NIGERIA BACKGROUND INFORMATION

Standard 7

The Physical Processes That Shape
the Pattern of Earth's Surface

Climatic Processes

One of the most critical processes that shapes the earth's surface is climate. In fact, climate variation is one of the main reasons why different parts of the world have such different physical and human characteristics. In this section, we will look at some of the basic climatic processes in Nigeria. These are some of the processes that make Nigeria distinct from places like Japan, Canada, or even South Africa.

The most basic characteristic about Nigeria's climate is that it is tropical. The term "tropical" generally refers to any region falling between the Tropic of Cancer and the Tropic of Capricorn. Nigeria is safely within this region, ranging between 4 and 11 degrees north latitude. Thus, because of its location we can intuitively get a sense of Nigeria's climate. As we will see, however, it is really necessary to talk about the climates of Nigeria. Even though it is a comparatively modest-sized territory by world standards, Nigeria contains a great deal of regional climatic variation. Nonetheless, it is possible to attribute Nigeria's temperature regimes to its tropical location. Tropical locations generally receive greater solar energy for two reasons. First, solar radiation is more direct near the equator. The same amount of radiation is received over a relatively smaller area in the tropics. Second, solar radiation in the tropics passes through relatively less atmosphere. Recall that the Earth's atmosphere reflects and absorbs much solar radiation before it strikes the Earth.

Latitude is only one determinant of temperature. Atmosphere, ocean currents, and altitude also influence temperature. The moisture content of the atmosphere (i.e. the type and density of clouds) also influences the amount of solar radiation that reaches the Earth's surface. In the case of Nigeria, many parts of the country often experience overcast skies. This is particularly true in the southern part of the country and during the rainy season in both northern and southern Nigeria. Ocean currents do not exercise a significant impact on Nigeria's temperature regime, since ocean waters in the tropics are warm. In the same way, altitude does not alter the country' temperature regimes much. In some tropical locations, high elevation promotes substantial cooling. Examples are parts of highland Latin America. Nigeria does not have areas of substantial elevation, however (few areas exceed 4,000 feet). Thus, adiabatic cooling does not substantially affect the country's climate. Because of these reasons, mean annual temperatures for the country range between 75 and 80 degrees Fahrenheit (25-27 degrees celsius). While some locations, like the extreme northwest, do experience some seasonal variation in temperature, the majority of the country experiences very little seasonal variation in temperature. Because Nigeria has relatively constant temperatures both seasonally and regionally, the main variable of climate differentiation is precipitation. The following section focuses on why precipitation varies among Nigeria's regions.

The major seasonal process that influences Nigeria's climate is the movement of the southern Nigerian low-pressure belt to the Sahara Desert. This belt begins its northward shift in January and returns to southern Nigeria in July. The location of the low-pressure belt in the dry season promotes the development of Harmattan winds. These are strong, dry winds that carry a substantial amount of dust from the Sahara to Nigeria (remember, air masses move from high to low air pressure). Harmattan winds are also important because they bring much drier air to Nigeria. During the dry season, relative humidity falls below 40 percent in northern Nigeria and falls 20 to 30 percent in the southern half of the country. As the low-pressure belt tracks northward across Nigeria, it pulls moisture-rich air masses from the coast. The Atlantic Ocean is, after all, the major source of water for Nigeria's rainfall. Because of its coastal location, Nigeria has advantages over interior nations like Niger or Chad. Not only is a coastal location beneficial for transportation and trade, proximity to the ocean provides the country a large source region for precipitation.

While Nigeria is a relatively well-watered country by global standards, the strongly seasonal nature of precipitation has important consequences for other physical and human processes. Vegetation is one phenomenon that is closely correlated with the seasonality and the amount of annual precipitation. Nigeria's vegetation is discussed in a later section. The spatio-temporal pattern of rainfall also has important consequences for humans. Farming patterns, drinking water availability, and human settlement are all influenced by the country's pattern of rainfall.

