Much of Nigeria's surface consists of ancient crystalline rocks of the African Shield. Having been subject to weathering and erosion for long periods, the characteristic landscape of this area is extensive level plains interrupted by occasional granite mountains. These features form a major landscape type of Nigeria and of West Africa as a whole. There are also smaller areas of younger granites found, for example, on the Jos Plateau.
Sedimentary strata dating from various periods overlay the older rocks in many areas. The sedimentary areas typically consist of flat-topped ridges and dissected plateaus and a characteristic landscape of extensive plains and no major rocky outcrops. This landscape is generally true of the basins of the Niger and Benue rivers as well as the depressions of the Chad and Sokoto basins in the far northeast and northwest of the country, respectively. The most dramatic of the sedimentary landscapes are in southeastern Nigeria, where thick sedimentary beds from the Abakaliki Uplift to the Anambra Basin have been tilted and eroded. This process has resulted in a rugged scarp land topography with east-facing cliffs at in the Udi Hills, north of Enugu, and in the area around Nanka and Agulu.
Although relatively little of the Nigerian landscape has been shaped by volcanic episodes, there are two main areas of volcanic rock. They are found on the Biu Plateau in the northeast, extending into some localized volcanic areas along the eastern border with Cameroon, and on the Jos Plateau in the northern center of the country.
The elevational pattern of most of Nigeria consists of a gradual rise from the coastal plains to the northern savanna regions, generally reaching an elevation of 600 to 700 meters. Higher altitudes, reaching more than 1,200 meters in elevation, are found only in isolated areas of the Jos Plateau and in parts of the eastern highlands along the Cameroon border. The coastal plain extends inland for about ten kilometers and rises to an elevation of forty to fifty meters above sea level at its northern boundary. The eastern and western sections of the coastal plain are separated by the Niger Delta, which extends over an area of about 10,000 square kilometers. Much of this is swampland, separated by numerous islands. The coastal plain region penetrates inland about seventy-five kilometers in the west but extends farther in the east. This region is gently undulating with elevation increasing northward and a mean elevation of about 150 meters above sea level. Much of the population of southern Nigeria is located in these eastern and western coastal plains and in some of the contiguous areas of the coast and the lower Niger Basin.
Separating the two segments of the coastal plain and extending to the northeast and northwest are the broad river basins of the Niger and Benue rivers. The upper reaches of these rivers form narrow valleys and contain falls and rapids. Most of the lower portions, however, are free from rapids and have extensive floodplains and braided stream channels. To the north of the Niger and Benue basins are the broad, stepped plateau and granite mountains that characterize much of northern Nigeria. Such mountains are also found in the southwest, in the region between the western coastal plains and the upper Niger Basin. The western wedge between Abeokuta and Ibadan and the Niger Basin reaches elevations of 600 meters or more, while the extensive northern savanna region, stretching from Kontagora to Gombe and east to the border, includes extensive areas with elevations of more than 1,200 meters or more at its center. The mountainous zone along the middle part of the eastern border, the Cameroon Highlands, includes the country's highest point (2,042 meters). In the far northeast and northwest, elevation falls again to below 300 meters in the Chad Basin in the far northeast and the Sokoto Basin in the northwest.
<>Climate
As in most of West Africa, Nigeria's climate is characterized by strong latitudinal zones, becoming progressively drier as one moves north from the coast. Rainfall is the key climatic variable, and there is a marked alternation of wet and dry seasons in most areas. Two air masses control rainfall--moist northward-moving maritime air coming from the Atlantic Ocean and dry continental air coming south from the African landmass. Topographic relief plays a significant role in local climate only around the Jos Plateau and along the eastern border highlands.
In the coastal and southeastern portions of Nigeria, the rainy season usually begins in February or March as moist Atlantic air, known as the southwest monsoon, invades the country. The beginning of the rains is usually marked by the incidence of high winds and heavy but scattered squalls. The scattered quality of this storm rainfall is especially noticeable in the north in dry years, when rain may be abundant in some small areas while other contiguous places are completely dry. By April or early May in most years, the rainy season is under way throughout most of the area south of the Niger and Benue river valleys. Farther north, it is usually June or July before the rains really commence. The peak of the rainy season occurs through most of northern Nigeria in August, when air from the Atlantic covers the entire country. In southern regions, this period marks the August dip in precipitation. Although rarely completely dry, this dip in rainfall, which is especially marked in the southwest, can be useful agriculturally, because it allows a brief dry period for grain harvesting.
From September through November, the northeast trade winds generally bring a season of clear skies, moderate temperatures, and lower humidity for most of the country. From December through February, however, the northeast trade winds blow strongly and often bring with them a load of fine dust from the Sahara. These dust-laden winds, known locally as the harmattan, often appear as a dense fog and cover everything with a layer of fine particles. The harmattan is more common in the north but affects the entire country except for a narrow strip along the southwest coast. An occasional strong harmattan, however, can sweep as far south as Lagos, providing relief from high humidities in the capital and pushing clouds of dust out to sea.
Given this climatological cycle and the size of the country, there is a considerable range in total annual rainfall across Nigeria, both from south to north and, in some regions, from east to west. The greatest total precipitation is generally in the southeast, along the coast around Bonny (south of Port Harcourt) and east of Calabar, where mean annual rainfall is more than 4,000 millimeters. Most of the rest of the southeast receives between 2,000 and 3,000 millimeters of rain per year, and the southwest (lying farther north) receives lower total rainfall, generally between 1,250 and 2,500 millimeters per year. Mean annual precipitation at Lagos is about 1,900 millimeters; at Ibadan, only about 140 kilometers north of Lagos, mean annual rainfall drops to around 1,250 millimeters. Moving north from Ibadan, mean annual rainfall in the west is in the range of 1,200 to 1,300 millimeters.
North of Kaduna, through the northern Guinea savanna and then the Sudan savanna zones, the total rainfall and the length of the rainy season decline steadily. The Guinea savanna starts in the middle belt, or southern part of northern Nigeria. It is distinguished from the Sudan savanna because it has more trees whereas the Sudan few trees. Rainy seasons decline correspondingly in length as one moves north, with Kano having an average rainy period of 120 to 130 days, and Katsina and Sokoto having rainy seasons 10 to 20 days shorter. Average annual rainfall in the north is in the range of 500 to 750 millimeters.
The regularity of drought periods has been among the most notable aspects of Nigerian climate in recent years, particularly in the drier regions in the north. Experts regard the twentieth century as having been among the driest periods of the last several centuries; the well publicized droughts of the 1970s and 1980s were only the latest of several significant such episodes to affect West Africa in this century. At least two of these droughts have severely affected large areas of northern Nigeria and the Sahel region farther north. These drought periods are indications of the great variability of climate across tropical Africa, the most serious effects of which are usually felt at the drier margins of agricultural zones or in the regions occupied primarily by pastoral groups.
Temperatures throughout Nigeria are generally high; diurnal variations are more pronounced than seasonal ones. Highest temperatures occur during the dry season; rains moderate afternoon highs during the wet season. Average highs and lows for Lagos are 31° C and 23° C in January and 28° C and 23° C in June. Although average temperatures vary little from coastal to inland areas, inland areas, especially in the northeast, have greater extremes. There, temperatures reach as high as 44° C before the onset of the rains or drop as low as 6° C during an intrusion of cool air from the north from December to February.