The Ethiopian Highlands, also known as the Abyssinian Highlands, represent a vast, rugged mountain range stretching across Ethiopia and Eritrea in Northeast Africa. This extensive area boasts the largest continuous expanse of high elevation on the continent, with most of its surface lying above 1,500 m (4,900 ft). Summits reach impressive heights of up to 4,550 m (14,930 ft). Due to its altitude and vastness, it is sometimes referred to as the "Roof of Africa".
Ethiopia is uniquely positioned in the region, possessing such a significant elevated landmass. The Great East African Rift System diagonally bisects this elevated surface, extending from Syria to Mozambique across the East African Lakes.
Most of the Ethiopian Highlands are part of central and northern Ethiopia, with the Eritrean Highlands as its northernmost portion.
Topographical map of Ethiopia showing the Ethiopian Plateau.
Location and Borders
Ethiopia is located in the Horn of Africa. It is bordered by:
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- Eritrea to the north
- Djibouti and Somalia to the east
- Sudan and South Sudan to the west
- Kenya to the south
Ethiopia's westernmost locality is Pibor River opposite the Sudanese village of Denjok. Between the valley of the Upper Nile and Ethiopia's border with Sudan and South Sudan is a region of elevated plateaus from which rise the various tablelands and mountains that constitute the Ethiopian Highlands.
On nearly every side, the walls of the plateaus rise abruptly from the plains, constituting outer mountain chains. The highlands are thus a clearly marked geographic division.
Geographic Divisions
The Ethiopian Plateau consists of the rugged Western Highlands and the more limited Eastern Highlands. The two sections are separated by the vast Eastern Rift Valley, which cuts across Ethiopia from southwest to northeast. The highlands are divided into northwestern and southeastern portions by the Main Ethiopian Rift, which contains a number of salt lakes.
The highlands are the most agriculturally productive and densely populated areas of both Eritrea and Ethiopia.
The Western Highlands extend from central Eritrea and northern Ethiopia to the basin of Lake Rudolf in the south and include the traditional lands of the Amhara and Tigray peoples. Within this section the Simen Mountains are marked. In the northwestern part of the highlands, the deep, steep-sided valleys of the major rivers separate blocks of mountains, and the upper courses of the big rivers such as the Tekezze and Abay (Blue Nile) plunge through deep gorges.
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The Eastern Highlands are more limited. Among the peaks of the Eastern Highlands stands Mount Batu.
Elevation is generally highest just before the point of descent to the Great Rift Valley, which splits the plateau diagonally. A number of rivers cross the plateau; notably the Blue Nile rising from Lake Tana. The plateau gradually slopes to the lowlands of the Sudan on the west and the Somali-inhabited plains to the east.
Outer Mountain Ranges
In Eritrea, the eastern wall of this plateau runs parallel to the Red Sea from Ras Kasar (18° N) to Annesley Bay (also known as the Bay of Zula) (15° N). About 9° N there is a break in the wall, through which the Awash River flows eastward. The main range at this point trends southwest, while south of the Awash Valley, which is some 1,000 m (3,281 ft) below the level of the mountains, another massif rises in a direct line south.
The two chief eastern ranges maintain a parallel course south by west, with a broad upland valley in between - in which valley are a series of lakes - to about 3° N, the outer (eastern) spurs of the plateau still keeping along the line of 40° E. The southern escarpment of the plateau is highly irregular, but has a general direction northwest and southeast from 6° N to 3° N. The western wall of the plateau from 6° N to 11° N is well marked and precipitous.
North of 11° N the hills turn more to the east and fall more gradually to the East Sudanian savanna plains at their base.
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The eastern escarpment is the best defined of these outer ranges. It has a mean height of 2,100 to 2,400 m (6,890 to 7,874 ft), and in many places rises almost perpendicularly from the plain. Narrow and deep clefts, through which descend mountain torrents that lose themselves in the sandy soil of the Eritrean coast, afford means of reaching the plateau, or the easier route through the Awash Valley may be chosen.
Topography and Key Features
The physical aspect of the highlands is impressive. It consists of a huge mass of Archaean rocks with a mean height of 2,000 to 2,200 m (6,562 to 7,218 ft) above sea level, and is flooded in a deep central depression by the waters of Lake Tana. Above the plateau rise several irregular and generally ill-defined mountain ranges which attain altitudes of from 3,700 m (12,139 ft) to just under 4,600 m (15,092 ft).
Characteristic of the country are the enormous fissures which divide it, formed over time by the erosive action of water. They are the valleys of the rivers, which rising on the uplands or mountain sides, have cut their way to the surrounding lowlands. Some of the valleys are of considerable width; in other cases the opposite walls of the gorges are but two or three hundred meters apart, and fall almost vertically thousands of meters, representing an erosion of many hundred thousands cubic metres of hard rock.
