THE INFLUENCE OF ROCK TYPES ON LANDSCAPES

The appearance and characterstic features of landforms are greatly influenced by the underlying rock type. Softer rocks like clay and shale are worn down much faster than harder rocks like granite.

The ancient rocks which dominate a great part of West Africa have been so greatly worn down by erosion that they now appear as peneplains. Granite domes (or inselbergs) have been formed where the old rocks resist erosion; and in some cases, such resistant rocks produce high relief, such as the Jos Plateau of Nigeria, the Akwapim-Togo-Atacora ridges and the Birrimian uplands of Ghana, and the interior plateaux and mountains of Sierra Leone. Shales, schists and sandstones being less resistant, form the much lower rounded hills. Recent river sediments form flat plains.

Volcanic activities in some parts of West Africa had produced landscapes of high elevation such as the Cameroon Mountains; the Cameroon and Bamenda Highlands; the Jos Plateau of Nigeria: Cape Manuel, Goree and the Mamelles of Cape Verde (Senegal). The islands of Fernando Po, Principle, Sao Tome and Annobon contain similar high reliefs too. The Sierra Leone Peninsula, which was formed from the basic intrusive rocks, now has erosion platforms of varying heights, whilst Cape Mount in Liberia is another relief feature formed from Pre-Cambrian (basic) intrusive rock.

Impressive scenic features in the form of gorges, scarps, etc ., have been produced where running water and other agencies of denudation have heavily dissected or cut through the basic rocks. Such features are commonly seen on the Fouta Djallon Highlands of Guinea, and the scarps of Mali, south-western Burkina Faso and Mampong of Ghana. Limestones, resistant because of their permeability, form prominent steep-sided hills such as those near Ipoh and in Perlis near Kuala Lumpur, Malaysia.

Uses of Rocks

Rocks are very important economically. The thin surface layer of the earth, formed by the break-down of the rocks in various ways and by various processes, is known as soil. The geographer is, among other things, interested in the soil as the medium in which plants grow, hence the scene of the production of most of man’s food and most of his raw materials.

Man’s vital water supply is connected with the nature of the rocks. Part of our drinking water is got by sinking wells to tap the sub-surface water which is filtered naturally during its passage through the rocks. The amount of surface runoff in the form of rivers and springs affects the siting of villages.

Rocks provide building materials. Granite is quarried in many parts of Nigeria for road construction, decking of houses and other building purposes.The Freedom and Development Company quarry at Okpella in Edo State is a fine example of a granite quarry.

Various limestones are burnt to produce lime, for mortar and for agricultural use. Cement is made by burning a mixture of two-thirds limestone or chalk and one-third clay, and grinding the result to fine powder. There are cement works in Ewekoro and Sagamu in Ogun State, Nkalagu in Anambra State, Okpella in Edo State, Sokoto in Sokoto State, Ashaka in Bauchi State, Yandev in Benue State and Calabar in Cross River State.

Some rocks are important to man as sources of fuel. Lignite, bituminous and steam coals, anthracite and mineral oil (petroleum) are examples of vital sources of fuel.

In various parts of the world, there are large deposits of salts. Examples are phosphates in Tunisia and Morocco, and potash at Stassfurt in Germany. Salt is obtained near Kaolack in Senegal and is refined in the town.

Of great economic significance is brick which is made from clay. The pottery industry also uses various types of clay.

Gravel is quarried to make concrete for use in the building industry. The commonest types are quartz and laterite gravel.

Rocks are sources of various economic metals. Metals such as gold, copper, tin, lead, zinc and silver, though extremely rare in ordinary rocks, are locally concentrated in ore deposits which can be economically extracted. Examples are tin and columbite in the Jos Plateau, gold of the alluvial type from Ilesa in Osun State of Nigeria, lead and zinc south of Abakaliki in Anambra State, and copper in Akjoujt in Mauritania.