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A laccolith is a large mound of igneous rock beneath the earth's surface caused by magma intrusion, sometimes attaining the size of a mountain. The strata (rock layers) above the intrusion are arched up and sometimes cause a surface deformation. Laccoliths are formed when magma is very viscous, and instead of spreading widely, heaps up at a particular level. Well-formed laccoliths have a flat bottom and a dome-shaped upper surface, and a postulated dike-like feeder beneath its thickest point. A laccolith is roughly circular in plan, but often very irregular in shape.
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