(a) Define the term sedimentary rock and describe any two processes of its formation (b) Discuss any three ways in which sedimentary rocks are important to ...

(a) Sedimentary rock and processes of its formation

A sedimentary rock is a rock formed from the accumulation, compaction and cementation of sediments (fragments of older rocks, mineral particles, or the remains of plants and animals) usually in layers or strata.

Two processes of formation:

• Mechanically formed (clastic): rocks are broken down by weathering and erosion; the fragments (gravel, sand, silt, mud) are transported by water, wind or ice, deposited in layers, then compacted and cemented to form rocks such as sandstone, conglomerate and shale.
• Organically formed: the remains of plants and animals accumulate and are compacted over time; plant remains form coal, while shells and skeletons of marine organisms form limestone. (Chemically formed sedimentary rocks such as rock salt and gypsum arise from evaporation and precipitation of dissolved minerals.)

(b) Importance of sedimentary rocks to man

1. They are the main source of fossil fuels - coal, petroleum and natural gas - which provide energy.
2. They yield valuable minerals and building materials such as limestone (for cement), sandstone, clay, gypsum and rock salt.
3. They weather into deep, fertile soils (e.g. from shales and limestone) that support agriculture, and their porous layers store underground water (aquifers).