Chapter Three: The Topography of Ethiopia and the Horn PDF

11/27/2024 CHAPTER THREE THE TOPOGRAPHY OF ETHIOPIA AND THE HORN Introduction  The topography of Ethiopia is largely determined by the geologic activities of the Cenozoic Era.  The uplifting of the Arabo-Ethiopian swell and the subsequent...

11/27/2024 CHAPTER THREE THE TOPOGRAPHY OF ETHIOPIA AND THE HORN Introduction  The topography of Ethiopia is largely determined by the geologic activities of the Cenozoic Era.  The uplifting of the Arabo-Ethiopian swell and the subsequent outpouring, spreading and thick accumulation of Trapean lava have given rise to an outward sloping highland plateau and mountains.  The major faulting resulted in the division of the plateau into two broad units and the formation of a great structural valley. 1  11/27/2024 Cont. Faulting elsewhere and on the floor guided part of the course of some rivers. They also formed depressions on which lakes were subsequently created. Geomorphic processes brought some modification to the structural landform by river dissection and roughening on the highlands, and deposition on the lower areas. General Characteristics of the Ethiopian Physiography The Ethiopian landform is characterized by great diversity:  plateaus,  high and rugged mountains,  deep river gorges and vast plains. Altitude ranges from;  125 meters below sea level (i.e. Kobar Sink)  to 4,620 m. above sea level (i.e. Mount Ras Dashen). 2  11/27/2024 Cont. Ethiopia has the largest proportion of elevated landmass in Africa (Roof of East Africa). – > 50% of the Ethiopian landmass is above 1,000 meters of elevation – > 44% of Ethiopian landmass is above 1,500 meters of elevation (22% of this is at 2000 meters a.s.l). – Half of this, in turn, is at more than 2,000 meters above sea level. Most of the Ethiopian Highlands are – part of central and – northern Ethiopia, and its northernmost portion extends into Eritrea. Physical map of Africa 3  11/27/2024 Cont. The highland core, on the other hand, is encircled by semi-arid and lowlands. The Ethiopian Highlands are rugged mass of mountains, situated in the Horn of Africa. They are the most significant physical features of the country, which form the largest continuous area of its elevation in the continent. They have been dissected by – several rivers and – gorges which have cut deep gorges. Cont. They have been divided into several regions and mountain systems. Most of the country consists of; – high plateau and mountain ranges – They are sources of many rivers and streams – “Water Tower of East Africa”. 4  11/27/2024 Cont. The diversity in topography brought about differences; in natural features (soil, climate, vegetation and wild life), socio-cultural and economic phenomena. 1000 meters contour line is used as the demarcation line between Ethiopian highlands and lowlands. Highlands (>1000 meters): characteristics  Moderate and high amount of rainfall (>600 mm per year)  Lower mean annual temperature (2,000 m.a.s.l) constituting nearly 21%. 5  11/27/2024 Cont.  In contrast to the highlands, the remaining 44% of the Ethiopian lowlands are characterized by:  Low amounts of rainfall  higher temperature.  High prevalence of tropical diseases.  Lower population densities.  Nomadic and semi-nomadic economic life.  Vast plain lands favorable for irrigation agriculture along the lower river basins. The physiographic division of Ethiopia Following the structural divisions brought about by the geologic processes of the Cenozoic Era, three major physiographic units can be identified in Ethiopia. These are: 1) The Western highlands and lowlands 2) The South-eastern (Eastern) highlands and lowlands 3) The Rift Valley 6  11/27/2024 Figure 3.1. Ethiopian Topography The Western Highlands and Lowlands: includes all the area west of the Rift Valley. It extends from north to south encompassing nearly the whole western half of Ethiopia. It makes up about 44% of the area of the country.  In the east the western escarpment of the Rift Valley bound it  in the west the western foothills and lowlands, along the Sudan and South Sudan border bound it. This region is further subdivided into; – four groups of highlands (76.3%) and – four groups of lowlands (23.7%) 7  11/27/2024 The Western Highlands The four groups of highlands consists of: 1) The Tigray Plateau 2) North Central Massifs 3) The Shewa Plateau/central highlands 4) The Southwestern Highlands The Tigray Plateau  It extends from the Tekeze gorge in the south to central Eritrean highlands.  The Tigray plateau is separated from the Eritrean plateau by the Mereb River.  It lies to the southeast of the upper course of the Mereb/Gash River and to the northeast of Tekeze River Gorge.  It constitutes about 13% of the area of the region.  It is an elongated highland with most of the land being in between 1,000 and 2,000 meters above sea level. 8  11/27/2024 The Tigray Plateau  Long period of denudation has created residual features of granite hills, rugged topography, and Ambas.  