Pyroclasts

Pyroclasts are rock fragments ejected during volcanic eruptions. They range in size from fine volcanic ash to large rocks and are a major component of tephra. Pyroclasts are produced by explosive fragmentation of magma and surrounding rock as volatile gases drive rapid expansion, often accompanied by column collapse or other explosive processes.

Size-based categories commonly used are ash, lapilli, and bombs or blocks. Ash consists of particles smaller than 2 millimeters that are carried by eruption clouds and fall as ash fall deposits. Lapilli are 2 to 64 millimeters in diameter and may fall to the ground or be transported a short distance. Bombs and blocks exceed 64 millimeters; bombs are often aerodynamically shaped and solidify in flight, while blocks are larger, massive fragments.

Formation mechanisms include fragmentation of magma during rapid decompression, vesiculation, and brittle failure of magma and conduit walls. Pumice is a vesicular, highly buoyant pyroclastic rock formed when highly gas-rich magma cools and depressurizes rapidly. Glassy shards arise from rapid quenching of molten material, producing angular particles with a glassy texture.

Transport and deposition vary with eruptive style. Pyroclastic fall plumes produce widespread ash deposition and layered tephras, while pyroclastic density currents generate ground-hugging flows and surge deposits called ignimbrites. Distinct hazards arise from ash clouds, ballistic ejecta, and fast-moving flows.

The study of pyroclasts aids tephrochronology, stratigraphy, and hazard assessment. Notable eruptions with extensive pyroclastic output include Vesuvius in 79 CE, Krakatoa in 1883, and Mount St. Helens in 1980.