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.
Would you expect a fine-grained igneous rock to have formed deep below the Earth's surface or at the surface?
A fine-grained igneous rock requires rapid cooling and this is more likely at the surface, where magma comes into contact with air or water, rather than in the hot interior of the Earth.
Generally speaking, the number and size of the crystals in an igneous rock depend on the amount of time available for their growth. The slower the cooling, the bigger the crystals. In the case of extrusive rocks, the amount of time is short – anything from a few seconds for droplets of magma flying through the air in an explosive volcanic eruption, to a few years for the interior of a thick lava flow. This results in small crystals (Figure 3e). For intrusive rocks, the cooling rate is much slower and there is time for larger crystals to grow (Figure 3d). (The times involved are not known for certain because the magma at depth cannot be observed.) Figure 4 illustrates this with two rocks of essentially similar chemical composition, but from different igneous settings. Figure 4a shows an intrusive rock containing crystals of mainly two minerals – one dark (this is the iron (symbol Fe) and magnesium (Mg) -bearing silicate mineral called pyroxene), the other pale (this is the calcium (Ca), sodium (Na), aluminium (Al) -bearing silicate mineral called plagioclase feldspar). The crystals are intergrown (cf. Figure 3d) and easily visible. In contrast, Figure 4b shows a rock collected from a lava flow. This rock has only a few crystals (pale) that are large enough to see, and this is because cooling and crystal growth were abruptly halted when the magma erupted and froze as lava. The rest of this rock is so finegrained that crystals are indistinguishable without the benefit of a microscope. There are, however, a few small round dark areas (e.g. near the top left edge) These are gas bubbles formed when the magma came to the surface and gases that were dissolved in the magma came out of solution as the pressure on the magma decreased.