CHAPTER TWO - THE GEOLOGY OF ETHIOPIA AND THE HORN

Chapter's Objectives

After completing this chapter, the students will be able to:
 Analyze the geologic processes and the resultant land forms of Ethiopia and the Horn.
 Examine the formation of the Rift Valley.
 Recognize the current status of Ethiopian mineral endowment associated with geologic processes.

2.1. Introduction

Geology is an Earth science that studies the evolution of the earth, the materials of which it is made of, and the processes acting upon them.  Therefore, this chapter will focus on the geology of Ethiopia and the Horn.

The earth’s continents were once bunched up together in to a single huge continent called Pangaea. The large super continent was then split into Gondwanaland (where Africa is a part) and Laurasia; and later into smaller fragments over the last million years.

Alfred Wegener, who was a German Climatologist proposed the hypothesis that argues the continents were once assembled together, and this theory is called the Continental drift theory.

Wegener’s principal evidences were obtained from the following observations:

Fit of the continents: The opposing coastlines of continents often fit together.

Match of mountain belts, rock types: If the continents are reassembled as Pangaea, mountains in West Africa, North America, Greenland, and Western Europe match up.

Distribution of fossils: The distribution of plants and animal fossils on separate continents forms definite linked patterns if the continents are reassembled.

Paleo-climates: rocks formed 200 million years ago in India, Australia, South America, and southern Africa all exhibited evidence of continental glaciations.

Pangaea
Distribution of fossils

2.2 The Geologic Processes: Endogenic and Exogenic Forces

Geology studies of how Earth’s materials, structures, processes and organisms have changed over time. These processes are divided into two major groups: internal and external processes.

The internal processes (endogenic) include orogenesis (mountain building processes such as volcanic activities and all the tectonic processes (folding, faulting), and epeirogenesis (slow rising and sinking of the landmass). These processes result in building of land features like plateaus, rift valleys, Block Mountains, volcanic mountains, etc.

The external (exogenic) processes include weathering, mass transfer, erosion and deposition. They act upon the volcanic and structural landforms by modifying, roughening and lowering them down. The landmass of Ethiopia, as elsewhere, is the result of the combined effect of endogenic and exogenic processes.

The internal processes
The external processes

2.3 The Geological Time Scale and Age Dating Techniques

The Earth is believed to have been formed approximately 4.5 billion years ago and the span of time before life appeared was termed the Precambrian Era. To describe the geology and history of life on Earth, scientists have developed the geological time scale.

The geological time scale measures time on a scale involving four main units which are known as eon, era, period and epoch from the largest to the smallest chronologically. An epoch is the smallest unit of time on the scale and encompasses a period of millions of years.

The Geological time scale

Era

Period

Began (in Million Years)

End (my)

Major Events (million years ago)

Cenozoic

Quaternary

1.6

Present

Major glaciers in North America and Europe (1.5)

Tertiary

70

1.6

Rocky Mountains (65), individual continents take shape.

Mesozoic

Cretaceous

146

70

Dinosaurs extinct (65), western interior seaway and marine reptiles (144 – 65)

Jurassic

208

146

Pangaea (one land mass) begins to break up (200)

 

Triassic

225

208

First mammals and dinosaurs

 

Permian

290

225

Greatest extinction on Earth (245)

 

Pennsylvanian

322

290

First reptiles

 

Mississippian

362

322

Coal-forming forests

Paleozoic

Devonian

408

362

First land animals and first forests (408)

Silurian

439

408

Life invades land

 

Ordovician

510

439

First fish appeared

 

Cambria

600

510

Great diversity of marine invertebrates

Precambrian

Proterozoic

2,500

600

Marine fossil invertebrates (600)

Archean

4,500

2,500

Earliest fossils recorded (3,500), earliest rock formation (4,000)

 

Age Dating Techniques

There are two techniques of knowing the age of rocks: Relative and absolute age dating.

Relative Dating

Relative dating uses geological evidence to assign comparative ages of fossils. Hence, we can use two ways to know the relative age of a rock: one way is to look at any fossils the rock may contain. If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period. The second way is to use the “What is on top of the older rocks?” When you find layers of rocks in a cliff or hillside, younger rocks are on top of older rocks. But these two methods only give the relative age of rocks -which one is younger and which is older.

Absolute Dating

This method is lso known as radiometric technique. This technique was developed with discovery of radioactivity in 1896. The regular rates of decay for unstable, radioactive elements were found to constitute virtual “clocks” within the earth’s rocks. Radioactive elements such as uranium (U) and thorium (Th) decay naturally to form different elements or isotopes of the same element. Every radioactive element has its own half-life.

