Of Desert and Delta. The Geology of the Kalahari
Beneath the Kalahari's sandy surface lie some of Africa's most ancient rocks. Overlying them is a far younger layer of sedimentary rock, which covers vast areas of the north and south.This mix of desert and delta is Botswana's most outstanding physical feature. Although burning within wildlife management areas is illegal, every year uncontrolled fires throughout the northern regions of Botswana threaten both the environment and the tourism infrastructure.
The Kalahari - Not a True DesertSituated centrally in southern Africa, Botswana also lies within the Kalahari Basin, an internal drainage system formed when the break-up of the super-continent, Gondwanaland, occurred over 100 million years ago. With the splitting of the southern continents, Africa experienced uplifting around its edges, which formed the mountain ranges and escarpments of southern and central Africa. Over millions of years these highland regions have endured wind and water erosion that has carried the removed sediment load inwards, to be deposited in the Kalahari Basin. This relentless accumulation has created the Kalahari sand mantle: the largest unbroken mass of sand that exists on the planet. Comprising ancient and well-leached soils that are mostly nutrient poor (particularly with regards to their phosphorus and nitrogen levels), the sand mass covers approximately 2.5 million square kilometres. It stretches from northern South Africa, across eastern Namibia and most of Botswana, through eastern Angola and western Zambia and ends up in the Democratic Republic of Congo in central Africa. Although the southern extent - covering South Africa, Namibia and Botswana - is known as the Kalahari Desert, it is in fact not a true desert, but rather an extended region of similar soil and mostly scrub- and acacia-type woodland vegetation, with low and erratic rainfall patterns. In certain areas the Kalahari sands that cover most of Botswana reach depths of over 300 metres.
Three Physiographic RegionsBeneath the Kalahari's sandy surface lie some of Africa's most ancient rocks. The oldest of these, the 3 500-million-year-old igneous and metamorphic Basement Complex, is to be found in the eastern and south-eastern regions of the country. Overlying them is a far younger layer of sedimentary rock, which covers the vast and the north and south. Because Botswana's surface is predominately an eroded one, the country is mostly flat or gently undulating, with an average elevation of just over 1 000 metres. The highest point is the Otse Hill in the southeast, which is a mere 1 491 metres above sea level.There are rocky outcrops in the east between Kanye and Francistown, and in the far west, where the Basement Complex reaches the surface. The country can be into three main physiographic regions: The wetland region of the Okavango Delta, which comprises approximately three percent of the country. The Hardveld, where the Basement Complex outcrops in the east and south-east, which comprises approximately 22 percent of the country. The Sandveld, which consists of thick Kalahari sands and ancient fossil valleys, covering the remaining 75 percent of the country.Although a tremor registering a magnitude of 6.7 on the Richter Scale was recorded in 1952, the seismic activity is generally not severe enough to be felt, and the depth of the Kalahari sands has an absorbing effect on the shock waves. Nevertheless, the crustal movement has created fault lines within the region that have resulted in the formation of both the Okavango Delta and the Makgadikgadi Pans.
At the Foot of the Rift ValleyAfrica is being torn apart by the Great African Rift Valley that extends from the Arabian Peninsula, through the Red Sea and down into East Africa, before it becomes ever shallower as it reaches into southern Africa. This major fault line has had a most dramatic influence on the geology of Botswana. While strictly speaking not a part of the Rift Valley, northern Botswana sits at the very south-western tip, which is sufficiently close to be affected by the ongoing tectonic movement associated with the rifting to the north.
Kalahari-Zimbabwe AxisApproximately seven million years ago the Okavango River was linked to the Limpopo River, or possibly even the Zambezi River, forming the greatest river system in Africa. That was until a few million years later, when a swath of ground known as the Kalahari-Zimbabwe axis was forced up by tectonic activity, separating the two rivers and cutting the Okavango's access to the Indian Ocean.A super-lake covering almost 200 000 square kilometres at its peak soon formed, as the waters of the Okavango became trapped to the west of the higher ground. This lake was also fed by other systems, the Chobe-Zambezi and Kwando-Linyanti in particular, but, when further crustal activity cut off the water supply from the Okavango River approximately 750 000 years ago, it began to dry up. The result was the formation of the Okavango Delta as it is today, and, once the lake had dried out totally, the Makgadikgadi Pans.
Four Drainage SystemsBesides the geological upheavals, the country's drainage patterns have also been affected by variations in the regional climate. Over the last 50 000 years, northern Botswana has undergone a series of alternating climatic cycles that has resulted in four drier periods and five wet ones. Presently in another dry cycle, the country's hydrology now consists of four major drainage systems: The Okavango River system, which includes the Selinda Spillway, the Thamalakane and Boteti rivers and the Makgadikgadi Pans. This system forms an inland delta with no access to the sea. At the southern extremity of the Okavango Delta the last of the flood waters pass through this floodplain channel during August and September, before feeding into the Thamalakane River. The Chobe River system, which includes the Kwando River and Linyanti Marsh. The water from this system in turn flows into the Zambezi River, immediately east of Kasane. Limpopo River system, which includes the Shasi and Maklautsi rivers in eastern Botswana. The Auob-Nossob and Molopo rivers in the far south. Only two of these systems, the Okavango and Chobe, are perennial, and both have their sources north of Botswana. The flow patterns of the others are totally dependent on local rainfall levels and in dry periods the Auob-Nossob and Molopo system may not flow for years, while the Limpopo River usually dries out by the end of summer. Of all the surface water in Botswana, almost 95 percent is found in the Okavango system.
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