An important point to remember about climate is that it is not static. Evidence for this is available from the West African Sahel. While many have been quick to blame farmers and herders for desertification, "there are those who believe that the climate of tropical Africa is changing and becoming drier, leading for example to the drought periods of the last three decades." Indeed, "between 1968 and 1989 in the Sahelian countries of West Africa, every annual rainfall total was below 'normal', if indeed 'normal' can be defined." It is not known what effect these climatic changes may have on humans and other physical systems in the future.

Geomorphologic Processes Past and Present

As mentioned in essay five, Nigeria is a country of plains and plateaus. Plateaus and highlands are particularly prominent in the west-central, north-central, and eastern parts of the country (see shaded relief map). In these areas, elevation exceeds 300 meters. The highest of these formations are the Jos Plateau in central Nigeria and the eastern highlands that border Cameroon. This section on geomorphology examines some of the broad outlines of Nigeria's geologic history. It divides this history into five major eras, the Precambrian, the Paleozoic, the Mesozoic, the Cainozoic, and the Quaternary eras.

The Precambrian and Paleozoic eras, which began at least 1.5 billion years before the present and 200 to 500 million years before the present, were marked by two major geologic processes. The first process involved the uplifting of large portions of the Earth's crust. Precambrian era rocks were uplifted to form substantial highland areas. The consequences of these changes are still evident in the contemporary landscape of Nigeria. In the latter part of the Paleozoic era, most geologists believe that the supercontinent of Gondwanaland split apart. This large land mass had formerly joined the regions of South America, Africa, Antarctica, Australia, and India. This continental reshuffling also likely led to the folding and fracturing of some rocks.

During the Mesozoic era , which began 120-180 million years before the present, wind and water were key forces of geomorphologic change. This was a period of intensive erosion due to strong winds and water currents. These weathering forces would have been particularly powerful if vegetation, including trees, was sparse. Many scholars believe that this was precisely the situation during this era. The sea also submerged all of the non-highland areas formed in the Precambrian era. This was likely associated with a warming of the globe's climate so that much of the polar ice caps melted. Toward the end of the Mesozoic era, the sea began to recede, however. This recession left substantial swamps, which later served as areas of sedimentation. In these and other areas, decaying marine organisms and other organic materials provided sediments for the formation of sandstones, shales, limestones, and localized coal deposits.

The next major era was the Cainozoic. During this era, the sea again submerged substantial portions of Nigeria. It is likely that water only covered approximately one-third of the present land mass during this period. Areas submerged during this era included the southern coastal strip and northwest and northeast corners of the country. In these areas, large amounts of clay and sand were deposited where the land met the sea. And, as the sea again began to recede, these areas of sediment deposition followed the retreat of the coastline. Other events in this period included the folding and tilting of older Cainozoic and Mesozoic rocks. A key outcome of these processes was the formation of the Niger-Benue trough. This is the valley that forms the U-shaped channel for West Africa's largest river, the Niger, and its major tributary, the Benue. Volcanic activity in central and east-central Nigeria also led to the formation lava sheets and volcanic cones.

In the most recent geologic era, the Quaternary era, there has been continuation and completion of processes initiated in the previous eras. First, the sea has receded further. "Lake Sokoto" and "Borno Lake," in the northwest and northeast respectively, have disappeared. Second, erosional processes have continued to play a prominent role in Nigeria. Because of Nigeria's abundant rainfall, erosion by water has been an important geological process of weathering. Sediments from highland areas continue to be worn away and deposited in lowland sediment basins. A highly visible area of sediment deposition is the Niger Delta. Much like the Mississippi Delta, this sprawling area of stream channels and swamps continues to grow. The most recent era has also been characterized by the deposition of sand from the Sahara Desert. This has particularly been evident in northern Nigeria during periods of short-term climate change. Greater aridity in the Sahara and Sahel regions has led to increased wind erosion and sediment deposition in northern Nigeria. This phenomenon is particularly evident in the dry season when the dry, dusty Harmattan winds blow from the desert into Nigeria.

