3.5 Intrusive Igneous Bodies

In most cases, a body of hot magma is less dense than the rock surrounding it, so it has a tendency to move very slowly up toward the surface. It does so in a few different ways, including filling and widening existing cracks, melting the surrounding rock (called country rock^[1]), pushing the rock aside (where it is somewhat plastic), and breaking the rock. Where some of the country rock is broken off, it may fall into the magma, a process called stoping. The resulting fragments, illustrated in Figure 3.5.1, are known as xenoliths (Greek for “strange rocks”).

Some upward-moving magma reaches the surface, resulting in volcanic eruptions, but most cools within the crust. The resulting body of rock is known as a pluton. Plutons can have various different shapes and relationships to the surrounding country rock as shown in Figure 3.5.2.

Figure 3.5.2 Depiction of some of the types of plutons. a: stocks (if they coalesce at depth then they might become large enough to be called a batholith), b: sill (a tabular body, in this case parallel to bedding), c: dyke (cross-cuts bedding), d: laccolith (a sill that has pushed up the overlying rock layers), e: pipe (a cylindrical conduit feeding a volcano). The two features labelled f could be pipes or dykes, but from this perspective it’s not possible to determine if they are cylindrical or tabular.

Large irregular-shaped plutons are called either stocks or batholiths. The distinction between the two is made on the basis of the area that is exposed at the surface: if the body has an exposed surface area greater than 100 square kilometers (km²), then it’s a batholith; smaller than 100 km² and it’s a stock. Batholiths are typically formed only when a number of stocks coalesce beneath the surface to create one large body. A grand example of a batholith is Pikes Peak, west of Manitou Springs, CO. The entire mountain is essentially an old magma chamber (~ 1 Ga in age). that has been thrusted up by tectonic forces, and sculpted through time by weathering.

Tabular (sheet-like) plutons are distinguished on the basis of whether or not they are concordant with (i.e., 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 (Figure 3.5.3.). 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 large dike can be seen in Figure 3.5.4.

Figure 3.5.3 A tan-colored sill composed of porphyry igneous rock, located in Eagle County, CO. Note that the sill trends parallel to the overlaying layer consisting of dark shales and sandstones.

A laccolith is a sill-like body that has expanded upward by deforming the overlying rock (Figure 3.5.5.).

Finally, a pipe is a cylindrical body (with a circular, elliptical, or even irregular cross-section) that served as a conduit for the movement of magma from one location to another. Most known pipes fed volcanoes, although pipes can also connect plutons. It is also possible for a dyke to feed a volcano.

Figure 3.5.6 shows a cross-section through part of the crust showing a variety of intrusive igneous rocks. Except for the granite (a), all of these rocks are mafic in composition. Indicate whether each of the plutons labelled a to e on the diagram below is a dike, a sill, a stock, or a batholith.

 

Figure 3.5.5

See Appendix 3 for Exercise 3.7 answers.

Media Attributions

• Figure 3.5.1.: Wikimedia Commons
• Figures 3.5.2, 3.5.6.: © Steven Earle. CC BY.
• Figure 3.5.3., 3.5.4., 3.5.5.: Colorado Geological Survey, Vince Matthews