Rock Classification Scheme (BGS).pdf - The Water, Sanitation and ...

1 INTRODUCTIONThe use of computers as primary tools for carrying out geologicalresearch and databases for storing geological informationhas grown considerably in recent years. In the sameperiod there has been a dramatic increase in the degree of collaborationbetween scientific institutions, universities andindustry, and between geologists working in differentcountries. To facilitate collaborative work amongst geologistsand to maximise efficiency in the use of geological databasesa common approach to classifying and naming rocks isessential. This publication presents a scheme for the classificationand nomenclature of igneous rocks that is practical,logical, systematic, hierarchical and uses clearly defined,unambiguous rock names.Producing a classification scheme with a hierarchicalstructure is an important objective for three reasons: firstly, it isa ‘user-friendly’ system in that the very wide range of igneousrock types can be divided and classified in a logical and readilyunderstood manner; secondly, the classification and naming ofrocks can be varied according to the expertise of, and the levelof information available to, the user — the more informationthat is available, the higher is the level of the hierarchy atwhich the rock can be classified and named; thirdly, it providesa convenient and simple system for inputting, storing andretrieving data on databases.The objective of this classification scheme is to introduce asystem of nomenclature for igneous rocks that is based as faras possible on descriptive attributes. Rock names that areconstructed of descriptive terms are more informative to bothspecialist and non-specialist users, and allow any rock to beplaced easily into its position in the hierarchy. The approachto rock nomenclature outlined below allows the vast majorityof all igneous rocks to be named adequately using a relativelysmall number of root names with or without qualifierterms.The basic classification system established by theInternational Union of Geological Sciences Subcommission onthe Systematics of Igneous Rocks (referred to hereafter asIUGS) (Le Maitre et al., 1989; Le Bas and Streckeisen, 1991;Woolley et al., 1996) is adopted here; parts of the text andseveral diagrams have been taken from these works. However,this scheme contains many refinements and changes to theIUGS recommendations, where it was considered necessaryand appropriate, and the resulting scheme is more logical, consistent,systematic and clearly defined. It is hoped that thescheme will receive widespread support and will be adoptedby a broad spectrum of geologists.A genetic approach to classification was not adoptedbecause it is less informative about the mineral/ chemicalcomposition of the rock, the information required to makean interpretation of rock genesis is not always available tothe geologist, and ideas about petrogenesis change withtime. Names that have a genetic meaning or connotation,and also those names which are more suitable for describingrock units rather than individual rock types, have beenexcluded wherever possible. However, in some cases agenetic aspect to rock classification and nomenclature isunavoidable, and attention is drawn to these exceptions atappropriate points in the text (see Section 8.2 Rock namesbased on processes ).Using the descriptive approach recommended here, itshould be possible to classify and name any igneous rockwithout knowledge of its field setting and without makingassumptions about its mode of origin.One of the main challenges in constructing this schemewas to make it equally appropriate to geologists working inthe field and in the laboratory. The level and type of informationavailable to geologists in these environments variesconsiderably. The hierarchical approach allows the user toassign a name to any igneous rock at the level of thehierarchy most appropriate to the type and level of informationavailable to them. In general, rock names in thelower and middle levels (Levels 1 to 7) of the hierarchywill be used by geologists in the field, while those in thehigher levels will be used where more detailed petrographicaland/or geochemical information is available.There are drawbacks to the scheme. For example theapproach to rock nomenclature that is recommended herecan result in rock names that are longer than equivalent‘traditional’ terms. Also, guidelines are introduced for usingqualifier terms and hyphens in constructing rock names,and users will need to familiarise themselves with these(see Section 2.1 Constructing a rock name and 2.2 Use ofhyphens).As with all previous attempts to create a taxonomy forigneous rocks, it has not been possible to keep the schemewholly logical. Despite significant improvements, there arestill instances where the range and complexity of naturalprocesses has produced igneous rocks that defy a logicalsystem of classification. In the future, refinements to thescheme will be made with advances in rock characterisationmethods and a better understanding of igneousprocesses.1.1 Principles behind the classification schemei The term ‘igneous rock’ is taken to mean ‘igneous origneous-looking’. Igneous rocks may have crystallised frommagmas or may have formed or been modified by eruptive,deuteric, metasomatic or metamorphic processes. Igneousrocks include fragmental rocks formed as a direct result ofvolcanic processes. Arguments as to whether a rock is igneous,sedimentary or metamorphic become irrelevant in this contextii Classification is based on descriptive attributes, such ascomposition and grain size, not interpreted attributes. Theclassification is non-genetic as theories on genesis canchange with time and fashion but observed or measuredparameters should stay the same.iii Classification is based on what rocks are, not whatthey might have been.iv In general, it should be possible to classify a rock fromfeatures observable in a hand specimen or thin section; itshould not be necessary to see field relationships. The maingroups of rocks can be classified without recourse to petrographicalor geochemical analysis. If such features cannot bedetermined due, for example, to the presence of glass or to thevery fine-grained nature of the rock, then other criteria such aschemical composition are used, where appropriate.v Types, or groups of rock types, are defined by boundaryconditions such as grain size and proportions of certainminerals or clasts. These boundaries generally follow naturalor well-established groupings to make application of thescheme easier and acceptable to a wide range of geologists.vi Well-established rock names are retained whereverpossible to avoid drastic changes of nomenclature andensure widespread adherence to the scheme, though someterms are defined more rigorously. Where many nameshave been used in the past for a particular rock type apreferred name has been chosen and the use of synonymsis discontinued.3