7.6: Intrusive Igneous Features
Intrusive Igneous Features
Once the magma has completely crystallized, an intrusive igneous body is formed that can be generally called a pluton Links to an external site. regardless of its shape or size. Sometimes these plutons are exposed at the Earth’s surface where direct observation of the rock is possible. They are composed of minerals that are large enough to see without the aid of a microscope. This is why any igneous rock that has visible crystals throughout (phaneritic, porphyritic-phaneritic, or pegmatitic) can be referred to as a plutonic rock.
Figure 7.15: The Sierra Nevada Mountains of eastern California. The batholith is a suite of intrusive igneous rocks representing old, cooled magma chambers that were originally beneath an ancient chain of subduction zone volcanoes. The rocks are mostly Cretaceous-aged granite and granite-like lithologies such as granodiorite. Some sedimentary and metamorphic rocks are present, representing roof rocks and altered roof rocks above the original magma chambers. (CC-BY 2.0; James St. John via Flickr Links to an external site.)
A batholith Links to an external site. is a large area (>100 square km) that consists of numerous types of plutons. Here in California, there are many famous batholith examples including the Sierra Nevada batholith Links to an external site., visible in Yosemite National Park Links to an external site. (Figure 7.15), Peninsular Range batholith Links to an external site., visible around the San Diego area and south into Baja California, and the Klamath Batholith, visible in northern California along the border with Oregon.
Sills and Dikes are tabular bodies of magma that intrude into a fracture. Sills Links to an external site. follow bedding planes, whereas dikes Links to an external site. cross-cut beds (Figure 7.16). Tabular (sheet-like) plutons are distinguished on the basis of whether or not they are concordant with (parallel to) existing layering (e.g., sedimentary bedding or metamorphic foliation) in the country rock. A sill is concordant with existing layering, and a dike is discordant. If the country rock has no bedding or foliation, then any tabular body within it is a dike. Note that the sill-versus-dike designation is not determined simply by the orientation of the feature. A dike can be horizontal and a sill can be vertical (if the bedding is vertical). A laccolith Links to an external site. is a sill-like body that has expanded upward by deforming the overlying rock (Figure 7.16). A stock Links to an external site. is a discordant (cuts across other layers), balloon-shaped intrusive rock body (< 100 square km) that is sometimes [confusingly] also called a pluton.
Figure 7.16: Intrusive igneous features. (CC-BY 4.0; Chloe Branciforte)