A laccolith is any of a type of igneous intrusion (or concordant pluton) that has been injected between two strata of a sedimentary bed. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like shape, the floor of which is usually horizontal.
Laccoliths tend to form at relatively shallow depths and are typically composed of granite. Cooling underground takes place slowly, giving time for larger crystals to form in the cooling rock. The surface rock above laccoliths often erodes away completely, leaving the core mound of igneous rock.
Devils Tower in Wyoming is thought to be the remants 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.
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