Sediments

Sediments are unconsolidated or partially consolidated accumulations of solid particles that originate from the weathering of rocks, microbial activity, or chemical precipitation. They accumulate at the Earth's surface and in bodies of water and may later be altered by diagenesis to form sedimentary rocks. Sediments are commonly classified by grain size into clastic (siliciclastic) sediments such as gravel, sand, silt, and clay; their names reflect particle size ranges (gravel > 2 mm, sand 0.062–2 mm, silt 0.004–0.062 mm, clay < 0.004 mm). They may exhibit sorting and roundness that record transport distance and energy conditions. Chemical and biochemical sediments arise from precipitation of minerals from solution or by organisms; examples include limestones, cherts, evaporites such as halite, and coal derived from plant material.

Sediments form where energy conditions allow deposition after transport by water, wind, or ice. River and delta deposits accumulate in fluvial and shoreline settings; lake and wetland beds form in quieter waters; aeolian dunes in deserts; deep-sea sediments gather on continental shelves and in basins. The character of a sedimentary deposit—grain size, composition, rounding, sorting, and fossil content—records its environment of origin, transport history, and time.

During diagenesis, burial and chemical alteration consolidate sediments. Compaction lowers porosity; cementation binds grains by minerals such as calcite, silica, or iron oxides; recrystallization may occur. Over geologic time these processes produce sedimentary rocks such as sandstone, shale, and limestone.

Sediments are central to geology and Earth history as archives of past climates and environments. They host vital groundwater and hydrocarbon reservoirs, provide raw materials for construction, and influence ecological and environmental processes through sedimentation and erosion.