Plutonic Igneous Rock Bodies

When magma solidifies under the earth's surface, it forms plutonic rock bodies or plutons. These plutons are classified as tabular or massive.

Tabular plutons

Tabular plutons form when magma is forced into a crack or between sedimentary beds. They are called tabular because they are sheet-like or table-like in shape.

Sills are tabular plutons that form parallel to sedimentary beds from magma that forced its way in.  They are largely horizontal, and because thay are parallel to the beds, they are also referred to as concordant tabular plutons.

Laccoliths are also concordant tabular plutons but differ from sills in that they bulge upward like a blister.  These bulges may be the size of a mountain.  The Henry Mountains in southern Utah are an example.

Dikes are discordant tabular plutons; they form from magma that is forced into cracks rather than between beds of sedimentary rock. They are generally oriented vertically--like a wall.

Massive plutons

Massive plutons differ from tabular plutons in having more or less equal dimensions; they are blobs of igneous rock.

Stocks are massive plutons that formed from magma chambers.  Most stocks are about the size of  mountains, and each probably represents a single magma chamber that fed one or a few volcanoes. An example of a stock is the source of the granodiorite currently being quarried from Little Cottonwood Canyon south of Salt Lake City.

Batholiths are also massive plutons, but are much larger than stocks. They probably formed and several different magma chambers are emplaced under a volcanic range--over time filling all the space between. Batholiths can span entire states--large states.  California's Sierra Nevada Range is a batholith.   The Idaho Batholith underlies the Snake River Plain.

Other Plutons

A Volcanic pipe or volcanic neck is the remains of the igneous rock-filled conduit leading from the magma chamber to the volcano.