Workings of the SCMR

Structure of the SCMR

augen gneiss, Shetland. Photo, D Fettes

The SCMR started operating in 1985. The Subcommission consisted initially of 33 members, distributed in 11 study groups devoted to special topics, and a working group of more than 100 earth scientists spread worldwide.

The study groups, in addition to Subcommission members, also drew membership from appropriate specialists worldwide. The main consultative work of the Subcommission was done initially by correspondence and during annual working meetings. Also, questionnaires were sent to members of the working group to improve prepared definitions and test international acceptance. The provisional recommendations were published on the SCMR website and critical comment encouraged. The final results and recommedations were then drawn up. The Subcommission’s work was conducted in English and all its recommendations and definitions are designed only for English language usage.

Scope of the SCMR

The SCMR has dealt with all metamorphic rocks. This was taken to include rocks which are quenched melts produced by, or closely associated with metamorphic processes and which are not defined by the Igneous Subcommission. In addition the SCMR has defined a number of structural terms and processes closely associated with metamorphic processes.

The SCMR also includes the systematics of impactites. Although many impactite products and processes are not strictly metamorphic it was considered expedient to deal with the group as an entity, especially as no part of the subject was being considered elsewhere by the CSP.

Basis of the recommendations

A prime objective is to provide a scheme for naming and describing metamorphic rocks, no attempt has been made to cover the terminology relating to the detail or theory behind metamorphic processes, mineral chemistry, graphical presentations, etc. It was agreed that rock names should, as far as possible be applicable at the hand-specimen scale, that they should be based on non-genetic criteria and that these criteria should be measurable in the field or under the microscope. Nomenclature based on criteria such as rock chemistry or metamorphic grade was obviously unsuitable and a systematic scheme was devised based on compound names with structural root terms and mineral qualifiers. This scheme allows a systematic name to be given to any rock. However, it was accepted that there were many well-established specific names, such as marble, amphibolite and eclogite, that would have to remain and could potentially be used as alternative names. In addition many metamorphic rocks can be named by reference to their protolith. Thus, metamorphic rocks may potentially have up to three acceptable names, that is a systematic name, a specific name and a protolith-based name, for example, carbonate granofelsmarblemetalimestone or hornblende-plagioclase schistamphibolitemetabasalt. The nomenclature scheme has taken account of this situation.