3.5 Intrusive Igneous Bodies

In most cases, a body of hot magma is less thick than the rock bordering it, for this reason it has actually a tendency to move an extremely slowly up towards the surface. That does so in a few different ways, consisting of filling and also widening currently cracks, melting the neighboring rock (called country rock<1>), pushing the rock aside (where the is somewhat plastic), and breaking the rock. Where few of the country rock is broken off, it may autumn into the magma, a procedure called stoping. The result fragments, illustrated in number 3.19, are well-known as xenoliths (Greek for “strange rocks”).

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Figure 3.19 Xenoliths that mafic rock in granite, Victoria, B.C. The fragments of dark rock have actually been broken off and incorporated right into the light-coloured granite.

Some upward-moving magma will the surface, causing volcanic eruptions, yet most cools in ~ the crust. The resulting body of absent is recognized as a pluton. Plutons have the right to have various different shapes and relationships come the surrounding country rock as presented in number 3.20.

Figure 3.20 depiction of few of the varieties of plutons. A: share (if castle coalesce at depth then they might constitute a batholith), b: sill (a tabular body, in this case parallel to bedding), c: dyke (cross-cuts bedding), d: laccolith (a sill that has actually pushed increase the overlying absent layers), e: pipeline (a cylindrical conduit feeding a volcano). The two functions labelled f could be pipes or dykes, yet from this view it’s not feasible to identify if they space cylindrical or tabular.

Large irregular-shaped plutons are called either stocks or batholiths. The difference between the 2 is made on the basis of the area the is exposed in ~ the surface: if the body has actually an exposed surface ar area better than 100 km2, climate it’s a batholith; smaller sized than 100 km2 and also it’s a stock. Batholiths are commonly formed only as soon as a variety of stocks coalesce beneath the surface ar to develop one large body. One of the biggest batholiths in the human being is the Coast range Plutonic Complex, i beg your pardon extends every the way from the Vancouver region to southeastern Alaska (Figure 3.21). Much more accurately, it’s numerous batholiths.

Tabular (sheet-like) plutons are identified on the basis of even if it is or no they room concordant with (parallel to) present layering (e.g., sedimentary bedding or metamorphic foliation) in the country rock. A sill is concordant v existing layering, and also a dyke is discordant. If the country rock has no bedding or foliation, then any tabular body within that is a dyke. Keep in mind that the sill-versus-dyke designation is not figured out simply through the orientation of the feature. A dyke have the right to be horizontal and a sill can be upright (if the bedding is vertical). A big dyke have the right to be viewed in figure 3.21.

A laccolith is a sill-like body the has expanded upward by deforming the overlying rock.

Finally, a pipe is a cylindrical human body (with a circular, ellipitical, or even irregular cross-section) that served as a conduit because that the motion of magma indigenous one location to another. Most recognized pipes fed volcanoes, although pipes can likewise connect plutons. It is also possible for a dyke to feeding a volcano.

Figure 3.21 The Stawamus Chief, part of the Coast variety Plutonic Complex, close to to Squamish, B.C. The cliff is about 600 m high. Most of the dark stripes are a result of algae and lichen development where the surface ar is generally wet, but there is a huge (about 10 m across) upright dyke that extends native bottom come top.

As questioned already, plutons can connect with the rocks right into which they room intruded, sometimes resulting in partial melting of the nation rock or come stoping and also formation of xenoliths. And, as we’ll check out in chapter 7, the warm of a body of magma have the right to lead come metamorphism the the nation rock. The country rock can additionally have an effect on the magma within a pluton. The most evident such result is the formation of a chill margin follow me the edge of the pluton, whereby it come in call with nation rock that was significantly chillier than the magma. In ~ the cool margin, the magma cooled more quickly than in the center of the dyke, so the structure is finer and also the colour might be different. An instance is displayed in number 3.22.

Figure 3.22 A mafic dyke v chilled margins in ~ basalt in ~ Nanoose, B.C. The coin is 24 mm in diameter. The dyke is around 25 cm across and the cool margins room 2 cm wide.

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Exercise 3.7 Pluton Problems 

The diagram below is a cross-section through part of the crust mirroring a variety of intrusive igneous rocks. Other than for the granite (a), every one of these rocks are mafic in composition. Indicate whether each of the plutons labelled a come e top top the diagram listed below is a dyke, a sill, a stock, or a batholith.