BGS Rock Classification Scheme

The BGS Rock Classification Scheme (RCS) is a corporate standard setting out a practical, logical and robust system for classifying and naming geological materials as they appear at the scale of a single exposure, hand specimen, or thin section.

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Classification helps to place materials in a wider geological context, and allows unambiguous and informative formal names to be assigned.

The RCS provides a comprehensive system for classifying and naming geological materials to act as a corporate standard in support of our digital geological maps, data dictionaries, and numerous other geological applications.

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Background

The rapidly increasing use of computers to store geological information and the production of maps from these databases has also driven the formulation of these schemes. Computers can integrate data very rapidly from a wide range of sources from properly constructed databases but it is vital that the data are input with a consistent meaning. This requires a clearly defined comprehensive rock classification scheme with master dictionaries of all the approved rock names.

Principles of the BGS Rock Classification Scheme

  • classification is based on descriptive rather than interpreted attributes
  • classification is based on what rocks are, not what they might have been; some flexibility is allowed for metamorphic rocks
  • in general it should be possible to classify a rock from features observable in a hand specimen or thin section; it should not be necessary to see field relationships
  • types or groups of rock types are defined by boundary conditions that will generally follow natural or well established groupings
  • well-established rock names are retained wherever possible although some terms are more rigorously defined
  • the scheme is essentially hierarchical
  • rock names are assigned using a system of approved root terms and qualifier terms

Explanatory Notes

The names and descriptions used in the BGS Rock Classification Scheme (RCS) are derived from a series of reports written by BGS staff. These reports contain the detail and reasoning behind the names used and should be consulted. These are freely available and may be downloaded below.

New names are added to the RCS as required. All new names follow the rules and conventions described in the BGS reports.

The RCS naming structure is hierarchical. Non-alphabetic characters at the start of the code identify high-level names in the hierarchy. For example the highest levels are TOP OF HIERARCHY, with the code +, and ROCK AND SEDIMENT with the code +@RSD.

Policy is that non-alphabetic character codes must not be used to identify rock or sediment types within coded digital data such as borehole logs or digital map polygons.

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The RCS volume dealing with igneous materials is based to a large extent on the recommendations of the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks (Le Maitre [editor] et al., 1st edition 1989; 2nd edition, 2002). However, the BGS scheme contains many changes to, and refinements of, the IUGS recommendations. Perhaps the most significant differences are that the RCS is built around a formal structure, namely a hierarchical classification tree in which igneous materials are classified in ranks, and that the RCS introduces a formal system for naming geological materials in a consistent, informative and unambiguous manner.

Rock Classification Scheme – Vol 1 – Igneous

The diversity of metamorphic rocks results from the combined effects of a range of tectonic and/or metamorphic processes acting on a wide range of protolilths. The names that have been given to metamorphic rocks are similarly diverse. The objective of this scheme is to introduce a system of nomenclature that is based as far as possible on descriptive attributes that will be informative to both specialist and non-specialist users and allow any rock to be placed easily into its position in the hierarchy. There are sections on nomenclature, rocks with sedimentary, volcaniclastic and igneous protoliths, rocks with unknown protolith, mechanically broken and reconstituted rocks, metasomatic and hydrothermal rocks, special case metamorphic rocks and qualifiers. The report, issued here as BGS Research Report RR 99-02, has nine figures, seven tables and an appendix listing approved rock names.

Rock Classification Scheme – Vol 2 – Metamorphic

Existing sediment and sedimentary rock nomenclature has been based on composition, texture and other physical attributes as well as depositional environment, genetic relationships and local economic importance. A consistent, workable classification for all sediments and sedimentary rocks needs a set of unifying boundary conditions: this classification attempts to do this. The sediment and sedimentary rock names used are descriptive and it should be possible to classify and name any sediment or sedimentary rock without knowledge of its field setting and without making assumptions about its mode of origin. There are sections on siliciclastics, carbonates, dolomites, phosphates, iron sediments and ironstones, organic rich sediments and rocks, non-carbonate salts, non-clastic siliceous sediments and sedimentary rocks, miscellaneous hydroxides and oxides, sediments and rocks based on grain size, hybrid sediments or sedimentary rocks, volcaniclastics and qualifiers. The report, issued here as BGS Research Report RR 99-03, includes 14 figures and 12 tables.

Rock Classification Scheme – Vol 3 – Sedimentary

This report describes the BGS classification schemes for superficial deposits of Pleistocene and Recent age comprising artificial ground (made through human activity) and natural superficial deposits. They are designed to be used and interpreted by both geologists and non-geologists to allow thematic material comprising identified classes of superficial deposits to be extracted from geological maps and other data-sets. The schemes can be developed further to other classes of materials particularly from overseas tropical and sub-tropical land, shelf and deep-sea areas. In Britain traditional onshore investigations of the superficial deposits have employed a morphogenetic and lithogenetic terminology resulting in an amalgam of terms with a dominantly genetic basis. This report adopts a similarly genetic approach, however it is recognised that an objective description involving sedimentological, structural, physical and chemical criteria to construct lithofacies associations is desirable and reference to such schemes is made where appropriate. The report, issued here as BGS Research Report RR 99-04, includes 26 tables and 16 figures.

Rock Classification Scheme – Vol 4 – Superficial

This report presents a scheme for classifying and naming geological discontinuities and their fillings, and provides guidance for describing discontinuities in rock masses. The scheme is a new BGS corporate standard analogous to more established standards like the BGS Rock Classification Scheme. Its purpose is to provide geologists with the means to collect and store discontinuity and filling data succinctly and consistently, in any setting and at any scale; to supply a set of approved terms and their definitions to the dictionaries that support BGS databases, and to permit BGS staff, customers and other users to better understand BGS outputs that incorporate discontinuity data.

Scheme for classifying discontinuities and fillings

Citation

If citing this online representation of the Rock Classification Scheme, please cite it as follows:

British Geological Survey. 2020. The BGS Rock Classification Scheme [online]. Keyworth, Nottingham. Available from https://www.bgs.ac.uk/technologies/bgs-rock-classification-scheme/

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Please contact Dr Tim McCormick for more information.

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