Intrusive igneous rocks originate from the solidification of magma (molten rock) within the Earth’s crust. They include both plutonic rocks, which crystallized slowly in large deep chambers at depths between 5 and 30 km (Figure 5.1a), and near-surface smaller intrusions, which crystallized relatively rapidly in fissures (Figure 5.1b). Intrusive igneous rocks are commonly related by composition and age to volcanic activity at surface; however, it is rare that the actual transition between them is preserved in the geological record. Following the cessation of the igneous activity, the volcanic edifice and the underlying rocks are uplifted and eroded away to expose the underlying plutonic rock and minor intrusions. Many kilometres of rock are worn away during this process, which may take several million years to complete. Figure 5.2 presents a schematic sketch of the relationships between the magma chamber, conduits, dykes, sills and volcanoes and shows a possible level of erosion necessary to expose the deep-seated roots to an igneous complex.