Like other parts of the world, Nigeria's geomorphology and topography are influenced by processes that both build and destroy. The preceding summary of geologic history discussed both of these processes. Prominent constructive forces have included volcanic activity and processes of crustal uplift. While Nigeria does not have active regions of volcanic activity, this constructive activity once had an important role in shaping Nigeria's morphology and topography. Geologists have identified several regions of remnant igneous rocks. These regions represent areas that were once subject to volcanic activity. Because Nigeria does not have any significant mountain regions, the main remnant igneous formations occur in highland plateau regions. These plateau regions roughly correspond to those areas that contain volcanic rocks and uplifted areas of basement rock. These igneous rocks have generally shown a greater resistance to erosion, and thus, have resulted in the formation of upland regions.

Important destructive processes have included wind and water erosion, chemical weathering, and crustal fragmentation. The discussion of ancient volcanic activity illustrates that one cannot understand constructive geological processes without understanding destructive processes simultaneously. The weathering processes that resulted in the formation of plateaus were mainly water erosion and wind erosion. Wind erosion is a more substantial destructive force in northern Nigeria. This is particularly true in the dry season when some locations do not receive any moisture for months at a time. Wind erosion is also more of problem in the North because vegetative cover is generally less in this region. Evidence of water erosion is abundant in southern Nigeria. In this part of the country, sedimentary rocks are very common. This indicates a gradual process of weathering, often resulting in the transport of rocks and sediment from the North to the South. As this last point indicates, however, erosion is a two-way process. If soil or sediment is removed from one location, it must be deposited in a new location. Nowhere is this process more evident than the Niger Delta. Just like the Mississippi River, the Niger is continually depositing loads of sediment at its mouth in south-central Nigeria.

Vegetation

Standards 4 and 5 describe the nature and distribution of vegetation in Nigeria. This section focuses on the physical processes that influence vegetation development within Nigeria. The study of vegetation will be our major topic of discussion from the Nigerian biosphere since standard 8 focuses on ecosystems.

We can speak about the geography of vegetation since plant diversity and abundance varies from one place to another. This variation exists mainly due to variability in climate, geomorphology, and soils. These factors interact to influence the distribution of vegetation on the Earth's surface. Here we are examining the unique combinations of these factors that have shaped Nigeria's vegetation. Vegetation also influences climate, soils, and topography. In the short-term, however, the effects of climate and soils on vegetation are of particular importance.

Of all the factors of vegetation development, climate is perhaps the most critical. Individual plant species require specific climatic conditions. Some plants and trees can only grow in warm, wet climates. Others thrive in semi-arid conditions. As these examples indicate, both precipitation and temperature regimes are important factors in the development of vegetation regions. This is evident by comparing a climate map of Nigeria with a vegetation map. You should be able to note two key points in this comparison. First the names of climate types and vegetation types are often the same. For example, the labels "rain forest" and "savanna" are both used to refer to a climate type and a vegetation region. Second, there is a great deal of similarity between these two maps. The spatial pattern of climates and the spatial pattern of vegetation regions are very close to one another. Both of these observations support the intuitive point that vegetation is closely dependent on the climate of a region. The spatial pattern of climate and vegetation in Nigeria exhibits this relationship. This helps us explain why a large rain forest region exists in southern Nigeria. It is this region that has a rain forest climate. Rainfall surpasses 80 inches per year (200 cm); relative humidity exceeds 75 percent throughout the year; and temperatures are warm enough to promote rapid growth of vegetation. In a similar way, savanna vegetation regions correspond to savanna climate regions. Within these regions, annual precipitation is lower, rainfall is distinctly seasonal in its occurrence, and relative humidity is lower. These climatic characteristics mean that less water is available for trees and plants, evapotranspiration is higher, and trees often enter into a semi-dormant stage in the dry season. During this season, many savanna trees lose their leaves. The most visible climatically-influenced contrast between the rain forest of the South and the savannas of the North is in tree spacing. In rain forest that has been relatively undisturbed by humans, tree density is high. The various layers of the rain forest combine to form a thick cluster of various species. In the savanna, however, there is insufficient water to support dense groves of trees. In the savannas, trees are spaced at a relatively regular interval (often in clusters), signifying the trees' competition for relatively scarce water resources. This influence of climate is clearly evident in the landscape as you travel from the dense rain forests of southern Nigeria to the Sahel savanna of northern Nigeria (see illustrations in standard 3). Tree height is also affected by the relative abundance of water. While the trend is not rigid, tree height generally declines from north to south, just as annual rainfall totals decline. A long dry season also allows fires to shape the savanna landscape. Fires keep the density of trees lower than it would otherwise be, and they promote the colonization of tree species that are adapted to the ravages of dry season fires.

