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A laccolith is a geological feature with a distinctive dome-shaped appearance. It forms when molten magma, which is less dense than the surrounding rock, intrudes into the Earth’s crust and forces the overlying rock layers to arch upward, creating a bulge or dome-like structure.

Granite laccolith
Granite laccolith. Al Copley photo.

Description: A laccolith is a geological landform that results from a specific type of igneous intrusion. It is typically composed of igneous rock, such as granite or diorite. The formation process begins deep within the Earth’s crust when molten magma, generated in the mantle, rises towards the surface. However, unlike volcanic eruptions where magma reaches the surface and forms volcanoes, laccoliths occur when the magma becomes trapped at a shallow depth below the surface.

The critical characteristic of a laccolith is its dome-shaped or mushroom-like appearance. As the molten magma intrudes into the surrounding sedimentary or metamorphic rocks, it encounters resistance from the denser overlying rock layers. This resistance causes the magma to push upwards, creating a bulge or uplift in the Earth’s crust. Over time, as the magma cools and solidifies, it forms a solid, lens-shaped intrusion with a flat base and a curved or domed upper surface.

Laccoliths can vary in size from a few meters to several kilometers in diameter and may be exposed at the Earth’s surface due to erosion or uplift of the surrounding rocks. Some famous examples of laccoliths include the Henry Mountains and La Sal Mountains in Utah.

They provide insights into the processes of magma intrusion, and crustal deformation.