Question

How do you think sedimentary rocks are formed?

Answer 1

Short answer: sedimentary rocks form when loose particles (sediment) or dissolved minerals are deposited, buried, and turned into solid rock by compaction and cementation — or when biological material accumulates and is preserved.

More detail, step by step:
- Weathering and erosion: existing rocks are broken down into sand, silt, clay, or dissolved ions by physical and chemical weathering. Biological activity also produces bits of shell, bone, plant matter.
- Transport: wind, water, ice, or gravity carries the particles and dissolved material away from their source.
- Deposition: when the transporting agent loses energy, sediments settle out and accumulate in layers in places like river beds, deltas, lakes, deserts, and the seafloor.
- Burial and compaction: additional layers bury earlier deposits; the weight of overlying material squeezes the grains closer together and expels pore water.
- Cementation (and chemical precipitation): minerals such as calcite, silica, or iron oxides precipitate from groundwater and glue the grains together. In chemical or organic cases, minerals precipitate directly from solution (evaporites, limestone) or organic matter is compacted into coal.
- Diagenesis: chemical and physical changes at low temperature and pressure further harden and alter the sediment into rock.

Main types:
- Clastic (detrital): made of fragmented rock grains (e.g., sandstone, shale, conglomerate).
- Chemical: formed by precipitation of minerals from solution (e.g., limestone, rock salt, gypsum).
- Organic: formed from accumulated biological material (e.g., coal, some types of limestone made of shells).

Clues these rocks preserve: layering (bedding), grain size and sorting, roundedness (transport history), and fossils — all useful for interpreting past environments. Sedimentary rocks typically form at low temperature/pressure and host most groundwater, petroleum, and many mineral resources.