Sedimentary rocks are produced by the lithification of sediments. Lithification turns individual sediment particles into rock in one or more of the following ways:
- Compaction of the grain framework, with expulsion of pore fluids. Muds compact more than sandstones.
- Pressure solution; for example, quartz or calcite grains dissolve at their points of contact.
- Cementation; a mineral material, typically quartz or calcite crystallises in the pore space between grains.
Five main types of sediments produce sedimentary rocks:
- Terrigenous clastics; comprising particles, mostly of silicate minerals, derived from pre-existing rocks. Also know as siliciclastics.
- Volcaniclastics; products of pyroclastic eruptions or the breakdown of volcanic rocks. (These are delt with in a separate section, as they overlap with volcanic rocks.)
- Carbonates; rocks containing >50% carbonate minerals, principally from hard parts of organisms.
- Evaporites; formed by precipitation of salts from water.
- Other non-clastics; for example, coal, ironstones, phosphates and siliceous sediments.
[Modified from Nigel Woodcocks NST 1A L25 and L30]