At the end of the period constituting one half-life, half of the original quantity of radioactive element has decayed; after another half-life, half of what was left is halved again, leaving one-fourth of the original, and so on. Two of the major techniques include:

  1. Carbon-14 Technique
  2. Potassium-Argon Technique

2.4. Geological Processes and the Resulting Landforms of Ethiopia and the Horn Activity

2.4.1. The Precambrian Era Geologic Processes (4.5 billion – 600 million years ago)

  • The Precambrian Era covers 5/6th of the Earth’s history. Due to its remoteness in time and the absence of well-preserved fossils, our knowledge of the events is limited. Nevertheless, some general description of the main geologic processes can be made.
  • The major geologic event of the Precambrian Era was As a result, the land was subjected to intense folding which was accompanied by intrusive igneous activity. The result was the formation of huge mountain ranges.
  • The Precambrian rocks are covered by younger rock formations. Therefore, in most parts of Ethiopia rocks belonging to this Era are found beneath all other rocks, forming the basement rocks.
  • Since, they had been subjected to pressure and heat from overlying rocks, earth movements (folding, orogenesis) and to intrusive igneous activity; the original rocks were altered into metamorphic rocks of varying stages of metamorphism. Since these same processes have allowed mineralization and crystal formation, the rocks are also collectively described as crystalline rocks.
  • The Precambrian rocks are overlaid by recent rock formations. However, as surface rocks covering 25% of the land mass of the country; they are found exposed in the following areas:

In the northern part: Western lowlands, parts of northern and central Tigray.
In the western Part: Gambella, Benishangul-Gumuz (Metekel and Asossa), western Gojjam, western Wellega, Illuababora, and Abay gorge.
In the southern Part: Guji, southern Omo, and parts of southern Bale and Borena.
In the eastern part: Eastern Hararghe.

2.4.2 The Paleozoic Era Geologic Processes (600 million – 225 million years ago)

The Paleozoic Era lasted for about 375 million years. The major geological process of this Era was denudation. The gigantic mountains that were formed by the Precambrian orogeny were subjected to intense and prolonged denudation. At the end, the once gigantic mountain ranges were reduced to a “peneplained” surface. Undulating plain with some residual features (inselbergs) here and there was formed. The sediments were transported southward and eastward to form continental (in Africa) and marine deposits, respectively. Because of the limited deposition within Ethiopia, rocks belonging to this Era are rare in the country.

2.4.3 The Mesozoic Era Geologic Processes (225-70 million years ago)

This Mesozoic era was an Era of alternate slows sinking and rising (epeirogenesis) of the landmass. This process affected the whole present-day Horn of Africa and Arabian landmass. At the same time the land was tilted eastward and therefore lower in the southeast and higher in the northwest.

The subsidence of the land began about 225 million years ago. As the land sank slowly the sea invaded it starting from Somalia and Ogaden and slowly spreading northwestward. This was in late Triassic. This phenomenon continued up to Jurassic period. As the shallow sea spread towards the land, sands were deposited over the peneplained Precambrian rock surface. As the depth of the sea increased, mud (shale), gypsum and later lime were deposited. Hence, Mesozoic rocks are considered to have the greatest potential for oil and gas deposits.

Through time, compression by the overlying rocks and by cementing minerals, the sands and lime were compacted to form sandstone and limestone layers respectively. These are known as the Adigrat sand stone and Hintalo limestone layers. They are named after place names in Tigray where they might have been first identified.

In the Horn of Africa and Ethiopia, the slow rise of the land and consequently the regression of the sea began in the Upper Jurassic. It continued throughout the Cretaceous period. With the retreat of the sea, another process of deposition occurred. Gypsum, shale and at last sands were laid over the Hintalo limestone. The uppermost layer is known as the Upper sandstone. By the end of the Mesozoic Era, when the land emerged out of the sea, three major sedimentary formations were laid and formed upon the Precambrian rock surface. The Mesozoic sedimentary rocks cover 25% of the land mass of the country.

Due to the tilting of the landmass during the transgression and regression of the sea, and due to the direction of the invading and retreating sea, the age and thickness of the Sandstone layers vary in a Southeast – Northwest direction. The Adigrat sandstone is older and thicker in the southeast and progressively decreases in age and thickness northwestward. The Upper sandstone, on the other hand, is thicker and younger (Upper Cretaceous) in the Southeast, while in the Northwest it is older and thinner.

