THE CONSTRUCTION OF THE TOTAL ALKALI-SILICA CHEMICAL CLASSIFICATION OF VOLCANIC-ROCKS

This paper describes the reasoning that lies behind the construction of the field boundaries in the Total Alkali-Silica diagram (TAS) for the chemical classification of volcanic rocks. It shows that by utilizing nomenclature in common use by petrologists in combination with a large computer database of geochemical analytical information derived from the published literature, it was possible to construct a Total Alkali-Silica diagram-for volcanic rocks of only 15 fields, which has straight-line boundaries and necessitates the definition of only 11 points to construct the diagram. One of the principal constraints on the positioning of the boundaries between the various named fields was to minimize the degree of overlap between adjacent fields.