Intrusive Igneous Activity
- The structures that result from the emplacement of igneous material at depth are called plutons, named for Pluto, the god of the lower world in classical mythology.
- Igneous rocks are classified according to their shape as either tabular or massive and by their orientation with respect to the host rock.
- Plutons are said to be discordant if they cut across existing structures and concordant if they form parallel to features such as sedimentary strata.
- Dikes are tabular discordany bodies that are produced when magma is injected into fractures.
- Sills are tabular plutons formed when magma is injected along sedimentary bedding surfaces.
- Columnar joints form as igneous rocks cool and develop shrinkage fractures that produce elongated, pillarlike columns.
- Loccoliths are similar to sills because they form when magma is intruded between sedimentary layers in a near-surface environment.