ABSTRACT
Rocks are classified in various ways, using indicators such as mineral content and chemistry, texture and grain size, to identify rock types and to infer their origins. The general idea is to fit the conclusions into a broad and simple template of three categories, igneous, sedimentary and metamorphic, with subdivisions, as shown in Figure 20.1, but this is not always obvious and easy to apply. The simple notion that igneous rocks are formed by solidification of melt, sedimentary rocks are aggregations of detritus from pre-existing rocks and metamorphics are those that have been seriously modified by heat, pressure or percolating water remains valid as a starting point. But, in many cases, more than one of these processes have contributed to rock formation, requiring judgement, or application of some rule or principle, to decide which is to be regarded as primary. The ‘rock cycle’ of transformations between rock types (Figure 20.2) is still simplistic but gives more scope for recognition of the complications and intermediate states that occur.
