TAS stands for Total Alkali Silica. The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali and silica contents. These chemical parameters are useful because the relative proportions of alkalis and silica are important in determining both normative mineralogy and actual mineralogy. The classification can be simple to use for rocks that have been chemically analyzed. Except for the following quotation from Johannsen (1937), this discussion is based upon Le Maitre et al (2002).