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how is sedimentary rock formed

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Answered by GPT-5 mini AI
Sedimentary rocks form from the accumulation and lithification of sediments. There are three main types (clastic, chemical, organic) and a common sequence of processes:

Main steps (clastic rocks)
- Weathering: rock at Earth’s surface is broken down by physical and chemical processes into particles (sand, silt, clay).
- Erosion and transport: wind, water, ice, or gravity carry the particles away from their source.
- Deposition: sediments settle out when the transporting medium slows (river floodplain, lake bottom, delta, beach, ocean floor, desert).
- Burial and compaction: overlying sediments squeeze out pore water and pack the grains closer together.
- Cementation (diagenesis): minerals precipitate from groundwater (common cements: calcite, silica, iron oxides) and bind the grains into solid rock. The combined compaction + cementation is called lithification.

Chemical and organic routes
- Chemical sedimentary rocks form when minerals precipitate from solution (evaporites like halite and gypsum, some limestones, chert).
- Organic sedimentary rocks form from accumulated biological material (coal from plant debris, some limestones and chalk from shells and skeletons).

Typical examples and features
- Sandstone (cemented sand), shale (compacted clay), conglomerate (rounded gravel), limestone (calcite; chemical or biological).
- Sedimentary rocks commonly show layering (bedding), fossils, ripple marks, cross-bedding, and sorting of grain sizes.

Why it matters
- They record past environments and climate, contain most fossils, and host resources like coal, oil, gas, and groundwater.