However; there are some high mountains with elevations of over 3000 meters in this plateau.  Mount Tsibet 3988 meters.a.s.l  Mount Ambalage 3291 meters.a.s.l,  Mount Assimba 3248 meters.a.s.l. The Tigray Plateau The famous monastery at Debre-Damo, a tableland that can only be climbed by a rope pulley is also located in this plateau region. 9  11/27/2024 North Central Massifs  Is the largest Physiographic in the western highlands  Much of its northern and southern limit follows the Abay and Tekeze gorges.  much of these plateau and tablelands are still covered by the Trappean lava.  In its central part, the physiographic unit also accommodates the Lake Tana basin surrounded by;  plains of Fogera and Dembia in the north and  an upland plain in its south Cont.  58% of the region is at an altitude of >2,000 meters.  It is the second highest physiographic division next to the Shewan Plateau  It consists of;  the Gonder,  Wello and  Gojjam Massifs. 10  11/27/2024 Cont. Out of the 26 mountain peaks with altitude of more than 4,000m.a.s.l in Ethiopia, 19 mountain peaks are found in this physiographic region. To mention a few; Name Height m.a.s.l Mountain System Mount Ras Dashen 4,620 m.a.s.l Mount Weynobar/Ancua 4462 m.a.s.l Mount KidisYared 4453 m.a.s.l Simen Mountain System. Mount Bwahit 4437 m.a.s.l Mount Guna 4,231m.a.s.l Debre Tabour Mountain System Abune Yoseph 4,260 m.a.s.l Lasta highlands of Wello Mount Birhan 4,154 m.a.s.l Choke Mountain System in Gojjam Cont. The Mountain systems in Gonder and Gojjam are separated from the eastern group of mountains in Wello by impenetrable and deep gorges. At one point though, they are connected by YejuWadla Delanta land bridge (ridge). This land bridge has been significant in history. It served as a route of penetration by the Turks, Portuguese, and Italians etc. 11  11/27/2024 Cont. The Woreta-Debre Tabor-Woldya road constructed to link; –the northwestern region with –Asseb through Woldya and Dessie took advantage of this land bridge. The Shewa Plateau/central highlands The Shewan plateau is bounded by; – the Rift Valley in the east and southeast, – the Abay gorge in its northern and western limit, and – the Omo gorge in the south and west. This plateau occupies a central geographical position in Ethiopia. It is the smallest of the Western highlands covering only 11% of the area of the whole physiographic region. Nearly three-fourth of its area is at an altitude of more than 2,000 meters above sea level. It has, therefore, the largest proportion of elevated ground 12  11/27/2024 Cont. The Shewa plateau is drained, outward in all directions by the tributaries of Abay (in the north), Omo, and Awash (in the south). – forms a water divide for these three river basins. Some of the highest mountain in the Shewan plateau Name Height Location Mount Abuye-Meda 4,000 m.a.s.l Northern Shewa Mount Guraghe 3,721 m.a.s.l In the South The Southwestern Highlands This physiographic division consists the highlands of Wellega, Illuababora, Jimma, Kaffa, Gamo and Gofa. This region is separated from the adjacent highlands by the Abay and Omo river valleys. It extends from; the Abay gorge in the north to the Kenya border and Chew Bahir in the south. Is the second largest in the Western highlands accounting for 22.7% of the area of the region. 13  11/27/2024 Cont. About 70% of its area is within 1,000-2,000 meters altitude. This plateau is the wettest in Ethiopia and drained by;  Dabus, Deddessa (tributaries of Abay),  Baro,  Akobo and  the Ghibe/Omo rivers. Guge Mountain (4,200 meters a.s.l.) is the highest peak. The Western Lowlands These are the western foothills and border plains that extend;  from the north western lowland  To southern Gamo-Gofa in the South. In certain places, ridges or part of the highlands protrude into the lowlands, interrupting their continuity. Make 11% of the area of the physiographic region. Elevation ranges between 500 and 1000 meters above sea level. 14  11/27/2024 Cont. This physiographic sub-region is further subdivided into four by the protruding ridges: these are  Tekeze lowland  Abay-Dinder lowland  Baro lowland, and  Ghibe lowland except ‘Baro lowland’, climatically the region is characterized by arid or semi-arid conditions accompanied by Pastoral or semi-pastoral economic activities. Cont. As one moves northwards, the degree of aridity increases, making rain-fed agriculture more difficult. Since one or two rivers cross all of these lowlands, irrigation agriculture is highly feasible. The Ghibe/Omo lowland, is classified;  in the Western lowlands from its geographical location.  But structurally it also belongs to the Rift Valley.  It is an area, which is both faulted and tectonically depressed. In the Western lowlands, there are small but important towns. These are Humera, Metema, Omedla, Kurmuk, Gambella etc. 15  11/27/2024 3.2.2. The Southeastern Highlands and Lowlands This physiographic region is the second largest in terms of area covering 37% of Ethiopia. The highlands make up 46% and Lowlands make up 54% of the physiographic division. These are further subdivided into; two units of highlands and two units of extensive lowlands.  