Laccolith

A laccolith is a mushroom-shaped mass of igneous rock, typically formed from magma that has intruded between two layers of sedimentary rock. The magma's viscosity prevents it from flowing far, and instead it domes the overlying strata, creating a bulge on the Earth's surface. Laccoliths are a type of concordant intrusion, meaning they parallel the existing rock layers.

The formation process begins with magma rising through the crust, often along pre-existing weaknesses like faults or fractures. Upon encountering a layer of relatively soft or pliable sedimentary rock (such as shale or sandstone), the magma may spread laterally rather than continuing vertically. The pressure from the intruding magma causes the overlying layers to bend upward, creating a domed structure. As the magma cools and solidifies, it forms a laccolith.

Over time, erosion can strip away the overlying sedimentary layers, exposing the igneous core of the laccolith. Because the igneous rock is often more resistant to erosion than the surrounding sedimentary rock, the laccolith may stand as a prominent hill or small mountain.

The size of laccoliths varies considerably, ranging from a few meters to several kilometers in diameter. Their composition is typically felsic to intermediate, such as diorite or monzonite, reflecting the higher viscosity required to form the characteristic domed shape.