(a) What are sedimentary rocks? (b) Name the three classes of sedimentary rocks according to their mode of formation. (c) Describe one of the classes of sed...

Answer Details

(a) What are sedimentary rocks?

Sedimentary rocks are rocks formed from the accumulation, compaction and cementation (lithification) of sediments, that is, particles of older rocks, mineral matter or the remains of plants and animals, usually deposited in layers or strata by water, wind or ice.

(b) Three classes according to mode of formation

• Mechanically-formed (clastic) sedimentary rocks - e.g. sandstone, conglomerate, shale.
• Chemically-formed sedimentary rocks - e.g. rock salt, gypsum, some limestones.
• Organically-formed (biogenic) sedimentary rocks - e.g. coal, chalk, coral limestone.

(c) Description of one class - Mechanically-formed sedimentary rocks

These are formed from fragments (detritus) of pre-existing rocks that have been broken down by weathering and erosion. The particles are transported by rivers, wind or ice and deposited in layers. Over time the weight of overlying material compacts them and mineral cement binds the grains together. They are classified by grain size: coarse (gravel forming conglomerate), medium (sand forming sandstone) and fine (mud and clay forming shale). They are typically arranged in visible strata (bedding planes).

(d) Four ways sedimentary rocks are important to man

• They are the main source of fossil fuels - coal, crude oil and natural gas.
• They yield valuable minerals and building materials such as limestone (for cement), sandstone, clay, gypsum and rock salt.
• They form fertile soils and often contain underground water (aquifers) used for water supply and agriculture.
• They preserve fossils that help geologists in the study of earth history and in the search for petroleum and other resources.