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: :r \ge \frac{2T\tau}{P_m-P_l} where P_m is the pressure of the magma, P_l is the
lithostatic pressure (weight of the overlying rock), T is the thickness of the overlying rocks, and \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 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. 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: :z = \frac{3(P_m-\rho_cgT)}{16BT^3}(r_0^2-r^2)^2 where z is the height of the laccolith roof, g is the acceleration of gravity, B is the elastic modulus of the host rock, r is the horizontal distance from the center of the laccolith, and 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. Over time, erosion can form small hills and even mountains around a central peak since the intrusive rock is usually more resistant to weathering than the host rock. Because the emplacement of the laccolith domes up the overlying beds, local
topographic relief is increased and erosion is accelerated, so that the overlying beds are eroded away to expose the intrusive cores. ==Etymology==