clasts

Clasts are fragments of pre-existing rocks or minerals produced by mechanical weathering. They are discrete pieces that can be transported and deposited as clastic sediments or ejected during volcanic activity. The term is used in both sedimentology and volcanology, with context indicating different processes.

In clastic sedimentary rocks, clasts are the main constituents alongside cementing material. They range in size from boulders to clay, commonly classified as boulder (>256 mm), cobble (64–256 mm), pebble (4–64 mm), sand (0.062–2 mm), silt (0.004–0.062 mm), and clay (<0.004 mm). Clasts are described by shape (angular to well-rounded) and by composition (quartz-rich, lithic, feldspathic, carbonate). Transport dynamics influence rounding and sorting: longer transport or higher-energy environments tend to yield more rounded and better-sorted clasts, whereas proximal or rapid deposition yields angular, poorly sorted clasts. The clast population records the source lithology and sedimentary history.

In volcanology, clasts refer to fragments ejected during eruptions, including juvenile magma fragments and lithic rock fragments, ranging from volcanic ash to large bombs.

Provenance analysis uses clast characteristics to infer tectonic setting and erosion history. Overall, clasts constitute the fundamental units of many sedimentary rocks and play a key role in interpreting geological processes and environments.