sediment, mineral or organic particles that are deposited by the action of wind, water, or glacial ice. These sediments can eventually form sedimentary rocks (see rock).
Classification of Sediments
Sediments are commonly subdivided into three major groups—mechanical, chemical, and organic.
Mechanical, or clastic, sediments are derived from the erosion of earlier formed rocks on the earth's surface or in the oceans. These are then carried by streams, winds, or glaciers to the site where they are deposited. Streams deposit sediment in floodplains or carry these particles to the ocean, where they may be deposited as a delta. Ocean sediments, especially in the form of turbidites, are usually deposited at the foot of continental slopes (see oceans). Glaciers carry sediment frozen within the mass of the ice and are capable of carrying even huge boulders (erratics).
Chemical sediments are formed by chemical reactions in seawater that result in the precipitation of minute mineral crystals, which settle to the floor of the sea and ultimately form a more or less chemically pure layer of sediment. For example, evaporation in shallow basins results in a sequence of evaporite sediments, which include gypsum and rock salt.
Organic sediments are formed as a result of plant or animal actions; for example, peat and coal form by the incomplete decay of vegetation and its later compaction. Deep-ocean sediment known as pelagic ooze consists largely of the remains of microscope organisms (mostly foraminifera and diatoms) from the overlying waters as well as minor amounts of windblown volcanic and continental dust. Limestones are commonly formed by the aggregation of calcite shells of animals.
Formation of Sedimentary Rock
Sediments form sedimentary rock by compaction and cementation of the particles. Thus, coarse sediments become conglomerates; sands become sandstone; and muds become shale. Sedimentary rocks make up only about 5% of all rocks of the earth's crust, yet they cover 75% of the land area in a veneer that averages 2.26 km (1.4 mi) in thickness, ranging from 0 to 12.9 km (0–8 mi).
sed·i·ment / ˈsedəmənt/ • n. matter that settles to the bottom of a liquid; dregs. ∎ Geol. particulate matter that is carried by water or wind and deposited on the surface of the land or the bottom of a body of water, and may in time become consolidated into rock. • v. [intr.] settle as sediment. ∎ (of a liquid) deposit a sediment. ∎ [tr.] deposit (something) as a sediment: the DNA was sedimented by centrifugation | [as adj.] (sedimented) sedimented waste. DERIVATIVES: sed·i·men·ta·tion / ˌsedəmənˈtāshən/ n.
A mixture of sand, silt , clay, and perhaps organic components. Soil eroded from one location and deposited in another is identified as sediment. The sedimentary fraction has the ability to carry not only the mineral (sand, silt, and clay) and organic (humus ) components, but also other components that may be attached such as nitrogen compounds, herbicides, and pesticides. These riders are of high concern to those involved in environmental studies. Products applied to the soil in one location and beneficial to that system may be transported to other locations where the effect is detrimental to the habitat of other life forms. Care must be exercised: 1) when applying supplementary items to the soil, and 2) to develop systems that keep sediment from finding its way into the streams and water bodies.