Types of rock - the IMS triangle

Rocks are solid mixtures of minerals (and occasionally other matter such as fossilized plant material - coal). Some rocks will consist almost entirely of one mineral; for instance, both limestone and marble are (by definition) rocks that contain more than 95% of the mineral calcite. Most rocks consist of mixtures of several minerals.
It is convenient to try to classify rocks as igneous, metamorphic or sedimentary depending on how they were formed. The definitions for these three types are:
  • Igneous rocks are those that have cooled and solidified from a molten (melted liquid) state, either on the surface (volcanic rocks) or deep underground (plutonic rocks).
  • Metamorphic rocks are those that have changed in their mineral composition and texture as a result of the heat and pressure as they have been buried deep under the ground, but without having melted. The original rock could have been igneous or sedimentary, and usually only an expert could identify which they once were.
  • Sedimentary rocks have formed from fragments of rock, minerals, biological materials (such as shells or plant matter) and dissolved substances, which have been transported to a particular place (e.g. by rivers), deposited as sediment and then formed into solid rock by a process in which the fragments become stuck together.
The diagram below shows the processes which relate the three rock types:


Borderline metamorphic rocks, Benmore
The boundaries between the three types are sometimes a bit unclear. This is most noticeable in the transition from sedimentary to metamorphic rocks – there are places where you can trace a particular suite of rocks over the kilometers and see a continuous transition from clearly sedimentary to clearly metamorphic rocks, but nowhere can you put your hand on a rock outcrop and say “This is the place where it becomes metamorphic”. The photo on the right shows a rock face above the Benmore hydro station; it has some characteristics of sedimentary rocks and some of schist (you could regard it as a slate, although a geologist would probably call it schist).
Sometimes classification is difficult. Should a recently erupted lahar (mudflow) from Mt Ruapehu be classed as igneous or sedimentary? It forms a hard, gritty rock rather like a coarse sandstone, with many of the features of a sedimentary rock, but is composed of mineral fragments that were recently molten. The IMS classification system is useful, but should not be treated as an absolute.

From the menu on the left you can find out more detail about the three types of rock.