Sediment forms when rocks and other materials are broken down and moved, then deposited and lithified into layers. The process involves weathering, erosion, transportation, deposition, and diagenesis, and sediments accumulate where the transporting agents lose energy or encounter environments that trap material. What sediment is and how it forms
- Weathering breaks down existing rocks and minerals into smaller pieces or dissolves them. This creates sediments such as sand, silt, clay, or chemical precipitates. Sedimentary particles may originate from land surfaces, rivers, lakes, oceans, or deserts.
- Erosion and transport move these particles from their source to new settings by water, wind, ice, or gravity. The distance and mechanism influence sediment size, sorting, and shape.
- Deposition occurs when the transporting energy decreases, causing sediments to settle out of the transporting medium. This happens in rivers, deltas, lakes, deserts, and sea floors.
- Diagenesis and lithification (compaction and cementation) transform loose sediments into solid sedimentary rocks, often over long timescales.
Where sediment forms on the landscape
- In rivers and floodplains, sediments are carried downstream and deposited as flow velocity drops, creating features like point bars, levees, and floodplain muds.
- In deserts, wind transports and sorts sands, forming dune fields; in loess regions, fine silt is deposited over vast areas.
- In lakes and oceans, sediments settle to the bottom from suspension, forming layered deposits that record past environments.
- Glacial and periglacial settings produce sediments via melting, subglacial grinding, and gravity-driven flows, leaving behind tills, gravels, and outwash sediments.
Key takeaways
- Sediments originate from preexisting rocks or once-living organisms, then are transported, deposited, and transformed into rocks through burial and chemical changes.
- The landscape location of sediment formation is tied to transport agents and energy: rivers and floodplains (water), deserts (wind), coastlines and basins (sea), and glacial areas (ice).
- Sedimentary rocks typically form in layers (beds or strata) and record past environmental conditions when lithified.