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.

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 ${\displaystyle P_{m}}$ is the pressure of the magma, ${\displaystyle P_{l}}$ is the lithostatic pressure (weight of the overlying rock), ${\displaystyle T}$ is the thickness of the overlying rocks, and ${\displaystyle \tau }$ 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 1 square kilometer (0.4 sq mi) 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 large enough for the pressure of the magma to force the overlying strata to dome upwards. Gilbert also determined that larger laccoliths formed at greater depth. Both laccoliths and sills are classified as concordant intrusions, since the bulk of the intrusion does not cut across host rock strata, but intrudes between strata.

More recent study of laccoliths has confirmed Gilbert's basic conclusions, while refining the details. Both sills and laccoliths have blunt rather than wedgelike edges, and sills of the Henry Mountains are typically up to 10 meters (33 ft) thick while laccoliths are up to 200 meters (660 ft) thick. The periphery of a laccolith may be smooth, but it may also have fingerlike projections consistent with Rayleigh-Taylor instability of the magma pushing along the strata. An example of a fingered laccolith is the Shonkin Sag laccolith in Montana, US. The critical radius for the sill to laccolith transition is now thought to be affected the viscosity of the magma (being greater for less viscous magma) as well as the strength of the host rock. A modern formula for the shape of a laccolith is:

where ${\displaystyle z}$ is the height of the laccolith roof, ${\displaystyle g}$ is the acceleration of gravity, ${\displaystyle B}$ is the elastic modulus of the host rock, ${\displaystyle r}$ is the horizontal distance from the center of the laccolith, and ${\displaystyle r_{0}}$ is the outer radius of the laccolith. Because of their greater thickness, which slows the cooling rate, the rock of laccoliths is usually coarser-grained than the rock of sills.

The growth of laccoliths can take as little as a few months when associated with a single magma injection event, or up to hundreds or thousands of years by multiple magmatic pulses stacking sills on top of each other and deforming the host rock incrementally.