A petrographic distinction between Cenozoic volcanics in and around the open oceans

Abstract

Geographic and petrographic definitions of ‘circumoceanic’ and ‘intraoceanic’ Cenozoic lavas are proposed and tested. The association between geography and petrography proves remarkably strong; a classification based on TiO2 content and the degree of alkali concentration relative to SiO2 and Al2O3 is more than 90 per cent accurate. Failures of the classification are frequently associated with unusual enrichment in H2O. The remarkable consistency of lava types in and around the oceans contrasts strongly with the complex mixtures of types characteristic of shallow-sea and continental areas of the globe.