Q:

What are intrusive volcanic landforms?

A:

Intrusive volcanic landforms are collections of cooled magma that formed within the Earth's crust and were later exposed by surface erosion and uplift. The collective term for these land forms is pluton.

Intrusions are classified as concordant or discordant. Concordant intrusions are parallel to the surface. Discordant intrusions are not parallel, and some are almost perpendicular. Batholiths, stocks and dikes are discordant intrusions, and sills, laccoliths and lopoliths are concordant intrusions.

Batholiths are massive discordant intrusions that have at least 100 square kilometers of exposed surface material and form deep within the crust. They are often comprised of multiple plutons containing various types of igneous rock. Stocks are discordant intrusions that are similar to batholiths except they have less than 100 square kilometers of surface exposure. Dikes are narrow, sheet-like discordant intrusions that formed as magma rose through vertical cracks in the crust.

Sills are concordant sheet-like intrusions that form between layers of sedimentary bedding surfaces. Laccoliths are mound-like concordant intrusions. They formed between layers of sediment and developed a dome shape as magma pressure from below pushed upwards into the rock above. Surface erosion can change a laccolith's shape. Devil's Tower in Wyoming is a well-known laccolith. Lopoliths are concordant intrusions similar to laccoliths, but their center has depressed to form a saucer shape.


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