The 8 million square kilometers and 17 countries covered by this atlas encompass a wide range of landscapes from alluvial valleys in Senegal and Ghana, sandy plains and low plateaus across the Sahel, and rolling hills of Togo to rugged mountains with summits reaching over 1,500 m in Guinea and 1,800 m in Niger. Covering approximately one quarter of Africa, West Africa contains a broad range of ecosystems, bioclimatic regions, and habitats from rain forest to desert.
West Africa can be divided internally through its natural features. Geology, relief, climate, vegetation, soils, and the responses of people to the patterns of its biophysical resources through human land uses all tend to be arranged along east-west belts. West Africa is remarkable for its geological variety. Like most of Africa, the region is largely composed of ancient Precambrian rocks (at least 541 million years old; the oldest rocks may be about 3 billion years old), which have been folded and fractured over hundreds of millions of years.
For most of West Africa, continental conditions have existed since the Eocene or Oligocene, that is, since the last 23 to 34 million years. Most of West Africa’s mountain massifs and highlands, such as the Aïr Mountains, the Tibesti Mountains, the Adrar des Ifoghas, and the Fouta Djallon, originated as Precambrian folds (Church, 1966). Much later, volcanic activity in many of these highlands deposited additional layers of igneous rock. Volcanic outpourings have occurred throughout West Africa’s geologic history, with major activity as recent as the Pliocene (2.5 to 3.6 million years ago), and even more recent activity in the Aïr and Tibesti Mountains.
Relief on its own is not the source of great regional diversity in West Africa. For the most part, West Africa is relatively flat and low, which sets it apart from the other major regions of Africa. Several major rivers, including the Niger — West Africa’s longest river — originate in the Guinea Highlands, where rainfall is heavy. Other major rivers rise from Guinea’s Fouta Djallon, including the Gambia and Senegal. The Senegal River drains a major basin — the third largest in West Africa after the Niger Basin and the Lake Chad Basin.
Many separate basins are defined by smaller rivers that drain the land between the Atlantic Ocean and the basins of the Senegal and Niger Rivers. Of these, two are worth mentioning: the Gambia, which drains central Senegal and the nation of The Gambia, and the Volta River, which starts at the confluence of the Nakanbé (White Volta) and the Mouhoun (Black Volta), and reaches into the Mossi Plateau in Burkina Faso. Ghana constructed the Akosombo Dam (completed in 1965) in a gorge where the Volta cuts through the Akwapim–Togo Range, creating the world’s largest artificial lake, Lake Volta.
Most of West Africa, from the southern Sahara to the humid coastal countries, has only one rainy season, which lasts from one to six months. The area of two rainy seasons, a long one and a shorter one, is limited to the southern portions of the coastal countries from Liberia to Nigeria. The climate is related to the advance and retreat of the intertropical front— the interface between two air masses — one hot and humid and the other cool and dry. In the summer the high pressure area is replaced by a depression, bringing warm, moist winds in from the Atlantic in the southwest. Generally, the dry season lengthens and annual rainfall decreases with increasing latitude. Conversely, in the southern latitudes, rainfall increases and the dry season shortens, often to just four months (December to March). Maximum temperatures and temperature ranges also increase with latitude. In the humid south, temperatures vary little, whereas in the arid north one temperatures range from 0˚C to more than 45˚C .
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