NOMENCLATURE AND CLASSIFICATION

Nomenclature Numerous means exist for classifying and naming rocks, all of which arise because as humans we feel we have to recognize and categorize common or contrasting features in related things.

Systems of nomenclature and classification may reflect:

genetic, textural, chemical or mineralogical features.

  1. GENETIC

    2. basic system which classifies rocks on the basis of where they form.

    plutonic - at depth

    hypabyssal - intermediate depth

    volcanic - on the Earth's surface.

    This system is not very practical, but it serves as a first approximation, it tells nothing about mineralogy, chemistry of the rocks and can not distinguish basalt from rhyolite.

  2. TEXTURAL


    3. relies on the grain size of individual minerals in the rock.

    aphanitic - fine grained < 1 mm

    phaneritic - medium grained 1 to 5 mm

    coarse grained (pegmatitic) > 5 mm

    This system has the same shortcomings as a genetic classification, however specific textures present may aid in classification, e.g., phenocryst, ophitic, coronas, but these are not indicative of a specific environment of formation or a specific lithology.

  3. CHEMICAL

    4. This type of classification requires a complete chemical analysis of the rock in order to pigeonhole a sample, and is not practical under field conditions where only a hand lens and hammer are available. A chemical classification system has been proposed for volcanic rocks and a comparable scheme for plutonic rocks is not available.

    This leaves us with a system based on mineralogy.

  4. MINERALOGICAL

    5. The one gaining application is the result of several years work by the IUGS Subcommission on the Classification of Igneous Rocks or Streckeissen Classification.

IUGS CLASSIFICATION SYSTEM

This is based on Modal Mineralogy (MODE - an accurate representation of the distribution and volume percent of minerals within a given rock).

The system is simple to use, can be applied in the field, using a hand lens and a hammer. It is based on the percentages of Q (quartz), A (alkali feldspar), P (plagioclase), and F (feldspathoids, e.g. nepheline, leucite). Further subdivisions are dependant on the type and percentage of mafic minerals present.

Two systems have been proposed - Handouts.

  1. Plutonic Rocks - widely used with universal application
  2. Volcanic Rocks - not as popular.