Sedimentary rocks form in two ways, and the first question in naming one is which way. Clastic rocks are piles of broken grains, the sediment of earlier rocks, pressed and cemented together. Chemical and biochemical rocks grow from minerals dissolved in water, either crystallizing as water evaporates or built into the shells and skeletons of living things.
| Clastic rock | Grains | Chemical or biochemical rock | Made of |
|---|---|---|---|
| Conglomerate | rounded gravel (over 2 mm) | Limestone | calcite, mostly from shells and skeletons |
| Breccia | angular gravel | Rock salt | halite, from evaporating water |
| Sandstone | sand, 1/16 to 2 mm | Rock gypsum | gypsum, from evaporating water |
| Siltstone | silt | Chert | microscopic quartz |
| Shale | clay; splits into thin sheets | Coal | compressed plant remains |
Why it works
Names follow how the rock formed. For clastic rocks, grain size is the record of energy, so it comes first; for gravel, shape adds a clue about distance, since rounded gravel traveled and angular gravel did not. For chemical rocks, the mineral tells the story: calcite means life in warm seas, halite and gypsum mean water evaporated, plant carbon means a swamp. Turning loose sediment into rock is called lithification: burial packs the grains (compaction), and minerals such as calcite or quartz crystallize between them as cement.
Key idea
Clastic rocks are named by grain size, and gravel rocks by shape: conglomerate, breccia, sandstone, siltstone, shale. Chemical and biochemical rocks are named by composition: limestone, rock salt, rock gypsum, chert, coal.
Worked examples
Worked example: Name a clastic rock
A rock is made of angular pieces of gravel, 5 to 30 mm, cemented together. Name it.
- Over 2 mm: gravel, so conglomerate or breccia.
- Angular pieces: breccia. The sharp corners mean they did not travel far, perhaps from a rockfall or fault zone.
Answer: Breccia.
Worked example: Name a chemical rock
A gray rock fizzes in dilute acid and holds broken shells. Name it.
- Fizzing in acid means calcite.
- Calcite with shells: limestone, a biochemical rock.
Answer: Limestone.
Common mistake
Many people picture limestone as sand cemented together. Most limestone is calcite, built by corals, shellfish and microscopic algae in warm, shallow seas and later cemented. Check with acid: limestone fizzes; a quartz sandstone does not.