(a) Outline the characteristics of sedimentary rocks. (b) Describe two ways in which sedimentary rocks can be formed

Answer Details

(a) Characteristics of sedimentary rocks:

• They are formed in layers (strata/beds) which are usually visible as bedding planes.
• They are non-crystalline and generally softer than igneous rocks.
• They frequently contain fossils of plants and animals buried during their formation.
• They are made of compacted and cemented sediments (particles of pre-existing rocks or organic matter), and are often porous and permeable.
• They are stratified and can be split along the layers.

(b) Two ways sedimentary rocks can be formed:

• Mechanically (clastic/detrital formation): rocks are weathered and eroded into fragments; these sediments are transported by water, wind or ice, deposited in layers, then compacted and cemented over time into rock such as sandstone and shale.
• Organically (biological formation): the remains of living things accumulate and are compacted, e.g. the shells and skeletons of sea organisms forming limestone/chalk, or plant remains forming coal. (Chemically formed sedimentary rocks, e.g. rock salt from evaporation, are also acceptable.)