Crystal-rich volcaniclastics—pyroclastic or epiclastic?

Abstract

Crystal-rich volcaniclastics are not uncommon in volcanic terrains or in the surrounding basins that derive their sediment from volcanic terrains. In this chapter we show that their origins may be diverse, involving the influence of pyroclastic processes, or epiclastic processes, or combinations of both. Having dealt with pyroclastic and epiclastic processes in preceding chapters, we can thus use this type of deposit to illustrate the relationships between, and the possible interface between these two groups of processes, and the importance of evaluating the role of each group of processes in volcanic terrains. Crystal-rich volcaniclastics are frequently called ‘crystal tuffs’ without careful evaluation of the possible diverse origins, or without consideration of the genetic implications of such terminology. In this consideration of crystal-rich volcaniclastics, and especially the use of the term ‘crystal tuff’, we can consider some of the problems of classification and terminology as an introduction to a comprehensive discussion on terminology in Chapter 12.