Definition: molten rock that has cooled and crystallized from a melt at depth. This rock is characterized by large mineral grains that can be seen without special equipment (Monroe and Wicander, 1995). The mineral grains are often well-formed, not aligned, of more than one size, and may be overgrown. Textural terms based on the grains are: aplite ( equigranular and < 1 mm), pegmatite (inequigranular and grains > 5 mm) and porphyry (two grain sizes with one < 1 mm in size)(Compton, 1985).

A note on mapping: In the GIS, the composition of sediment in intermontane areas or alluvial plains may be classified as a plutonic rock type. This is not to imply the landform is rock. This designation is meant to indicate that the landform is primarily composed of sediments of that composition. It may also have rock fragments of other compositions and/or have windblown sediments from undetermined origins as well.


(modified from Press & Siever, 1986)
Batholith: molten rock cooled at depth with a surface area greater than 100 square km (Blatt & Tracy, 1996).

Stock: small rounded body of cooled molten rock with an exposed surface area of less than 100 square km (Blatt & Tracy, 1996). A stock can be a small body or an exposed portion of a batholith (Monroe & Wicander, 1995).

Dike: narrow body of molten rock cooled at depth. The rock cut across any sedimentary beds that may have been present. This rock forms at an angle to the earth's surface other than horizontal. This term is used if the direction in which it formed is not known (Blatt & Tracy, 1996).

Sill: Tabular igneous body that has squeezed between rock strata. Slab thickness runs from millimeters to tens of meters, and more. Orientation, in the sense of dip, can vary, depending on local structure; but horizontal to near horizontal is common. Examples of sill-controlled landforms include: caprocks, mesas, hogbacks, cuestas, and escarpments, such as the thick sill forming the Palisades of the Hudson River, near New York City (Desert Processes Working Group).

Undifferentiated pluton


                 Silicic                  Granitoid                                          Intermediate                 Dioritoid         Mafic                 Gabbroid         Ultramafic                 Peridotite         Silica undersaturated                 Foidolite
        Undifferentiated Plutonic
Return to Igneous

Return to Composition

Return to MDEI2000 Main Page
(last modified 11/22/99)