Petrographic aspects of volcanic rocks

Abstract

The normal preliminary petrographic study of rocks, that is to say the identification of the minerals and the examination of grain size and texture, enables rocks to be classified and named. The present chapter is not concerned with this routine exercise but with some of those other petrographic aspects of volcanic rocks capable of giving information of direct petrogenetic value. Amongst these features probably the most important is the phenocryst assemblage, that is to say, what it consists of and what the relations are between the various phases and the groundmass. The determination of the order of crystallisation is a related topic, and other features may give information about cooling rates, pyroclastic origins, and other diverse subjects of more or less petrogenetic interest. The field is, however, wide and ill-defined and we cannot hope to be entirely comprehensive in this treatment.