utah, USA.