To classify volcanic rocks by this method requires a microscope and thin sections. Hundreds of mineral grains are identified and carefully counted before using this diagram. Today the diagram is useful mainly to keep the various rock names straight and to follow some of the older literature. The procedure is the same as with the QAP diagram for plutonic rocks.
Many volcanic rocks aren't suited for this classification method:
- Aphanitic rocks must be classified by chemical, not mineral content.
- Rocks with isolated larger grains (phenocrysts) may yield distorted results.
- Rare rocks including carbonatite, lamproite, keratophyre and others are "off the chart."
