Clastic

Clastic is a geological term used to describe sediments and sedimentary rocks composed primarily of fragments, or clasts, derived from preexisting rocks or minerals. The term contrasts with chemical or biochemical sedimentary rocks, which form by precipitation from solution or accumulation of organic material. Clasts are transported by water, wind, or ice and deposited in layers before being converted into rock through lithification, which involves compaction and cementation.

In clastic rocks, the basic components are clasts, a fine-grained matrix that may fill the spaces between clasts, and a cement that binds the clasts together. Grain size is a central criterion in classification. Common categories include clay (<4 micrometers), silt (4–62 micrometers), sand (0.062–2 millimeters), and gravel (>2 millimeters). Typical clastic rocks are shale or claystone (dominated by clay), siltstone, sandstone (sand-sized grains), conglomerate (rounded gravel clasts), and breccia (angular gravel clasts).

Sorting and rounding reflect the energy and distance of transport. Well-sorted rocks have nearly uniform grain sizes, while poorly sorted rocks contain a range of sizes. Rounding indicates abrasion during transport; more rounded clasts usually imply longer transport.

Formation involves weathering, erosion, transport, deposition, burial, compaction, and cementation. Common cementing minerals include quartz, calcite, and iron oxides, which help bind clasts into solid rock. Clastic textures and structures are central to sedimentary geology and help reveal past environments and processes.