clastos

Clastos, from the Greek klastos meaning broken, is a term used in geology and sedimentology to describe fragments of pre-existing rock or mineral grains that have been separated from their source and transported as discrete pieces. In general usage, clasts are the building blocks of clastic sedimentary rocks, which form when these fragments are deposited and lithified.

A clast can vary widely in composition, including quartz grains, feldspar grains, lithic fragments, or a mixture. They also vary in size, shape, and texture. Clasts may be angular, subangular, subrounded, or well rounded, with the degree of rounding reflecting the distance and energy of transport. Size classifications commonly used for clasts in sedimentary geology include clay (<0.004 mm), silt (0.004–0.062 mm), sand (0.062–2 mm), and gravel (>2 mm), with gravel subdivided into pebbles (2–64 mm), cobbles (64–256 mm), and boulders (>256 mm).

Clasts are cemented together with minerals such as silica, calcite, or iron oxides to form clastic rocks, in which the clasts are the primary constituents and the matrix or cement binds them. Common clastic rocks include shale and mudstone (dominated by clay or silt clasts), sandstone (sand-sized clasts), conglomerate (rounded gravel clasts), and breccia (angular gravel clasts).

In addition to sedimentary geology, the term also appears in volcanic contexts as volcanic clasts or tephra, which are fragments ejected during eruptions and may constitute pyroclastic deposits. Clast analysis helps infer transport histories and past environments.