<P> A laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock . The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom - like form with a generally planar base . </P> <P> Laccoliths tend to form at relatively shallow depths and are typically formed by relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite . Cooling underground takes place slowly, giving time for larger crystals to form in the cooling magma . The surface rock above laccoliths often erodes away completely, leaving the core mound of igneous rock . The term was first applied as laccolite by Grove Karl Gilbert after his study of intrusions of diorite in the Henry Mountains of Utah in about 1875 . </P>

Where do we find laccoliths in south africa