Volcaniclastics

Volcaniclastic rocks are clastic rocks whose components are derived from volcanic sources. They include rocks formed by fragmentation of volcanic rocks and glass during eruptions as well as sediments produced by the reworking of volcanic debris in air, water, or gravity-driven processes.

Particle sizes in volcaniclastic deposits range from fine volcanic ash (<2 mm) to lapilli (2–64 mm) and larger blocks or bombs (>64 mm). These rocks originate from explosive fragmentation of magma and volcanic rocks and are transported by wind, water, or gravity. Airfall deposits settle from eruption plumes as ash beds and pumice layers; pyroclastic density currents yield ignimbrites and volcanic breccias; fluvial, deltaic, lacustrine, and marginal-marine settings rework ash into volcaniclastic sandstones, siltstones, and conglomerates. Some deposits weld when hot ash accumulates, forming welded tuff or ignimbrite.

Common lithologies include tuffs (beds dominated by volcanic ash and pumice), ignimbrites (deposits from hot pyroclastic flows, often welded), volcanic breccias (coarse, angular fragments), agglomerates (coarse pyroclastic deposits with bombs), lapilli-tuffs, and crystal tuffs.

Volcaniclastic rocks occur in volcanic arcs, flood-basalts provinces, and in sedimentary basins receiving volcanic input. They provide important tephrochronological markers and help reconstruct eruption histories. Diagenesis and cementation influence their porosity and permeability, which can affect reservoir or aquifer properties in some basins. The term “volcaniclastic” thus covers a broad spectrum of pyroclastic and reworked volcanic rocks used in stratigraphy and sedimentary geology.