Fragmentation Processes in Magmas and Volcanic Rocks

In this chapter, we consider the many processes by which magma and volcanic rock can be fragmented. This comprehensive review describes the processes, dynamics, and characteristics of the diverse fragmental products, especially clast textures. Irrespective of their origin, all of these products can in the first instance be called volcaniclastic deposits. Volcaniclastic deposits can be produced by a spectrum of processes, ranging from primary, syn-eruptive autoclastic and explosive processes, syn- to post-eruptive surface sedimentary processes, subsurface hydrothermal processes, and even tectonic processes. Autoclastic processes involve non-explosive, syn-eruptive fragmentation of magma, usually associated with the flow of lavas in subaerial and subaqueous settings. The products include monomictic lava autobreccia, and in aqueous settings, quench fragmented hyaloclastite breccia. There are many different situations in which explosive eruptions and fragmentation of erupting magma and country rock can occur. Explosive eruptions can be driven by the exsolution of magmatic volatiles from magma, and the explosive growth of the resulting vesicles (magmatic explosive eruptions), or the explosive superheating of external water (phreatic explosive), or a combination of the two (phreatomagmatic explosive eruptions), or by overpressured hydrothermal fluids (hydrothermal explosive eruptions). Pyroclastic deposits and their pyroclasts can be enormously variable in grain size, clast and aggregate texture, and componentry. Secondary fragmentation can be caused by surface sedimentary processes. Volcanic terrains are often very dynamic surface sedimentary regimes, and therefore syn- to post-eruptive sedimentary processes produce a wide range of volcaniclastic deposit types, from mass wasting to mass-flow resedimented to tractionally reworked. In addition, post-depositional hydrothermal or hydraulic brecciation or fracturing of volcanic rocks, driven by sub-surface, overpressured hydrothermal or metamorphic fluids, and tectonic fracturing of volcanic rocks commonly produce volcanic breccias in volcanic settings. Although all the processes are distinctive, the characteristics of the resulting deposits can often be very similar, and not always easily distinguishable, especially in older volcanic successions. It is therefore important to establish criteria to distinguish the deposits and the processes from each other.