Large edge step
The Great Escarpment ( English Great Escarpment ) is a terrain and Escarpment in southern Africa , the inland highlands against the coastal plains to the Atlantic and Indian Ocean separates.
Emergence
The Great Rim Stage contains some of the geologically oldest areas on earth. The edge step itself was not formed until after the breakup of the original continent of Gondwana around 120 million years ago, but it consists at least partially of rock units with an age of over two and a half billion years. Today these are the relic of a relatively small supercontinent, covered by a blanket of weathering, which existed before Gondwana was formed.
In the course of the breakup of Gondwana, the continental crust in the area of the edges of the newly emerging African plate was expanded , thinned out and subsided. Soon afterwards the first oceanic crust formed on the new plate edges. The subsidence of the young fringes of the African continental block, including southern Africa, continued. In addition, the central regions of southern Africa also fell, albeit less sharply. All of these processes, which took place predominantly in the Cretaceous and Tertiary periods , were associated with fractional tectonics.
The sunken areas on the continental margin form today's shelf areas and coastal plains, the sunken central areas form the basins in the interior of southern Africa (e.g. today's Kalahari Basin). The areas in between, which were not or hardly affected by subsidence movements, now form the Great Edge Step.
In the course of the past millions of years, the Great Marginal Step has migrated inwards through the interplay of subsidence of the continental margin and receding erosion and will continue to do so in the coming millions of years.
course
Namibia
The Great Edge Step runs through Namibia from north to south as one of the three predominant landforms. It separates the coastal plain from the inland highlands.
The part of the country to the west of the peripheral step , close to the coast, stretches as an approximately 2000 km long, but only 80 to 130 km wide strip from southern Angola to the South African province of North Cape and essentially corresponds to the area of the coastal plain. This coastal strip is mainly occupied by the Namib Desert and only rises slowly from the sea level to 600 m. In the north the coastal strip is predominantly stony, partly rocky (see Skeleton Coast ) and in the south predominantly sandy desert (especially south of the dry river Kuiseb ) with large shifting dunes up to over 300 m high.
The natural area of the Namib is followed by the steep ascent of the Great Edge step to the east, which leads up to a stratification step reaching up to over 2000 m above sea level . In the north, between the Kunene and the Huab rivers, the Great Edge Step is formed by the Hartmann , Baynes and Joubert Mountains , in the south by the Khomashochland , the Rantberge , the Naukluftberge and the Tsarisberge ,Schwarzrand and Tirasberge . Broad and deep river valleys facing the coast cut through the Great Rim and open it to the Namib. In central Namibia, in the area of the 19th to 23rd parallel, the edge step is eroded and interrupted over several hundred kilometers: the so-called edge step gap . It is replaced by a sloping, continuously rising level and the terrain rises continuously to the level of the inland highlands. Outside the large marginal step are the Brandberg massif , the Erongo mountains and individual island mountains such as the Small and Large Spitzkoppe .
From the Naukluft Mountains to the Oranje River , the Great Edge Step runs over 400 km in north-south direction as a layer step ( red edge) and on the edge of the diamond restricted area the granite mountains of the Great Edge Step rise from the wide plains of the Namib.
To the east, the Great Marginal Step gradually descends into the mountainous lands of the Randschwelle and the central inner-African highlands and merges into the sand-filled basin of the Kalahari Desert , which is part of the basin landscapes of the Karoo supergroup in southern Africa. In the north, the inland highlands are criss-crossed by wide valleys, with individual mountains protruding from the high plateaus. In the south the highlands are mostly flat and the landscape is only structured by a few deep valleys.
