TAS classification

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.

(The subtitle of the classification chapter by Johannsen (1937) is "Chacun a son goût"—to each his own taste).

The TAS classification cannot be applied to all volcanic rocks—as discussed in detail by Le Maitre et al (2002).

Before classifying rocks using the TAS diagram, the chemical analyses must be recalculated to 100% excluding water and carbon dioxide.

The names provided by Le Maitre et al. (2002) for fields in the TAS diagram are listed below.