The unity of Africa is structural, that is, it depends on its geological constitution. The continent is in fact a gigantic mass of lands that have preserved their substantial structural integrity since the origins. Already when it is said that Africa is “immense and monotonous”, its structural features are implicitly defined. From this point of view it recalls other compact terrestrial plates: Siberian Asia, the Canadian Shield, the Brazilian one.. But while these marginalize in geologically unstable areas or affected by more or less ancient corrugations, Africa is the only continental mass not to have been deeply affected by great tectonic movements: the only tormented areas are found in the north-western sector, where they rise. the chains of the Atlas, formed following the tertiary orogenetic movements connected with the Mediterranean geosyncline, and in the extreme south, where the dislocations above the high continental slope gave rise to the Capidi. Elsewhere there are no wrinkles, but subsidence, subsidence, fractures; the most marked tectonic element is in fact constituted by the large fracture that opens in the eastern section of the continent with a meridian trend, from Lake Malawi or Niassa to the Ethiopian depressions to continue in the Red Sea and in the Syropalestinian trenches: it is a very important element in African geography because it is connected to that vast uplift of the eastern and southern lands that lead to the recognition of a high and a low Africa.
Its origin is framed in the geological history of the continent, in its very ancient link with the Asian mass (the continent called Gondwana), a link that seems confirmed by the geological continuity existing on both sides of the Red Sea. The formation of the pit dates back to the periods between the Mesozoic and the Cenozoic and was due to a large arching of the crust with rupture and subsidence along the anticline axis, extended between Africa and Arabian Peninsula; in the opposite syncline, the detachment of Madagascar from the continent would have occurred at the same time. The origin of the imposing volcanoes that rise next to it and which represent the highest African mountains, with a summit in the 5895 m of Kilimanjaro, is linked to this gigantic fracture; furthermore, along the trench there are some of the few active volcanoes in Africa (Virunga). The other volcanic buildings (Ahaggar, Tibesti, Jabel Marrah, Cameroon) are connected to more local but similar cratogenic phenomena, that is to fractures that have promoted the release of magma. The antiquity and integrity of the continent are testified by the presence of vast outcrops of archaic rocks (which explain the mineral wealth of Africa) mainly represented by granite and gneiss. They overlap on large surfaces sedimentary formations of different origin: the most common are made up of paleozoic arenaceous-clayey deposits of continental origin (such as karroo that covers the plateaus of the extreme edge of southern Africa) and powerful stratifications of sandstones (like the nubic sandstone) formed by wind sedimentation in dry periods of the Mesozoic. Between the end of this epoch and the Cenozoic, the vast lava expansions of the Ethiopian acrocore and the formation of the main volcanic massifs took place, while in the areas of subsidence the marine entrances began prolonged sedimentation processes. All this reveals a simple geological history, with few episodes, the most important of which, for the purposes of current African geography, are represented – in addition to the fractures and the consequent volcanic activities – by the formation of large depression basins that give a mark to the surface of the continent and are today the foundations of the regional division of Africa. The main ones are those of Niger, del Lake Chad, the Nile, the Congo, the Zambezi and the Kalahari; the dividing elements are generally low and uncertain, tabular plateaus of modest elevation and whose limits are not easily defined. On the bottom of these basins lake and river sediments accumulated in the past and still accumulate today which therefore represent the youngest soils of the whole together with those of the few plains that border the coasts (the Nile deltas, Niger etc.), very often high and steep and almost uniformly rectilinear.
MORPHOLOGY
According to countryaah, the African continental surface already reveals in the particular morphological motifs the antiquity of its origin and the poverty of its tectonics. Tabular formations prevail, flat horizons from which isolated rocky forms often emerge, the Inselberge, the “witness rocks”, the last remnants of ancient peneplanation processes that have eroded and leveled the major roughness. The erosive activity is currently regulated almost everywhere by the mechanism of the areas subjected to high temperatures, with the formation of steep slopes that sometimes create monumental forms. In the Sahara, erosion takes on particular aspects linked to the arid climate, which is responsible for the denudation of vast rocky surfaces (hamada) in which the previously eroded ancient structures stand out; around them the debris spread out like a fan forming pebbly surfaces (reg and serir) and, further away, sandy ones (erg); in the subarid or pre-desert areas, on the other hand, there are vast surfaces of fossil dunes (qoz) fixed by vegetation. In Africa there are also traces linked to Quaternary glacialism, limited to the highest areas of the Atlas and Ahaggar; connected to the humid climates of the Quaternary are also many fossil elements of African morphology, in particular Saharan: among these the spectacular engravings by Tassili and the uidian which evaporating leave characteristic halos of saline deposits (chott etc.) in the depressions.