South Africa
In the border area of Namibia and South Africa, the Great Rim Stage crosses the canyon of the Oranje and continues as a western slope of the mountain scenery from the Richtersveld to the south, past Springbok over the Bokkeveldberge (840 m) inland. Here the large marginal step divides the Karoo into its two important parts. It starts around the Hantamsberg massif (1799 m) along the chains from the Roggeveld Mountains (1742 m), the Komsberg Mountains (1902 m) and the Nuweveld Mountains (2070 m) to the Molteno Pass not far from the town of Beaufort West . From here the ridge runs through the barely inhabited area of the semi-arid Karoo to the region west of Graaff-Reinet, where it stretches from the southern to the eastern mountain slope of the Sneeuberg massif (2504 m) with a turn to the north. Here it arches around the Fish River basin and forms on its eastern edge the Bamboesberge (2376 m) and the Stormberg massif (2290 m) north of Queenstown .
Now the great marginal step follows a line in the province of Eastern Cape , which runs roughly along the villages of Dordrecht , Elliot with the Barkly Pass , Maclear and Elands Height , and goes over to the western foothills of the Drakensberg . Here the ridge reaches the border area between South Africa and Lesotho . The border following, she pulls on the eastern slope of the Drakensberg (3482 m) as the largest collection in their overall course along. The greatest difference in altitude is at the Sani Pass in KwaZulu-Natal . The Ukhahlamba-Drakensberg Park runs here over a long distance and parallel to the edge step . At the northern end of the Drakensberg, the altitude level turns almost at right angles from them and, not far from Harrismith , runs north through the Mpumalanga province , past Swaziland to Mbombela in the Limpopo province , over the small Strydpoortberge (2208 m) to the Soutpansberg - Massif (1876 m) near Louis Trichardt . Here it sinks for the time being to the Limpopo river valley . Its continuation can still be seen within Zimbabwe and reaches the Mozambique area further north .
Mozambique
The Great Peripheral Step begins in Mozambique with the Lebombo Mountains , near the western state border, and rises from the lowlands to an altitude of 2400 meters. A mountain formation through collision of tectonic plates and connected folds or thrust did not take place here, so that here the rocks have largely been preserved in their original storage conditions. The Great Lakes , of which Lake Malawi borders Mozambique, are also located on the edge of the escarpment itself .
North of the 20th parallel, the course of the Great Peripheral Step approaches the coast of the Indian Ocean and reaches significantly higher heights than in the south of the country. The transition area from the edge step to the flat land and in the upstream plateau of the province of Manica to the east are characterized by the tropics. Striking island mountains rise here from the largely flat landscape.
literature
- Erich Obst , Kurt Kayser: The great marginal step on the east side of South Africa and its foreland. A contribution to the history of the young prominence of the subcontinent (= special publication of the Geographische Gesellschaft zu Hannover. 3, ZDB -ID 978146-8 ). Geographical Society of Hanover, Hanover 1949.
- Herbert Abel : Contributions to the morphology of the large marginal stage in southwest Africa. In: German geographical sheets. Vol. 48, No. 3/4, 1959, ISSN 0070-4024 , pp. 133-268.
- Jürgen Kempf: Land degradation in Namibia northeast of the tropics. A sketch of the problem. In: The Earth . Vol. 127, No. 4, 1996, pp. 265-278, ( digital version (PDF; 4.76 MB) ).
Individual evidence
- ↑ The Ugab Terraces in Western Namibia. Research project at Heidelberg University .
- ↑ Christina Wolkenhauer: Land use comparison Namibia. Institute for General Botany Department of Biology, Hamburg 2003, http://www.landnutzungsvergleich-namibia.de/H_Untersuchungsgebiet.htm ( Memento from October 4, 2009 in the Internet Archive ) .
- ↑ Octavian Catuneanu, Helmut Wopfner, Patrick G. Eriksson, Bruce Cairncross, S. Bruce Rubidge, Roger MH Smith, Phillip J. Hancox: The Karoo basins of south-central Africa. In: Journal of African Earth Sciences. Vol. 43, 2005, ISSN 1464-343X , pp. 211-253, here p. 212 (map), ( digitized version (PDF; 3.06 MB) ).