Intrusive Structures

Intrusions are also classified according to size, shape, depth of formation, and geometrical relationship to the country rock. Intrusions that formed at depths of less than 2 kilometers are considered to be shallow intrusions, which tend to be smaller and finer grained than deeper intrusions.

Dikes. A dike is an intrusive rock that generally occupies a discordant, or cross‐cutting, crack or fracture that crosses the trend of layering in the country rock. Dikes are called pegmatites when they contain very coarse‐grained crystals—a single such crystal can range in size from a few centimeters to 10 meters in diameter.

Sills. Sills are formed from magmas that entered the country rock parallel to the bedding (layering) and are thus concordant with the country rock. Sills can sometimes look like volcanic flows that were interbedded with sedimentary units.

Laccoliths. A laccolith resembles a sill but formed between sedimentary layers from a more viscous magma that created a lensshaped mass that arched the overlying strata upward.

Volcanic necks. A volcanic neck is the rock that formed in the vent of a volcano at the end of its eruptive life and remains “standing” after the flanks of the volcano have eroded away.

Plutons. Plutons are discordant intrusive rocks that formed at great depths. They tend to be large, coarse grained, and irregular in shape. If the intrusion occupies less than 100 square kilometers (60 square miles) at the earth's surface it is called a stock; if it is larger than 100 square kilometers, it is termed a batholith. Batholiths are usually composed of granite. They have formed over long periods through the accumulation of smaller magma blobs called diapirs, which result from localized melting of the crust; the diapirs then slowly move upward toward the surface and coalesce into a larger mass. Granitic batholiths usually form the cores of mountain complexes and are a result of plate tectonic action.