The transgressing sea and Mesozoic sediments nearly covered the whole of Ethiopia. The northwestern limit was as far as central Tigray, and western slopes of Western highlands. In most parts of Ethiopia, the Mesozoic rocks are overlaid by the Cenozoic rocks. As surface rocks, these old marine sediments are extensively found in the Southeast lowlands. Other exposures include central Tigray, and along the gorges of Abay and Wabishebelle rivers.

2.4.4. The Cenozoic Era Geologic Processes (70million years ago – Present)

The Cenozoic Era is the most recent of the geologic Eras. The tectonic and volcanic activities that took place in this Era have an important effect in the making of the present-day landmass of Ethiopia and the Horn of Africa. The land was subjected to two major geologic events and other geologic processes of lesser magnitude but still important. These geologic activities are:

  1. Uplifting of the Arabo-Ethiopian landmass and outpouring of huge quantity of lava.
  2. Formation of the Rift Valley.
  3. Quaternary volcanism and deposition.

     

Uplifting of the Arabo-Ethiopian landmass and outpouring of lava flood

The uplifting of the whole of the Arabo-Ethiopian landmass is a continuation of the slow rise that began in the Upper Jurassic and Cretaceous periods. Where the uplifting was of greater magnitude, the land was pushed up to a maximum height of 2,000 meters above sea level. This uplifting was of an epeirogenic character. The whole of the Arabo-Ethiopian landmass was pushed up in blocks as one mass. The greatest uplift was in central Ethiopia. This immense tectonic force also fractured the crust at many places. Huge quantity of lava came out through these fractures. The out pouring of this flood of basalt spread widely and extensively and covered a large part of the Mesozoic sedimentary layer to form the Ethiopian plateau surface and also the floor of the present-day Rift Valley. At that time the Rift Valley was not yet formed. The mass of lava was so immense, that it formed a thick layer of volcanic rocks on the plateau, which amounted to more than 1,000 meters above sea level in the north Central Highlands. This volcanic material is known as Trappean lava or Trapean Series lava.

The Formation of the Rift Valley

The formation of the Rift Valley is said to be related with the theory of plate tectonics. According to the theory, the Rift Valley may be lying on the Earth’s crust below which lateral movement of the crust in opposite directions producing tensional forces that caused parallel fractures or faults on the sides of the up-arched swell. As the tension widened the fractures, the central part of the landmass collapsed to form an extensive structural depression known as the Rift Valley.

  • The major faulting movement probably began in the late Oligocene and Miocene Epochs of the Cenozoic Era. This rifted the Red Sea trough, which began to be flooded from the north. But the major rifting, affecting the whole African Rift System, including that of Ethiopia and the Gulf of Aden took place in the Miocene Epoch.
  • Rifting and faulting, however, continued all the time throughout the Pliocene and even the Pleistocene Epochs.
  • The Red Sea and the Gulf of Aden were connected as a result of the rifting and faulting of the land bridge that separated them. At the same period (Pliocene), the Afar depression (including the Gulf of Zula) was down-faulted allowing the Red Sea water to penetrate far inside.
  • As a result, thick saline materials accumulated. During the same period, the area between the Danakil Depression and the Red Sea was uplifted to form the Afar Block Mountains.

The Spatial Extent of the Rift Valley

  • The Ethiopian Rift Valley is part of the Great East African Rift system that extends from Palestine-Jordan in the north to Malawi-Mozambique in the south, for a distance of about 7,200 kilometers. Of these, 5,600 kilometers is in Africa, and 1,700 kilometers in Eritrea and Ethiopia.
  • On land, the widest part of the Rift Valley is the Afar Triangle (200-300 km). The Red Sea, the Gulf of Aden, and the East African System meet and form the triangular depression of the Afar where the Kobar Sink lies about 125 meters below sea level.
  • The formation of the Gulf of Aden and the separation of the Arabian Peninsula from the Horn of Africa also took place during the Tertiary period.
  • The Rift Valley region of Ethiopian is the most unstable part of the country. There are numerous hot springs, fumorales, active volcanoes, geysers, and frequent earthquakes.

The formation of the Rift Valley has the following structural (physiographic) effects:

  • It divides the Ethiopian Plateau into two.
  • It separates the Arabian landmass from African landmass.
  • It causes the formation of the Dead Sea, Red Sea and the Gulf of Aden troughs.
  • It creates basins and fault depressions on which the Rift Valley lakes are formed.

