In most cases, a body of hot magma is less dense than the rock surrounding it, so it has a tendency to creep upward toward the surface. It does so in a few different ways:
- Filling and widening existing cracks
- Melting the surrounding rock
- Pushing the rock aside (where the rock is hot enough and under enough pressure to deform without breaking)
- Breaking the rock
When magma forces itself into cracks, breaks off pieces of rock, and then envelops them, this is called stoping. The resulting fragments are called xenoliths. In Figure 7.18, the dark patches are xenoliths.
Some of the magma reaches the surface, resulting in volcanic eruptions, but most cools within the crust. The resulting body of rock is called a pluton. Plutons can have different shapes and different relationships with the surrounding country rock (Figure 7.19). These characteristics determine what name the pluton is given.
Large, irregularly shaped plutons are called stocks or batholiths, depending on size. Tabular plutons are called dikes if they cut across existing structures, and sills if they do not. Laccoliths are like sills, except they have caused the overlying rocks to bulge upward. Pipes are cylindrical conduits.
Types of Plutons
Stocks and Batholiths
Large irregular-shaped plutons are called either stocks or batholiths, depending on the area exposed at the surface. If the body has an exposed surface area greater than 100 km2, then it’s a batholith, otherwise it’s a stock. Batholiths are typically formed when a number of stocks coalesce beneath the surface to create one large body. One of the largest batholiths in the world is the Coast Range Plutonic Complex, which extends all the way from the Vancouver region to southeastern Alaska (Figure 7.20). Because this naming scheme depends on the area of rock that is exposed, it is still possible that a stock is a small part of a larger batholith-sized complex deeper within the Earth.
Dikes, Sills, and Laccoliths
Tabular (sheet-like) plutons are classified according to 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 could be horizontal and a sill could be vertical- it all depends on the orientation of features in the surrounding rocks.
A laccolith is a sill-like body which has expanded upward by deforming the overlying rock.
A pipe, as the name suggests, is a cylindrical body with a circular, ellipitical, or even irregular cross-section, which serves as a conduit (or pipeline) for the movement of magma from one location to another. Pipes may feed volcanoes, but pipes can also connect plutons. It is also possible for a dike to feed a volcano.
As discussed already, plutons can interact with the rocks into which they are intruded. This might lead to partial melting of the country rock, or to stoping and formation of xenoliths. And, as we’ll see in Chapter 10, the heat of a body of magma can lead to metamorphism of the country rock, causing mineralogical and textural changes. However, it is also the case that the country rock can affect the magma.
The most obvious effect that country rock can have on magma is a chilled margin along the edges of the pluton. The country rock is much cooler than the magma, so magma that comes into contact with the country rock cools much faster than magma toward the interior of the pluton. Rapid cooling leads to smaller crystals, so the texture along the edges of the pluton is different from that of the interior of the pluton, and the colour may be different. An example is shown in Figure 7.21.
Exercise 7.5 Pluton Problems
The diagram below is a cross-section through part of the crust showing a variety of intrusive igneous rocks. Indicate whether each of the plutons labelled a to e on the diagram below is a dike, a sill, a stock, or a batholith. (Note the trees for scale.)
- From the Greek words xenos, meaning "foreigner" or "stranger," and lithos for "stone." ↵
- After Pluto was demoted from planet status, astronomers tried to come up with a name for objects like Pluto. For a while they considered "pluton" however geologists rightly objected that they had first claim on the word. In the end the International Astronomical Union settled on "dwarf planet" instead. ↵
- Also referred to as the Coast Range Batholith ↵
- Sedimentary bedding refers to the layers in which sedimentary rocks form. Metamorphic foliation refers to the way minerals or other elements in a rock are aligned as a result of being deformed by heat and pressure. Bedding and foliation will be discussed in more detail in later chapters. ↵
- Also spelled dyke. ↵