Terminology for Volcanic Deposits and Rocks

In many modern active settings where eruptions and transportation processes have been witnessed, the naming of volcanic deposits and rocks can be quite straightforward using genetic terminology. However, for prehistoric eruptions and sequences, and particularly for ancient successions, the origin of many deposit types may be uncertain or, at best, ambiguous, particularly where successions have been subject to hydrothermal alteration, metamorphism, and deformation. In most cases, therefore, a cautious approach to deposit classification and nomenclature is required. Our approach to terminology incorporates documentation of components, textures and assessing the degree and effects of alteration on both components and original emplacement textures. This allows a purely descriptive composite rock or deposit name to be constructed first, incorporating components, textural, and compositional petrological characteristics, free of any biases about emplacement settings and processes. Then important facies features such as depositional structures, contact relationships, and settings are assessed, leading to a composite descriptive and interpretive genetic name for the facies or rock unit that summarises key descriptive characteristics, emplacement processes, and settings. Colour coded flow charts summarising the key observations and steps in developing a progressive descriptive to genetic terminology are provided for both coherent (volcanic and subvolcanic) and volcaniclastic facies, including pyroclastic, autoclastic, and epiclastic, as well as hydraulic fracture deposits/rock units. These flow charts can be copied and used in the field (including drill core logging observations), and in conjunction with petrographic data, provide a useful checklist of essential observations that should be made to develop an understanding of the possible origins of any volcanic succession.