The Southeastern Highlands consists of; A. The Arsi-Bale-Sidama Highlands and B. The Hararghe Plateau A. The Arsi-Bale-Sidama Highlands Found to the east of the Lakes Region. Located in the south western section of the physiographic region. They make up 28.5% of the area of the region and 62% of the south - Eastern Highlands. The Arsi Highlands are made up of flat rolling uplands and dissected mountains. Mountains in this area include; Name Height m.a.s.l Mount Kaka 4,180 m.a.s.l Mount Bada 4,139 m.a.s.l Mount Chilalo 4,036 m.a.s.l 16  11/27/2024 Cont. The Bale highlands are separated from the Arsi highlands by the head and main stream of Wabishebelle. The highest mountain peaks in this region are. Name Height m.a.s.l Tulu-Demtu (4,377 m.a.s.l) Mount Batu (4,307 m.a.s.l) The Arsi-Bale Highlands are; important grains producing areas with still high potential. Cont. The Sidama Highlands are separated from the Bale Highlands by the Ghenale river valley. They occupy the southwestern corner of this region. The prominent feature here is; – the Jemjem plateau, an important coffee growing area. Rivers Wabishebelle and Ghenale along with their tributaries have dissected this physiographic region. 17  11/27/2024 Cont. Specially, Weyb River, tributary of Ghenale, has cut an underground passage (Sof Omar cave) through the Mesozoic Limestone rocks. The cave is found near Bale Mountains. It is one of the World's most spectacular and extensive underground caverns creating a magnificent view accessible only by an underground stream. B. The Hararghe Plateau  This plateau is a north-easterly extension of the south-eastern highlands.  It extends from the Chercher highlands in the south-west to Jigjiga in the east.  It makes up;  38% of the South Eastern highlands and  17.4% of the whole physiographic region. 18  11/27/2024 Cont.  It has the smallest proportion of upper highland (>2,000 meters). It is a low lying and elongated region.  Rising sharply from the Rift Valley floor, it immediately but gently descends east and southeastward. Much of the Trappean lava is removed and the Mesozoic rocks are extensively exposed. The highest mountain here is Mount Gara-Muleta (3,381 m.a.s.l). The Southeastern Lowlands  The Southeastern lowlands are located in the southeastern part of the country and they are the most extensive lowlands in Ethiopia.  make up 54% of the area of the physiographic region and one- fifth of the country.  This region consists two plains: namely;  Wabishebelle plain (60%) and  Ghenale Plain (40%).  They include the plains of;  Ogaden,  Elkere, and  Borena. 19  11/27/2024 Cont. Because of the harsh climatic conditions, these lowlands are little used and support very small population. They are sparsely inhabited by pastoral and semi- pastoral communities. The economic potential for this region includes;  animal husbandry,  irrigation,  agriculture and  exploitation of petroleum and natural gas 3.2.3: The Rift Valley  The Rift Valley;  is a tectonically formed structural depression.  is bounded by two major and more or less parallel escarpments.  The formation of the Rift Valley has separated the Ethiopian Highlands and Lowlands in to two  extends from the Afar triangle in the north to Chew Bahir for about 1,700 km.  covers 18% of the total area of Ethiopia.  is elongated and funnel shaped, with a NE-SW orientation.  opens out in the Afar Triangle, where it is the widest, and narrows down to the south. 20  11/27/2024 The Ethiopian Rift Valley Cont. The floor of the Rift Valley is made up of interconnected troughs, grabens and depressions. Volcanic rocks, fluvial and lacustrine deposits cover the floor. In many places, numerous volcanic domes, hills and cinder cones rise from the floor. Altitude in the floor ranges from 125 meters b.s.l. at Dallol Depression, to as high as 2,000 meters a.s.l. in the Lakes region. The bounding escarpments are also of varying heights. From the floor to the edge of the escarpment, the heights vary from 200 to 1500 meters. 21  11/27/2024 Cont. Climate varies from warm, hot and dry to cool and moderately moist conditions. Similarly, the social and economic life reflects this pattern. There are;  sparsely inhabited areas by pastoralists and  in others parts people practice rain-fed agriculture. The Rift Valley is further subdivided into three physiographic sub- regions. These are; the Afar Triangle, the Main Ethiopian Rift, and the Chew Bahir Rift. The Afar Triangle The Afar Triangle;  Is the largest and widest part of the Rift Valley.  makes up 54% of the Rift Valley area.  is bounded by the high western and eastern escarpments, and by the Afar and Aisha Horst in the northeast.  It is generally of low altitude (300-700 meters).  