klastes

Klastes, from the Greek klastos meaning broken, is a term used in geology and sedimentology to denote discrete fragments of pre-existing rocks or minerals that have been weathered, eroded, transported, and redeposited. In modern English usage the term clast is standard, but klastes appears in some academic discussions with emphasis on its Greek origin.

A clast/klaste is defined by its lithology (mineral composition) and its textural attributes. Size categories are commonly reported as clay (<0.004 mm), silt (0.004–0.0625 mm), sand (0.0625–2 mm), granule (2–4 mm), pebble (4–64 mm), cobble (64–256 mm), and boulder (>256 mm). Clasts may be quartzose (dominated by quartz) or lithic (composed of various rocks). Shapes range from angular to subrounded or rounded, reflecting transport distance and mechanisms.

Formation and transport: Clasts form through weathering and fragmentation of parent rocks. They are transported by rivers, glaciers, wind, or gravity, and may experience abrasion and sorting before final deposition. In sedimentary rocks, clasts are embedded in a matrix and their abundance, size distribution, and mineralogy are used to interpret depositional settings and provenance.

Analytical use: Geologists study clast content through field observations and petrographic analysis, including point counting in thin sections. Clast data support reconstructions of ancient environments, such as rivers, deltas, glacial deposits, and desert aeolian systems, and inform diagenetic history and sediment maturity.