(a) Explain briefly the processes of formation of (i) igneous rocks; (ii) sedimentary rocks; (iii) metamorphic rocks. (b) Give two examples of each of igneous and metamorphic rocks and three examples of sedimentary rocks.
Explanation
(a)(i) Igneous rocks are formed as a result of hot molten magma escaping to earth surface and cooling. On reaching the surface, the rocks cool to form glassy o.r coarsely crystalline textured mass. Some of the magma may cool before reaching the earth surface thus forming plutonic rocks while some may reach the surface to form volcanic rocks. Rapid cooling gives rise to fine grained rocks, while slow cooling gives raise to larger grained rocks.
(ii) Sedimentary rocks are formed when pre-existing rocks are broken down by geological denudation agents. The materialsproduced are moved from their places of origin and deposited elsewhere as solid particles or in solution. The particles are usually laid down in strata or beds and they are usually cemented together by organic matter, cementing agents or pressure. They contain fossils.
(iii) Metamorphic rocks are derived from pre-existing sedimentary or igneous rocks by the agent of metamorphism such as heat, pressure and chemical change. The changes often take place at some depth within the earth's crust, e.g. earth movements and folding subject rocks to great heat and pressure and new rocks of new types called metamorphic rocks are produced. Under the new conditions, some of the original minerals are no longer stable and they give place to other minerals fitted for the new environment.
(b) Examples of igneous rocks are: Granite, diorite, gabbro, basalt, syenite, pumice. Examples of metamorphic rocks are: Gneisss, schist, quartzite, marble, slate. Examples of sedimentary rocks are: limestone, sandstone, coal, clay, dolomite, shale, mudstone, peat, lignit, gypsum.