     

Quaternary Volcanic Eruptions and Depositions

  • They include recent volcanic activities that took place after the formation of the Rift Valley. This occurred in the Pliocene-Pleistocene Epochs.
  • This activity was generally limited to the floor of the Rift Valley and the region south of Lake Tana, where the lava covers an area of more than 3,000 km .
  • Aden volcanics and recent faulting are more extensively developed in the Afar region. The latter phenomenon is also widely manifested in the main Ethiopian Rift, especially in its northern section. Because of their recent occurrence, the Aden volcanics have relatively well-preserved and visible morphological features.
  • The basic volcanic features of the Aden series include the following:
  • Numerous and freshly preserved volcanic cones, many of which have explosive craters. Some of these are active Dubi, Erta Ale, Afreraetc.Of these, Erta Ale is the most active volcano in Ethiopia.
  • Volcanic hills and mountains, some of which are semi-dormant (Fantale, Boseti-Gouda near Adama, Aletu north of Lake Ziway, Chebbi north of Lake Hawassa etc.).

Quaternary Deposition

  • During the Quaternary period of the Cenozoic Era, the Earth experienced a marked climatic change, where warmer and dry periods were alternating with cooler and wet periods. This was the time of the last “Ice Age” in the middle and high latitude areas and the time of the “Pluvial Rains” in Africa.
  • The heavy Pluvial Rains eroded the Ethiopian plateau and the eroded materials were deposited in the Rift Valley lakes. The excessive rain resulted in an excessive surface flow; rivers were many and large. They carried a lot of water and sediments. Lake and marshy areas became numerous and deep. Many were enlarged and covered much area and even merged together. For example, Ziway-Langano-Shalla; Hawasa-Shallo; Chamo-Abaya; and Lake Abe and the nearby smaller lakes and marsh basins formed huge lakes.
  • After the “Pluvial Rains”, the Earth’s climate became warmer and drier. Thus, it increased the rate of evaporation that diminished the sizes of the lakes. Today, there are lacustrine deposits of continental origin around many of the Ethiopian lakes, river valleys and lowlands.
  • According to the place and manner of deposition and depositing agents these deposits are divided as follow.
  1. Lacustrine deposits: Deposits on former lakebeds, and swampy depressions.
  2. Fluvial deposits:Deposits on the banks of rivers, flood plains both in plateau, foothills etc.
  3. Glacio-fluvial deposits and erosional features: These are occurred on high mountains, such as Bale and Kaka Mountains.
  4. Aeolian deposits:Arewindblown deposits.
  5. Coastal and marine deposits: Deposits on sea invaded and sea-covered places.
  • The quaternary deposits are mainly found in the Rift Valley (Afar and Lakes Region), Baro lowlands, southern Borena, and parts of northwestern low lands.
  • Generally, the Cenozoic rocks cover 50% of the land mass of the country. These include Highland Tertiary volcanics (basalts), Tertiary as well as Quaternary volcanics, and sediments of the rift valley.
Geology of Ethiopia
Geology of Ethiopia

2.5. Rock and Mineral Resources of Ethiopia

The occurrence of metallic minerals in Ethiopia is associated with the Precambrian rocks. Although not in sufficient concentration and extent, a great variety of such minerals occur in the basement rocks. These rocks contain most of the metallic deposits known at present.

The exploitation and search for mineral deposits in Ethiopia has been taking place for the past 2,000 years or so. However, presently mineral production from Ethiopia has been negligible by World standards.

2.5.1. Brief Facts and Current State of Main Minerals in Ethiopia

Geological surveys proved that Ethiopia has abundant mineral resources of metals and precious metals, coal, and industrial minerals.

Gold:

Gold has been mined in Ethiopia for quite long time, mainly from Benishangul-Gumuz (Metekel) and Adola. Operating mines produce gold from primary sources in such localities as Dermi-dama, Sakoro and Lega-dembi. Mechanised alluvial working is confined to the state-operated gold field of Adola. Secondary gold deposits are common in the following localities: Adola, Murmur Basin, Shakiso, Awata Basin, Dawa Basin, Ghenale Basin, UjamaBasin,Makanisa (Guba and Wombera), Kaffa. In Gambella and Illuababora (Akobo River), in Sidama (Wondo), Borena (Negele-Yabelo area) and in Benishangul-Gumuz (Sherkole), west Wellega, Mengi-Tumat-Shangul areas to the Sudanese border, and the drainage of the Didessa and Birbir.

