Rock identification and classification


The study of geology is the study of the Earth, and so is ultimately the study of rocks. Geologists define a rock as:

A bound aggregate of minerals, mineraloids, or fragments of other rocks.


The use of the word 'bound' means that a rock must have structural integrity, e.g. an aggregate of sand does not become a rock until the grains are bound together. Typical binding agents are very fine grained minerals (e.g. calcite, clay) or mineraloids (e.g. chert, glass), though in some rock types the crystals are intergrown and no binder is required.


There are three major groups of rocks:

  1. Igneous rocks are those that have formed by the cooling and crystallisation of magma, either at the Earth's surface or within the crust;
  2. Sedimentary rocks are those that have formed when eroded particles of other rocks have been deposited (on the ocean floor, stream/lake beds, etc) and compacted, or by the precipitation of minerals / mineraloids from water;
  3. Metamorphic rocks are those that have formed when existing rocks have undergone pressure and / or temperature changes so that their original mineralogy has been changed.

Each of these rock groups contains many different types of rock, and each can be identified from its physical features.

Being able to describe and name rocks is one of the fundamental skills of a geologist. Important information regarding the nature of rocks is communicated through concise, accurate descriptions. This information allows the geologist to identify the rock, and, in the process, to learn about its history and the geological environment in which it was formed.

A knowledge of field relationships between different rock units is fundamental to the study of rocks. It is gained from mapping and observing rocks in the field. In depth analysis of rocks using a microscope or sophisticated analytical laboratory equipment provides important information on their composition. In between these extremes is the observation and description of hand specimens. The term hand specimen refers to an easily manageable piece of rock that can be picked up and easily transported back to the geologist's base for further investigation.


Rock identification is a systematic process, requiring concise, accurate descriptions of physical characteristics. This process is called petrography. Geologists use petrographic descriptions to communicate the essential features of rocks in writing (with illustrations / photographs if appropriate). Petrographic descriptions also summarise these characteristics for future reference. They should contain sufficient information to allow identification of the rock.

Systematic petrographic descriptions, as the name suggests, should follow a systematic pattern detailing the necessary information in a set order. Geologists need to be able to determine the physical properties of rocks based on observations and simple tests that can be conducted in the field.