A laccolith is a body of intrusive igneous rock that forms from magma pushing its way into the Earth’s crust but not erupting onto the surface. This material cools and solidifies entirely beneath the ground, becoming a type of pluton. The formation is governed by the magma inserting itself between existing layers of sedimentary rock. This geological process creates a distinct structure where the overlying rock is significantly deformed.
Defining the Laccolith Structure
A laccolith is characterized by a distinctive geometry that resembles a mushroom or a dome when viewed in a geological cross-section. The magma intrusion is generally concordant, meaning its base lies parallel to the bedding planes of the underlying rock strata. This flat floor is a defining feature that distinguishes it from other types of intrusions.
The magma forces the overlying rock layers to arch upward into a convex, dome-like shape. This uplift can cause the surrounding sedimentary layers to be tilted at the edges of the intrusion. Laccoliths are relatively small features in the crust, often less than 16 kilometers in diameter, with a thickness ranging from hundreds to a few thousand meters. The relationship between the flat base and the domed roof creates what geologists describe as a plano-convex structure.
The Process of Formation
The formation of a laccolith begins when magma rises through a vertical fracture or conduit in the Earth’s crust. As the magma ascends, it encounters a plane of weakness, such as the contact between two sedimentary rock layers, and begins to spread horizontally. This initial spreading creates a sheet-like intrusion similar to a sill.
The further development into a laccolith depends on a combination of high pressure and the physical properties of the magma. The molten rock is highly viscous, which prevents it from spreading out into a thin, wide sheet. Because it cannot flow easily, the continuously supplied magma accumulates in one spot, building up intense pressure on the overlying rock.
This internal pressure forces the rock above to deform plastically, arching it upward into a dome without breaking through to the surface. The magma solidifies within this created chamber, forming the hard igneous rock core of the laccolith. The entire process of formation can occur relatively quickly over a few months through a single injection, or incrementally over thousands of years through multiple pulses of magma.
Notable Examples and Geographic Context
Laccoliths are found in areas where the Earth’s crust is being subjected to compression, which limits the ability of the magma to fracture the rock vertically. This setting promotes the lateral spreading and subsequent upward doming between pre-existing, stable sedimentary layers. The most famous and widely studied examples are located in the western United States, particularly across the Colorado Plateau.
The Henry Mountains in Utah are the classic example, where geologist Grove Karl Gilbert first described and named this type of intrusion in the 1870s. Other prominent laccolithic mountain ranges in Utah include the La Sal Mountains and the Abajo Mountains. Torres del Paine in Patagonia, Chile, is another spectacular example, showcasing a well-exposed, solidified magma chamber.