Platinum:

The Yubdo area in Wellega, is the only active Ethiopian Platinum mine. Platinum occurrences have been reported from Delatti in Wellega, and the valley of Demi-Denissa and Bone Rivers as well as Tullu Mountain area in Sidama.

Tantalum:

Significant deposit of tantalum and niobium is found in southern Ethiopia. It occurs in Adola area where Kenticha Tantalum mine with resources of more than 17,000 metric tons of world class ore reserve is found.

The sedimentary and volcanic rock activities are also resourceful. Extensive lignite deposits in Ethiopia are found in Nedjo (Wellega), and in small amounts in Chilga (Gonder) are found in the sedimentary formations laid in between Trapean lava. However, important Lignite, one of the lowest ranked coal, is known to occur in many localities such as in the Beressa Valley and Ankober (North Shewa), Sululta (nearAddis Ababa),Muger Valley (West Shewa), Aletu valley (near Nedjo), Kariso and Selmi Valleys (Debrelibanos), Zegawodem gorge (near Fiche), Didessa Valley (southwest of Nekemte), Kindo and Challe Valley (Omo confluence), Adola,Wuchalle (north of Dessie), Chukga area (on Gonder-Metema road), Dessie area (near Borkena River). These areas are promise to be a good prospect to meet some of the local industrial and domestic needs.

Gemstones: Gemstones, including amethyst, aquamarine, emerald, garnet, opal, peridot, sapphire, and tourmaline occur in many parts of Ethiopia, mainly in Amhara and Oromia Regional States. Quality Opal was first discovered by local people in Wadla and Dalantaworedas, North Wello in Amhara Regional State.

Potash: The potash reserve in the Danakil (Dallol Depression) of the Afar region is believed to be significant.

Gypsum and Anhydrite: A limited amount of gypsum is produced for domestic consumption in Ethiopia, mainly for the cement industry, but very large deposits are known to occur in sedimentary formations of the Red Sea coastal area, Danakil Depression, Ogaden, Shewa, Gojjam, Tigray, and Hararghe.

Clay: Ethiopia is endowed with industrial clay material. Alluvial clay deposits for bricks and tile, pottery and pipe industry occur in Adola, Abay gorge, and the Rift Valley lakes region. Ceramic clay for the production of glasses, plates, bricks is found at Ambo and Adola. Tabor ceramic industry in Hawassa gets most of its raw materials from local sources.

Marble: Crystalline limestone is widespread in the basement rocks of Ethiopia. Marble has been quarried in such localities as west of Mekelle and south of Adwa in Tigray. In the east in Galetti, Soka, Ramis, Rochelle, Kumi and other valleys of Chercher Mountain in West Hararghe. In the northwestern also in areas built of Precambrian schist in Gonder, and the Dabus River and other neighboring river basins in Benishangul-Gumuz and Gojjam.

Construction stones: Basalt, granite, limestone and sandstone are important building stones. For the surfacing of roads and compaction, basalt, scoria and other volcanic rocks are extensively used. Mesozoic limestone is an important raw material for cement and chalk production. The earlier cement works at Dire Dawa and the recent ones at Muger Valley, Abay gorge (Dejen), Tigray (Messebo) are using similar raw materials from these rock formations.

2.5.2. Mineral Potential Sites of Ethiopia

According to the Ethiopian geological survey, the geologic formations that host most mineral potentials of Ethiopia includes three major greenstone belts and other formations .These are:

  1. The Western and South-western-greenstone belt: They contain various minerals: primary gold occurrences (Dul,Tulu-Kape,Oda-Godere, Akobo,Baruda,Bekuji-Motish and Kalaj);Yubdo Platinum, Base metals of AzaliAkendeyu, Abetselo and Kata;FakushuMolybdenite and the iron deposits of Bikilal, Chago, Gordana and Korre, Benshagul-Gumuz-Marble, Akobo and Asosa placer gold deposits and etc.
  2. The Southern greenstone belt:It is known as the Adola belt, which comprises the primary gold deposits and occurrences of Lega-dembi, Sakaro, Wellena, Kumudu, Megado-Serdo, DawaDigati, Moyale and Ababa River;thecolumbo-tantalite of kenticha and Meleka, andtheAdola nickel deposit and other industrial minerals.
  3. The Northern greenstone belt(Tigray): This belt comprises of the primary gold occurrences of Terakemti, Adi-Zeresenay, and Nirague.The base metals of Terer, Tsehafiemba and other parts of Tigray, Placer gold occurrences of Tigray.

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