Unit 1: Geological History and Topography of Ethiopia

This unit introduces the foundational concepts of geography and explores the physical environment of Ethiopia, including its spatial position, geological evolution, and diverse landforms . Students will learn about the scientific methods used to describe the Earth’s surface and how internal and external forces have shaped the “Roof of Eastern Africa” over millions of years. Understanding these geographic themes is essential for analyzing the country’s sociocultural, political, and economic conditions .

1.1 Geography: Meaning, Scope and Branches

• Meaning of Geography: Derived from the Greek words Geo (Earth) and Graphos (Writing) .
• Scholarly Definitions:
  • Eratosthenes: The description of the Earth.
  • Alexander Von Humboldt: A synthesizing discipline connecting the general with the particular through mapping and regional emphasis.
  • Richard Hartshorne: Orderly and rational description of distributions on the Earth’s surface.
  • Modern Definition: The scientific study of Earth that analyzes spatial and temporal variations of physical, biological, and human phenomena.
• Scope of Geography: The scope is wide and dynamic, focusing on features in the hydrosphere, atmosphere, lithosphere, biosphere, and anthroposphere .
• Branches of Geography:
  1. Physical Geography: Studies natural features like climate, landforms, soil, and water resources. Sub-fields include climatology, geomorphology, and oceanography.
  2. Human Geography: Studies human aspects including cultures, population, economic activities, and political systems. Sub-fields include population geography, urban geography, and economic geography.

1.2 Location, Size and Shape of Ethiopia

• Location:
  • Relative Location: Position in relation to other features.
    • Vicinal Location: Position relative to neighboring countries (Sudan, South Sudan, Eritrea, Djibouti, Somalia, and Kenya) .
    • Strategic Location: Position relative to global features (e.g., Horn of Africa, Nile Basin, Red Sea route) .
  • Absolute (Astronomical) Location: Expressed in latitudes and longitudes. Ethiopia is located between $3^\circ N – 15^\circ N$ latitude and $33^\circ E – 48^\circ E$ longitude.
• Size: Ethiopia covers a total area of 1,106,000 square kilometers, making it the tenth largest country in Africa.
• Shape: Ethiopia has a compact shape (essentially circular) because its north-south and east-west spans are comparable .
  • Compactness Indicators: Measured by ratios like Boundary-Circumference (B/C) and Area-Boundary (A/B) .
  • Militaristic Advantage: A compact shape allows a smaller borderline to protect a larger hinterland .

1.3 Geological History of Ethiopia

Scientists use a geological time scale to measure the history of Earth through eras, periods, and epochs .

• Precambrian Era (4.5 billion – 600 million years ago): The oldest and longest era. Characterized by orogenic movements, volcanic activities, and the formation of the basement complex rock (crystalline rock) .
• Paleozoic Era (600 – 250 million years ago): Characterized by massive denudation and peneplanation. No significant rock formations occurred, leaving a gap in the rock record.
• Mesozoic Era (250 – 70 million years ago): An era of epeirogenesis (sinking and uplifting) and sedimentary rock formation .
  1. Triassic Period: Land sank, followed by transgression of the sea; formed Adigrat sandstone .
  2. Jurassic Period: Continued sea transgression; deposited Hintalo limestone.
  3. Cretaceous Period: Land began to rise, sea started to regress; deposited Upper Sandstone .
• Cenozoic Era (70 million years ago – present): The most recent era, shaping current landforms .
  • Tertiary Period: Characterized by massive uplifting and volcanic eruptions (Trappean lava series) which formed the highlands . Faulting created the Great East African Rift Valley System.
  • Quaternary Period: Recent volcanic activities and the evolution of modern man.

1.4 Landforms of Ethiopia (Topography)

Ethiopia’s topography is the result of endogenic forces (internal) and exogenic forces (external) . It is divided into three main physiographic divisions:

1. Western Highlands and Associated Lowlands: The biggest region (44% of the country) . Sub-divisions include the Plateau of Tigray, North Central Massif (containing Ras Dashen, the highest peak at 4620m), Plateau of Shewa, and Southwestern Highlands.
2. Southeastern Highlands and Associated Lowlands: Located southeast of the Rift Valley. Includes the Hararghe Plateaus, Arsi Plateau, Bale Massif (containing Mount Tulu Dimtu), and Sidama Highlands.
3. The Rift Valley: A narrow strip (18% of the country) dividing the highlands. It includes the Afar Triangle (containing the Kobar Sink, 116m below sea level), the Main Ethiopian Rift (Lakes Region), and the Chew-Bahir Rift.

Key Terminology

• Absolute location: Position expressed in latitudes and longitudes.
• Adigrat sandstone: The oldest sedimentary rock formed during the Triassic period.
• Anthroposphere: The part of the environment modified by humans.
• Basement complex rock: The oldest crystalline rocks formed during the Precambrian era.
• Compact shape: A shape where the distance from the center to any border does not vary greatly.
• Denudation: Long-term wearing away of the Earth’s surface by water, ice, and wind.
• Endogenic force: Pressure originating inside the Earth leading to volcanism and faulting.
• Epeirogenic: The vertical uplifting or sinking of the Earth’s surface.
• Exogenic force: Forces deriving strength from the Earth’s exterior, such as erosion.
• Geology: The study of the Earth’s physical structure and history.
• Hintalo limestone: Sedimentary rock deposited during the Jurassic period.
• Orogenic: Mountain-building processes associated with volcanic or tectonic activity.
• Peneplanation: Formation of a low-relief plain through protracted erosion.
• Regression: Exposure of submerged seafloor above sea level as water retreats.
• Relative location: The position of a place in relation to other geographic features.
• Relief: The vertical and horizontal dimensions of a land surface.
• Transgression: A geologic event where sea level rises and floods the land.
• Upper Sandstone: The youngest sedimentary rock formed during the Cretaceous period.
• Vicinal location: A type of relative location considering neighboring countries.