Volcaniclastic deposits: fragmentation and general characteristics

Abstract

Volcanic successions can consist of both coherent lavas and a range of volcaniclastic deposits. Lavas, their characteristics and the parameters which control their characteristics, are discussed in Chapter 4. In this chapter we consider how volcaniclastic deposits (particularly those formed by primary volcanic processes) are formed, and some of their general characteristics. The use of the non-genetic term ‘volcaniclastic’, opens the way to explore the range of origins of all fragmental volcanic rocks. Although some have explosive pyroclastic origins, many do not. Misinterpretation of fragmental volcanic deposits as explosive pyroclastic rocks is commonplace, and usually results from a lack of understanding of the types of fragmentation processes and the characteristics of the deposits from each process, and on whether the deposits have been redeposited from the site of fragmentation. Nomenclature of volcaniclastic rocks is then further discussed in Chapter 12.