Morphologically the Afar depression is a triangular- shape lowland, 22  11/27/2024 Cont. elevation drops uniformly from approximately 1,000 meters in the southwest to 125 meters b.s.l. in the north (Danakil depression) and in the east. Maximum temperatures can exceed 50°C during the summer wet season; Dallol, at the northern tip of Afar. ; Cont. The area is characterized by; – faulted depressions (grabens), – volcanic hills, active volcanoes, volcanic ridges, – lava fields and – low lava platforms. Lakes (Abe, Asale, and Afrera) occupy some of these basins. A prominent feature in this region is the; – Denakil Depression (Kobar Sink). Separated from the Red Sea by a 200 meters high land barrier, much of it lies below sea level. covered by thick and extensive salt plain. Lake Asale and Lake Afrera occupy the lowest parts of this sunken depression. 23  11/27/2024 Cont. The Afar Triangle is generally hot and dry. The only source of water for this area is from the Southern part of the Awash River. The economic importance of this region includes  Salt extraction,  irrigation along the Awash River and  electric potential from geothermal energy. The Main Ethiopian Rift/ Central Rift  It refers to the narrow belt of the Rift Valley that extends from; Awash River in the north to Lake Chamo in the south.  This part of the Rift Valley is the narrowest and the highest.  It has an average width of 50-80 kilometers and  elevation of 1,000-2,000 meters above sea level.  The floor in many places is dotted by cinder cones and volcanic mountains, such as;  Mount Fentale,  Boseti-guda (near Adama),  Aletu (north of Lake Ziway) and  Chebi (north of Lake Hawasa). 24  11/27/2024 Cont.  The prominent features, however, are the numerous lakes formed on tectonic sags and fault depressions.  Because of altitude, the lakes region of the Main Ethiopian Rift is generally milder and watery.  Importance of this part of Rift Valley are;  Rain-fed agriculture.  the recreational value of the lakes, and  the geothermal energy potential. The Chew Bahir Rift This is the smallest and the southern-most part of the Rift Valley. Gneissic highlands of Konso and the surrounding highlands separate it from the Main Ethiopian Rift to the north. The characteristic feature of this region is;  the broad and shallow depression, which is a marshy area covered by tall grass, into which the Segen and Woito streams empty. 25  11/27/2024 The Impacts of Relief on Biophysical and Socioeconomic Conditions A. Agricultural practices  Relief influences farm size and shape; for example  in an area of rugged terrain the farmlands are;  small in size and fragmented and tend to be irregular in shape.  Choice of farming techniques and farm implements are highly influenced by relief.  Relief influences type of crop production;  higher altitudes crops (barley, wheat) and  low altitude crops (sorghum, maize).  Relief also influence the type of animal husbandry;  sheep are reared in the higher altitudes and  camels and goat are well adapted to lower altitudes Cont. B. Settlement pattern  Highlands of Ethiopia that experience a temperate type of climatic condition that are mainly free from most of the tropical diseases are densely settled.  Rugged and difficult terrain hinders the development of settlement and its expansion.  The highlands of Ethiopia are characterized by sedentary life and permanent settlements while lowlands that are inhabited by pastoralists have temporary settlements. 26  11/27/2024 Cont. C. Transportation and communication  The highly dissected nature of the landscape is a barrier to the development of internal surface transportation that resulted in the long-term isolation of many communities  The difficult terrain makes infrastructure development and maintenance costly.  TV and radio communications are also highly influenced by relief.  The rugged topography rendered rivers less navigable due to the waterfalls, deep gorges and steep cliffs Cont. D. Hydroelectric power potential  The great difference in altitude coupled with high rainfall created suitable conditions for a very high potential for the production of hydroelectric power in Ethiopia. E. Socio-cultural feeling  People who live in the highlands have been identifying themselves as degegnas (mountaineers) and those who live in the lowlands as kollegnas (lowlanders). 27  11/27/2024 Cont. F. Impacts on climate  The climate of Ethiopia is a result of the tropical position of the country and the great altitudinal variation of the general topography.  Highlands with higher amount of rainfall and lower rate of evapo-transpiration tend to be moisture surplus compared to the moisture deficit lowlands. G. Impacts on soil  Steep mountain slopes provide low angle of rest, unstable surface materials and subject to degradation processes and relatively form shallow and little developed soils. H. Impacts on natural vegetation  Relief through its effect on climate and hydrology affect the type of natural vegetation grown in an area. 28