Definitions and images to illustrate geological terms, links to images and website articles


Laccoliths are moderately large concordant intrusions that cause uplift and folding of the preexisting, country rocks above the intrusion. Laccoliths tend to form at relatively shallow depths beneath relative light sedimentary rocks, ensuring that the pressure of intruding magma is sufficiently high to lift overlying rock strata, forming a domed or mushroom-like form with a generally planar base. Laccoliths typically arise from relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite.

[images laccolith, 2, 3, 4, 5; laccolith (l) and set of sills (r); monocline and syncline; Henry Mt. laccoliths, Capitol Reef, UT, anaglyphic images (needs 3D glasses); Mt Hillers, Sawtooth Ridge; Mt Ellesworth, UT; DEM Black Mesa laccolith; diagram laccolith Black Hills, SD; Bear Butte, SD, photo; Mt. Elden laccolith; Los Cuernos; topo map of Mount Katahdin laccolith]

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