The highest peaks are found in the Semien and Bale ranges.
The Semien Mountains lie northeast of Lake Tana and culminate in the snow-covered peak of Ras Dashen, which has an altitude of 4,550 m (14,928 ft). The Simien Mountains are the highest parts of the Ethiopian Plateau (more than 2,000 meters; or 6,560 feet). They are surrounded by a steep, ragged escarpment (step), with dramatic vertical cliffs, pinnacles, and rock spires. Included in the range is the highest point in Ethiopia, Ras Dashen at 4,533 meters (14,926 feet).
The Bale Mountains are separated from the larger part of the Ethiopian highlands by the Great Rift Valley, one of the longest and most profound chasms in Ethiopia. The southeastern portion of the Ethiopian Highlands includes the Sidamo, Bale, Arsi and Harerge Mountains. Its highest peaks are located in the Bale Zone of Ethiopia's Oromia Region. The Bale Mountains, also designated a national park, are nearly as high as those of Semien. It is the main source of the Wabishebelle and Genalle (Juba). The range includes peaks of over 4,000 m.
Rivers and Drainage
Most of the Ethiopian uplands have a decided slope to the northwest, so that nearly all the large rivers flow in that direction to the Nile, comprising some 85% of its water. The Blue Nile watershed is the largest basin in Ethiopia. Rivers of this basin drain the great central plateau and the Blue Nile descends about 1,450 m in a distance of only 350 km from its source to Khartoum.
Such are the Tekezé River in the north, the Abay in the center, and the Sobat in the south, and about four-fifths of the entire drainage is discharged through these three arteries. Another large river is the south-flowing Omo, with 14% of the entire drainage the largest river outside the above-mentioned three main arteries discharging to the west, and by far the main feeder of the endorheic Turkana Basin with Lake Turkana.
The Tekezé River, which is the true upper course of the Atbarah River, has its headwaters in the central tableland; and falls from about 2,100 to 750 m (6,890 to 2,461 ft) in the tremendous crevasse through which it sweeps west, north, forming part of the border with Eritrea, and west again down to the western terraces, where it passes from Ethiopia to Sudan.
During the rains the Tekezé (i.e. the "Terrible") rises some 5 m (16.4 ft) above its normal level, and at this time forms an impassable barrier between the northern and central regions.
The Gash or Mareb, which forms part of the border with Eritrea, is the most northerly of the highland rivers which flow toward the Nile valley. Its headwaters rise on the landward side of the eastern escarpment within 80 km of Annesley Bay on the Red Sea. It reaches the Sudanese plains near Kassala, beyond which place its waters are dissipated in the sandy soil.
The Abay - that is, the upper course of the Blue Nile - has its source near Mount Denguiza in the Choqa mountains, around 11°0′N 37°0′E. It first flows for 110 km (68.4 mi) nearly due north to the south shore of Lake Tana.
At the southeast corner the rim of the crater is, as it were, breached by a deep crevasse through which the Abay escapes, and here makes a great semicircular bend like that of the Tekezé, but in the reverse direction - east, south and north-west - down to the plains of Sennar, where it takes the name of Bahr-el-Azrak or Blue Nile.
The chief river of Ethiopia flowing east is the Awash River (or Awasi), which rises in the Shewan uplands and makes a semicircular bend first southeast and then northeast. It reaches the Afar Depression through a broad breach in the eastern escarpment of the plateau, beyond which it is joined on its left bank by its chief affluent, the Germama (Kasam), and then trends round in the direction of the Gulf of Tadjoura.
After a winding course of about 800 km (497 mi), the Awash River passes (in its lower reaches) through a series of badds (lagoons) to Lake Abbe (or Abhe Bid) on the border with Djibouti and some 100 to 110 km (62 to 68 mi) from the head of the Gulf of Tadjoura. In this lake the Awash river is lost.
Another lacustrine region extends from the Shoan heights southwest to the Samburu (Lake Turkana) depression. To the same system belongs the neighbouring Lake Turkana, which is larger than all the rest put together. This lake receives at its northern end the waters of the Omo, which rises in the Shoan highlands and is a perennial river with many affluents.
Three major highland lakes, Lakes Hayq, Ardebo and Ashengie, lie near the edge of the western escarpment of the rift valley at altitudes between 2,000 and 2,500 m. Lake Hayq, located in northern Ethiopia’s Wollo region, has an area of 5 km2 and a maximum depth of 23 m, and is noteworthy for its extremely clear water.
Lake Ardebo is located about 5-km southeast of Lake Hayq. This lake is smaller in size than Lake Hayq and flows into Hayq via the Anchercah River.