Topography also impacts the development of vegetation. The physical relief of the land influences the shape and the characteristics of watersheds. This fact is particularly important in regions of savanna vegetation and climate. In these regions, "gallery forests" often form along rivers. These linear tree groves often line the banks of savanna region rivers. These uncharacteristically dense areas of trees are supported by the greater availability of water. Nigeria's topography is important in another way. The country's relatively flat relief means that it has relatively little montane vegetation. Given Nigeria's lack of high mountains, it is not surprising that very little montane vegetation is found in the country. Areas of exception are parts of the Jos Plateau and the eastern highlands. In these areas, grasses are shorter and trees are less abundant than at lower altitude.

Soil is a factor that is highly localized in its effects on vegetation. Just like agricultural crops, different plant species thrive in different soils. For example, mangrove species do well in swampy areas near the coast. In contrast, the acacia tree does well in the semi-arid, well drained soils of the Sahel savanna. Overall, however, Nigeria's soils are quite poor in their quality. Paradoxically, the rain forests have some of the poorest soils in the whole country. The next section addresses this and other issues of soil formation.

Soils

Iloeje provides a concise summary of the factors of soil formation:

Soils are a mixture of rock particles loosened by weathering, mineral salts and dead vegetable matter . . . the formation and nature of soils depend chiefly on three factors - the nature of parent rock, the vegetation and the climate. The parent rock produces the mineral salts and rock particles, the vegetation is the source of the organic matter, while the climate makes sure that the rocks weather, and by promoting bacterial action, ensures that the vegetable matter decays.

Thus, we can see that soil formation is a function of interactions between the biosphere (organic matter), the lithosphere (parent rock), and the atmosphere (climate). Parent rocks and decaying organisms provide the materials for soil formation; climate influences the nature of the process. An important soil characteristic derived from the parent rock is the size of component grains. Based on inorganic grain size, soils can be described as sandy, clayey, or loam soils. Sandy soils have large component grains and are typically derived from sedimentary parent rock. These soils are easy to cultivate, well-drained, and typically are less fertile. Clayey soils have much smaller component grains. These soils tend to be darker in color and retentive of water. Clayey soils are also derived from basement complex rocks.

The organic content of soil is also important. Because of the region's thicker vegetation, southern Nigeria's soils tend to have more humus, and are potentially more fertile. Southern Nigeria's soils are also subject to leaching, however. In this process, rainfall percolates through the top layer of organic material, dissolving the minerals and carrying them downwards. The humid, hot climate of the South also promotes chemical weathering. The combination of high temperatures and high relative humidity enhances the decay of both rocks and organic matter. Thus, while the region is rich in humus, leaching and chemical weathering both make the soil vulnerable if trees and vegetation are cut.

"In the north the dominant process of rock weathering is exfoliation. This process produces screes which are worn down over a long period of time and eventually from [sic, form] soils. During the dry season the water in the soil rises by capillary attraction and as it reaches the surface it evaporates, leaving the mineral salts it has carried up to accumulate just below ground level."

Laterite soils are found throughout much of the central one-third of Nigeria. These soils are thought to be formed through the alteration of processes of capillary attraction and leaching and chemical corrosion. This alternating pattern parallels the alternation of wet and dry seasons. Laterite soils are of low fertility. Further, when they are cleared of vegetation they become extremely hard.

In northern Nigeria, sandy soils dominate, with the major exception of the Zaria loam region. The rise and fall of the sea and sandy sediment deposition are two geologic processes that have been critical in the formation of the northern sandy soils. These soils are particularly well suited for growing groundnuts, which is a major cash crop of the North.