Question

How are the three types of sedimentary rocks different in their formation?

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Answer 1

Sedimentary rocks form in three main ways — clastic (detrital), chemical, and organic (biochemical). The key difference is what is deposited and how it becomes rock:

- Clastic (detrital)
- Made of broken rock and mineral fragments produced by weathering and erosion.
- Transported by water, wind, or ice, deposited, then buried and lithified by compaction and cementation.
- Typical rocks: conglomerate (coarse), sandstone (medium), shale/mudstone (fine).

- Chemical
- Form when minerals precipitate directly from aqueous solution (often by evaporation or chemical change).
- Common in closed basins, evaporating seas, or spring settings.
- Typical rocks: rock salt, gypsum, some limestones and chert formed by inorganic precipitation.

- Organic (biochemical)
- Made from accumulation of biological material (plant debris or shells/skeletons of organisms).
- Organic matter or biogenic mineral fragments accumulate, compact, and lithify.
- Typical rocks: coal (plant remains), coquina and chalk (shell/skeletal accumulations), some limestones formed mostly of organisms’ CaCO3.

In short: clastic = fragments of other rocks, chemical = minerals precipitated from solution, organic = remains of living things.