(a) Outline the characteristics of sedimentary rocks.
(b) Describe two ways in which sedimentary rocks can be formed
(a) Characteristics of Sedimentary Rocks
(1) Sedimentary rocks are stratified rocks or rocks that show distinct layers or strata
(2) They are non-crystalline
(3) They are usually soft
(4) Not resistant to erosion
(5) They are permeable
(6) They may be coarse or fine grained depending on the nature of materials deposited.
(7) They are fossilized rocks / they contain fossils of plants and animals.
(8) Examples are sandstone, coal, limestone.
(b) Sedimentary rocks can be formed mechanically, organically and chemically
(i) Mechanically formed Sedimentary Rocks
These are derived from sediments of pre-existing rocks. The rock debris are usually deposited by agents of erosion such as water, wind or glacier. The deposited materials are pressed together to become solid by the weight of overlying deposits. This process of solidification is called compaction or cementation. They may form conglomerates.
(ii) Organically formed Sedimentary Rocks
These are formed from the remains of living organisms such as plants and animals. The remains of animals like corals or shellfish form calcareous rocks. Those formed from the remains of vegetation / plants such as forest or swamps are called carbonaceous rocks. Examples are limestone, chalk, etc.
(iii) Chemically formed Sedimentary Rocks These are rocks formed from precipitation from other rock solutions or certain minerals from solution of salt. They are often referred to as evaporates. They are also from crystallization. Examples include potash, gypsum etc.
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