Age Dating Techniques

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 5/6^th 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.