Back to: Geography of Ethiopia and The Horn – Freshman Courses
2.4. Geological Processes and the Resulting Landforms of Ethiopia and the Horn
- Analyze the geologic processes and the resultant landforms of Ethiopia and the Horn.
- Examine the formation of the Rift Valley.
- Recognize the current status of Ethiopian mineral endowment associated with geologic processes.
The landforms of Ethiopia and the Horn are shaped by the interplay of endogenic (internal) and exogenic (external) forces over billions of years. Endogenic forces—such as tectonics, volcanism, and faulting—build major structural features. Exogenic forces—like weathering and erosion—modify them. The geological evolution is best understood through four eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
2.4.1. The Precambrian Era (4.5 billion – 600 million years ago)
This era constitutes ~5/6 of Earth’s history. Intense orogenesis (mountain-building) created massive ranges, later reduced by denudation to a near-level peneplain. These ancient, metamorphosed rocks now form Ethiopia’s basement complex, exposed in:
- North: Western Tigray, western lowlands
- West: Gambella, Benishangul-Gumuz, western Gojjam
- South: Guji, southern Omo, Bale, Borena
- East: Eastern Hararghe
These rocks host Ethiopia’s major metallic minerals (gold, platinum, tantalum).
2.4.2. The Paleozoic Era (600 – 225 million years ago)
Characterized by prolonged denudation of Precambrian mountains. The landscape was reduced to an undulating peneplain with scattered inselbergs. Due to minimal sedimentation, Paleozoic rocks are rare in Ethiopia.
2.4.3. The Mesozoic Era (225 – 70 million years ago)
Alternating epeirogenic uplift and subsidence tilted the Horn eastward. A shallow sea transgressed from Somalia (~225 Ma), depositing:
- Adigrat Sandstone (early transgression)
- Hintalo Limestone (peak sea depth; marine fossils)
- Upper Sandstone (sea regression)
These layers cover ~25% of Ethiopia and are exposed in the **Southeastern Lowlands**, central Tigray, and Abay/Wabishebelle gorges.
2.4.4. The Cenozoic Era (70 million years ago – Present)
a. Uplifting of the Arabo-Ethiopian Landmass and Trap Series Lavas
Massive epeirogenic uplift (~60 Ma) raised central Ethiopia to ~2,000 m, fracturing the crust. Flood basalts (Trap Series lavas) covered Mesozoic sediments, forming the **Ethiopian Plateau**.
b. Formation of the Rift Valley
The **Nubian and Somali plates diverged**, creating tensional forces that caused parallel faulting. The central block subsided, forming the Rift Valley—a structural depression extending ~1,700 km in Ethiopia.
| Sub-region | Extent | Key Features |
|---|---|---|
| Afar Triangle | North (widest) | Kobar Sink (–125 m), Erta Ale, salt flats, hot springs |
| Main Ethiopian Rift | Awash River to Lake Chamo | Lakes Ziway, Langano, Awassa; volcanoes Fentale, Boseti |
| Chew Bahir Rift | Southernmost | Shallow, marshy depression |
c. Quaternary Volcanism and Deposition
Renewed volcanism (<5 Ma) produced active cones (Erta Ale, Dubi) and lava fields (>3,000 km²). Climatic shifts caused **pluvial rains**, depositing sediments in Rift lakes. Today, Cenozoic rocks cover ~50% of Ethiopia.
📝 Review Quiz
Q1. Which geological era was dominated by the denudation of Precambrian mountain ranges, resulting in a peneplain?
A) Mesozoic Era
B) Cenozoic Era
C) Paleozoic Era
D) Quaternary Period
Explanation: Section 2.4.2 states the Paleozoic Era (600–225 Ma) was characterized by prolonged denudation, reducing Precambrian mountains to a near-level peneplain.
Q2. The Adigrat Sandstone and Hintalo Limestone were deposited during which era due to a marine transgression?
A) Precambrian
B) Paleozoic
C) Mesozoic
D) Cenozoic
Explanation: As per Section 2.4.3, these sedimentary rocks formed during the Mesozoic (~225–70 Ma) when a shallow sea covered much of Ethiopia.
Q3. What caused the formation of the Ethiopian Rift Valley?
A) Volcanic hotspot activity
B) Collision of tectonic plates
C) Divergence of the Nubian and Somali plates
D) Epeirogenic uplift alone
Explanation: Section 2.4.4 explains the Rift Valley formed due to tensional forces from plate divergence, causing faulting and subsidence.
Q4. The Trap Series lavas of the Cenozoic Era primarily formed which major landform?
A) Rift Valley lakes
B) Ethiopian Plateau
C) Afar Depression
D) Blue Nile Gorge
Explanation: Flood basalts (Trap Series) covered Mesozoic layers, creating the flat, elevated surface of the Ethiopian Plateau.
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