A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apart the host rock strata. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith its dome-like form. Over time, erosion can expose the solidified laccolith, which is typically more resistant to weathering than the host rock. The exposed laccolith then forms a hill or mountain. The Henry Mountains of Utah, US, are an example of a mountain range composed of exposed laccoliths. It was here that geologist Grove Karl Gilbert carried out pioneering field work on this type of intrusion. Laccolith mountains have since been identified in many other parts of the world. Description A laccolith is a type of igneous intrusion, formed when magma forces its way upwards through the Earth's crust but cools and solidifies before reaching the surface. Laccoliths are distinguished from other igneous intrusions by their dome-shaped upper surface and level base. They are assumed to be fed by a conduit from below, though this is rarely exposed. When the host rock is volcanic, the laccolith is referred to as a cryptodome. Laccoliths form only at relatively shallow depth in the crust, usually from intermediate composition magma, though laccoliths of all compositions from silica-poor basalt to silica-rich rhyolite are known. A laccolith forms after an initial sheet-like intrusion has been injected between layers of sedimentary rock. If the intrusion remains limited in size, it forms a sill, in which the strata above and below the intrusion remain parallel to each other and the intrusion remains sheetlike. The intrusion begins to lift and dome the overlying strata only if the radius of the intrusion exceeds a critical radius, which is roughly: where is the pressure of the magma, is the lithostatic pressure (weight of the overlying rock), is the thickness of the overlying rocks, and is the shear strength of the overlying rock. For example, in the Henry Mountains of Utah, US, the geologist Grove Karl Gilbert found in 1877 that sills were always less than in area while laccoliths were always greater than 1 square kilometer in area. From this, Gilbert concluded that sills were forerunners of laccoliths. Laccoliths formed from sills only when they became