### Become an OU student

Practising science: Reading the rocks and ecology

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

# 1.3.2 Texture of igneous rocks

What texture might we expect an igneous rock to have? An igneous rock will contain crystals that grew as the magma cooled. Each crystal will have started to grow unhindered by neighbouring crystals, so an igneous rock therefore has a crystalline texture in which the crystals are randomly oriented.

To picture this, consider magma at an initial temperature of perhaps 1000 °C, as it slowly cools underground (see Figure 3, path (a) to (d)). Initially the magma is completely molten (Figure 3a) but, unlike water placed in the ice box of a freezer, magma doesn't turn from being totally liquid to totally solid at a single temperature when it is cooled. Instead, different minerals crystallise over a range of temperatures (in fact over one or two hundred degrees Celsius). So, when the temperature of magma falls by a small amount, only a few mineral crystals will form (Figure 3b). On further cooling these crystals grow larger, and new minerals also start to crystallise (Figure 3c). Eventually, these crystals form an interlocking network, with the last crystals to grow filling the spaces between. When totally solidified, the rock has the crystalline texture shown in Figure 3d.

In contrast, very fast cooling allows crystallisation to occur by the nucleation of many small crystals rather than the steady growth of a few crystals. The resultant igneous rock contains innumerable tiny crystals that may be so tiny as to be indistinguishable except under the high magnification of a microscope (Figure 3, path (a) to (e)). In the most extreme case crystallisation is totally inhibited and the starting liquid is quenched to form volcanic glass.

Figure 3 The number and size of crystals that grow in magma depend on the temperature and the cooling rate. Starting with a liquid (a) that undergoes very slow cooling, the magma solidifies by the gradual growth of large crystals of different minerals (indicated by different patterns of shading) to produce an igneous rock with large crystals with random orientations (cooling path (a) to (d)). When cooling is fast, many very tiny crystals, rather than a few large crystals, grow with the result that a fine-grained igneous rock is formed (cooling path (a) to (e))

## Activity 1

Would you expect a fine-grained igneous rock to have formed deep below the Earth's surface or at the surface?