A laccolith is a large blister or igneous mound with a dome-shaped upper surface and a level base fed by a pipe like conduit from below.[1] [2] They form after an initial sheet-like intrusion (or concordant pluton) has been injected within or between layers of sedimentary rock (when the host rock is volcanic, the laccolith is referred to as a cryptodome[3]). The pressure of the magma is high enough that the overlying strata are forced upward and folded, giving the laccolith a dome or mushroom-like form (or possibly conical or wedge-shape) with a generally planar base. Over time, erosion can form small hills and even mountains around a central peak since the magma rock is likely more resistant to weathering than the host rock. The growth of laccoliths can take as little as a few months when associated with a single magma injection event,[4][5] 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.[6]

Basic types of intrusions:
1. Laccolith
2. Small dike
3. Batholith
4. Dike
5. Sill
6. Volcanic neck, pipe
7. Lopolith
Note: As a general rule, in contrast to the smoldering volcanic vent in the figure, these names refer to the fully cooled and usually millions-of-years-old rock formations, which are the result of the underground magmatic activity shown.
Cross section of a laccolith intruding into and deforming strata

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