clastische

Clastic, often translated as clastische in some languages, is a geological term used to describe sedimentary rocks whose primary constituents are clasts—fragments of preexisting rocks or minerals produced by weathering and erosion. Clasts vary from clay-sized particles to boulders, and the overall character of a clastic rock depends on grain size, sorting, rounding, and mineral composition.

Formation: Clastic rocks form when worn fragments are transported by water, wind, or ice and accumulate as layers. Over time, burial leads to diagenesis, during which compaction reduces porosity and crystals precipitate cementing minerals (such as quartz, calcite, or iron oxides) to bind grains into a solid rock.

Classification: By clast size, clastic rocks include shale or mudstone (very fine), siltstone, sandstone (sand-sized), conglomerate (rounded clasts), and breccia (angular clasts). Sorting describes how uniform the grain sizes are; rounding describes the shape of clasts. Compositional labels include quartz arenite, arkose, and lithic sandstone, reflecting dominant mineral content.

Environment and interpretation: The distribution of clastic rocks records past environments and transport distances, such as rivers, deltas, beaches, deserts, and offshore settings. Their porosity and permeability vary with grain size and cementation, influencing groundwater flow and hydrocarbon reservoirs.

Relation to other rocks: Clastic rocks differ from chemical and biochemical sedimentary rocks, which form by precipitation of minerals from solution or from organic matter, and from metamorphic rocks formed under pressure and temperature conditions that recrystallize existing materials.