The planet we live on: The beginnings of the Earth Sciences

Figure 1.40: A close up view of a piece of gabbro.the surface cool more quickly, forming medium-grained rocks with individual crystals thatare harder to see. If magmas reach the surface, they are extruded and can flow out aslavas. Such extrusive igneous rocks cool down very quickly, either in the air or underwater, forming fine-grained igneous rocks. <strong>The</strong> crystals in fine-grained igneous rocksare almost impossible to see without using a hand lens.<strong>The</strong>se processes show how igneous rocks of different compositions and grain sizes areformed. <strong>The</strong>ir colour can help us to distinguish them, since the richer in silicon a rock is,the paler it is. Some igneous rocks are more common than others. <strong>The</strong> coarse-grained ultramaficrock that forms much of the mantle is called peridotite. A coarse-grained maficrock, dark in colour, is called gabbro, shown in Figure 1.40, and its fine-grained equivalent,that forms many lava flows, is dark-coloured basalt (Figure 1.41). <strong>The</strong> fine-grainedrock forming intermediate lavas is andesite, which is paler in colour than basalt. Finegrainedsilicic lavas are uncommon, but the coarse-grained equivalent is very common.This coarse-grained, pale-coloured, silicic igneous rock is granite (Figure 1.42).This fairly simple picture is complicated by how lavas erupt, since the composition affectsthe viscosity of the magma and this in turn affects the type of eruption. Mafic magmasthat produce basalts are relatively rich in iron but poor in silica. Silica-poor magmas havelow viscosity and these runny lavas can flow quickly out of cracks in the ground, calledvolcanic fissures (Figure 1.43), and spread over wide areas. Small bubbles of volcanic27