Igneous Rocks

Igneous rocks are formed by the cooling and solidification of molten rock (magma) from beneath the earth's crust. They are normally crystalline in structure. They do not occur in strata (layers) nor do they contain fossils. Igneous rocks may be subdivided on the basis of mineral composition. When they contain a high proportion of silica they are said to be acid. Acid igneous rocks, such as granite, are less dense and are lighter in colour than basic rocks. These contain a greater proportion of basic of basic oxides, e.g. of iron, aluminum or magnesium, and are thus denser and darker in colour.
In terms of origin there are two main classes of
igneous rocks.

1.     Plutonic rocks. These are igneous rocks, formed at some depth in the earth's crust. They have cooled and solidified slowly so that large, easily-recognized crystals have been able to form. These intrusive rocks, such as granite, diorite and gabbro, are exposed to the surface by the processes of weathering and erosion.

2.     Volcanic Rocks. These are molten rocks poured out of volcanoes as lavas. They solidify rapidly on the earth's surface and the crystals are small.
Basalt is a common volcanic or extrusive rock and forms lava flows, lava sheets and lava plateaux, e.g. those of Antrim in Northern Ireland, the Deccan Plateau of India, the Columbia-Snake Plateau in U.S.A., and the Cameroon Mountains. Traces of lava flows which have formed basalt plateaux are found in northern Nigeria: they are the Longuda Plateau, north of Benue and west of the Gongola river, and the Biu Plateau east of Gongola; so also are small volcanic cones with lava flows. The air of Niger Republic shows volcanic features in form of lavas, ashes and tuffs over a very wide area. Some kinds of basalt solidify in a very peculiar manner to form long Polygonal columns. A well-known example is the columnal basalt of Giant's Causeway in Antrim. Similar examples in Nigeria are the Assob basalt of the south-west of Jos through which the Kafanchan-Jos railway and the Kaduna-Jos road pass; and in Senegal are the basalt of Goree, Cape Manuel, the islets of Madeleine, and of the Mamelles which form the most westerly point in Africa - the Almadi Point. Some of the molten lava may push its way to the surface through clefts and passages, solidifying as vertical dykes or horizontal sills. Their origin and occurrence will be discussed in my later posts of volcanism and earthquakes.
Most igneous rocks are extremely hard and resistant. For this reason, they are quarried for road-making and polished as monuments and grave-stones.