Theory question 206
Rocks(a) Outline the characteristics of sedimentary rocks. (b) Describe two ways in which sedimentary rocks can be formed
Show the model answer
(a) Laid down in layers (strata) separated by bedding planes; usually contain fossils; non-crystalline; often porous and permeable; relatively soft and easily eroded; may be folded or tilted; formed mostly under water; varied particle sizes.
(b) Mechanically formed: particles from weathered rock are carried by rivers, wind, ice or waves, deposited in layers and compressed and cemented (sandstone, shale, clay, conglomerate). Organically formed: remains of plants (coal) or of shells and skeletons of animals (limestone, chalk, coral) pile up and are compressed. Chemically formed: minerals precipitate from solution when water evaporates (rock salt, gypsum, potash).
For each mode give the process and examples.