Lake Ashengie is located north of Lake Hayq in the Tigray region, and sits at an altitude of 2,460 m. The lake covers an area of 25 km2 with a maximum depth of 20 m and a mean depth of 14 m. The lake is fed by a number of small streams from the surrounding areas and there is no drainage out of the lake.
Geology
The Ethiopian Highlands began to rise 75 million years ago, as magma from the Earth's mantle uplifted a broad dome of the ancient rocks of the Arabian-Nubian Shield. Around 30 million years ago, a flood basalt plateau began to form, piling layers upon layers of voluminous fissure-fed basaltic lava flows. Most of the flows were tholeiitic, save for a thin layer of alkali basalts and minor amounts of felsic (high-silica) volcanic rocks, such as rhyolite.
The Ethiopian Highlands were eventually bisected by the Great Rift Valley as the African continental crust pulled apart.
The metamorphic rocks compose the main mass of the tableland, and are exposed in every deep valley in Tigre and along the valley of the Blue Nile. Mica-schists form the prevalent rocks. Hornblende schists also occur and a compact feldspathic rock in the Suris defile.
In the region of Adigrat the metamorphic rocks are invariably overlain by white and brown sandstones, unfossiliferous, and attaining a maximum thickness of 300 m. They are overlain by the fossiliferous limestones of the Antalo group. Around Chelga and Adigrat coal-bearing beds occur, which Blanford suggests may be of the same age as the coal-bearing strata of India.
The fossiliferous Antalo Limestones are generally horizontal, but are in places much disturbed when interstratified with Mekelle Dolerite.
Above a height of 2,400 m (7,900 ft) the country consists of bedded traps belonging to two distinct and unconformable groups. The lower (Ashangi group) consists of basalts and dolerites often amygdaloidal. Their relation to the Antalo limestones is uncertain, but Blanford considers them to be not later in age than the Oolite. The upper (Magdala group) contains much trachytic rock of considerable thickness, lying perfectly horizontally, and giving rise to a series of terraced ridges characteristic of central Ethiopia.
Of more recent date (probably Tertiary) are some igneous rocks, rich in alkalis, occurring in certain localities in southern Ethiopia. Of still more recent date are the basalts and ashes west of Massawa and around Annesley Bay and known as the Aden Volcanic Series.
With regard to the older igneous rocks, the enormous amount they have suffered from denudation is a prominent feature.
Climate
The climate of Ethiopia and its dependent territories varies greatly. It is temperate on the plateau and hot in the lowlands. The country lies wholly within the tropics, but its nearness to the equator is counterbalanced by the elevation of the land.
At Addis Ababa, which ranges from 2,200 to 2,600 m (7,218 to 8,530 ft), maximum temperature is 26 °C (78.8 °F) and minimum 4 °C (39.2 °F). The weather is usually sunny and dry, but the short (belg) rains occur from February to April and the large (meher) rains from mid-June to mid-September.
Over the greater part of Ethiopia as well as the Oromia highlands, the climate is very healthy and temperate. However, in the deep valleys of the Tekezé and Abay, and generally in places below 1,200 m (3,937 ft), the conditions are tropical and diseases such as malaria are prevalent. The Somali Region and the Danakil Depression in the Afar Region have a hot, sunny and dry climate producing fully desert or semi-desert conditions.
In the uplands, the air is cool and bracing in summer, and in winter very bleak. The mean range of temperature is between 15 and 25 °C (59 and 77 °F). On the higher mountains the climate is alpine in character.
The atmosphere on the plateaus is exceedingly clear, so that objects are easily recognizable at great distances.
In addition to the variation in climate dependent on elevation, the year may be divided into three seasons. Winter, or the cold season, lasts from October to February. It is followed by a dry hot period, which about the middle of June gives place to the rainy season. In the former provinces of Gojjam and Welega, heavy rains continue till the middle of September. October is occasionally a wet month. There are also spring and winter rains; indeed rain often falls in every month of the year.
But the rainy season proper, caused by the southwest monsoon, lasts from June to mid-September, and commencing in the north moves southward. In the region of the headwaters of the Sobat the rains begin earlier and last longer.
The rainy season is of great importance not only to Ethiopia but to the countries of the Nile valley, as the prosperity of the eastern Sudan and Egypt is largely dependent upon the rainfall.
Flora and Fauna
The Ethiopian Highlands share a similar flora and fauna to other mountainous regions of Africa; this distinctive flora and fauna is known as Afromontane, but from the time of the last ice age the region has been populated with some Eurasian (palearctic) flora. The highlands themselves are divided into three distinct ecoregions, distinguished by elevation.
