Grade 10 Geography Notes

Earth's History and Geological Time Scale

  • Geologists divide Earth's history into four major eras, each with periods and epochs.
  • Subdivisions are based on tectonic activity, life forms, and environmental changes.
  • Eras are major divisions defined by life form differences.

Four Eras

  • Precambrian Era
  • Paleozoic Era (ancient life)
  • Mesozoic Era (middle life)
  • Cenozoic Era (recent life)
  • Periods are subdivisions of eras, also based on life forms.
  • Cenozoic periods are further divided into epochs.

Precambrian Era

  • Longest geological time unit (88% of geological time scale).
  • Lasted from 4.5 billion to about 600 million years ago.
  • Little is known due to deeply buried and transformed rocks.
  • Precambrian rocks form foundation rocks worldwide.
  • Metamorphic rocks are crystalline, transformed from sedimentary or volcanic rocks by heat and pressure.

Paleozoic Era

  • Began about 600 million years ago with the appearance of organisms with hard parts.
  • Warm, shallow seas covered much of Earth's surface.
  • Most life forms were of marine origin.

Mesozoic Era

  • Began about 250 million years ago.
  • Continents were joined as Pangaea.
  • Pangaea separated into Laurasia (north) and Gondwanaland (south) during the Triassic period.
  • Reptiles were dominant in the Jurassic period.

Cenozoic Era

  • Began about 70 million years ago.
  • Dinosaurs and many other life forms became extinct.
  • Mountain ranges began to form.
  • Climate became cooler, and ice ages occurred.
  • The present-day period is the Quaternary.
  • We live in the Holocene epoch, after the last ice age.
  • Homo Sapiens appeared about 500,000 years ago, becoming dominant only about 10,000 years ago.

Geological Events in Ethiopia

Precambrian Era

  • Ethiopia consisted of huge folded and faulted mountains.

Paleozoic Era

  • Land was affected by persistent denudation (erosion) and peneplanation.
  • No significant rock formation in many parts of Ethiopia.

Mesozoic Era

  • Land surface sank, and the sea invaded from the southeast towards the northwest.
  • Layers of sandstone (Adigrat sandstone) and limestone (Hintalo limestone) were deposited.
  • Landmass started uplifting, emerging from the sea toward the end of the era.
  • The sea retreated southeast, depositing Upper sandstone.
  • Many parts of Ethiopia were covered by three layers of Mesozoic marine sediment.

Cenozoic Era

  • Uplift of the Ethiopian landmass, crustal deformations, and immense lava flow (Trapean lava series).
  • Formation of the Ethiopian highlands.
  • Major vertical displacements formed the Ethiopian Rift Valley during the middle tertiary.
  • The Rift Valley divided the Ethiopian highlands into Western and Southeastern Highlands.

Quaternary Period

  • Marked climatic change resulting in Pluvian Rains in Africa.
  • Ethiopian plateaus affected by heavy erosion.
  • Dry climate increased evaporation and reduced rift valley lake sizes.
  • Salt, potash, and saline water were left behind in the Afar depression.

Continental Drift

  • Alfred Wegener proposed the theory of continental drift in 1915 in "The Origin of Continents and Oceans".
  • Pangaea (supercontinent) existed during the Mesozoic era.
  • About 200 million years ago, Pangaea began to break apart into Gondwanaland and Laurasia.
  • Gondwanaland: South America, Africa, Arabian Peninsula, Indian subcontinent, Australia, and Antarctica.
  • Laurasia: North America and Eurasia.
  • Continents 'drifted' to their present positions.

Earth's Physical Environment

  • Divided into four major parts: hydrosphere (water), atmosphere (gases), lithosphere (solid Earth), and biosphere (life).
  • Highly integrated environment with continuous interactions between air, rocks, and water.

Hydrosphere

  • Includes all of Earth’s water, circulating among oceans, continents, and the atmosphere.
  • Oceans cover nearly 71% of the globe, with an average depth of about 3.8 km.
  • Oceans account for about 97% of Earth's water.
  • About 1.8% of Earth’s water is frozen in glaciers.
  • About 0.63% is found underground.
  • Only 0.01% forms streams and lakes.

Atmosphere

  • Gaseous envelope surrounding Earth, mostly nitrogen and oxygen.
  • One-half of the mass lies below 5.6 km altitude, and 90% within 16 km.
  • About 99% is concentrated in the first 30 km above the surface.

Biosphere

  • Zone inhabited by life; concentrated near the surface extending from the ocean floor up several kilometers into the atmosphere.
  • Includes the uppermost lithosphere, the hydrosphere, and the lower parts of the atmosphere.

Lithosphere

  • Solid Earth; consists of the crust, mantle, and core.
  • The outermost layer (crust) is composed of solid rock.
  • The mantle lies beneath the crust and contains small pools of liquid rock (magma).
  • The core is a dense, hot, partly molten region composed mainly of iron and nickel.

Components of the Lithosphere: Rocks

  • Rocks are naturally formed solid aggregates of one or more minerals.

Types, Formation, and Characteristics

  • Geologists group rocks into igneous, sedimentary, and metamorphic types.
Igneous Rocks
  • Form when molten rock (magma) cools and solidifies, either in the crust or on the surface.
  • Magma cools and hardens before and erupting onto the earth’s surface.
  • Magma that gets to the surface is called lava.
  • Two main groups: intrusive and extrusive.
Extrusive igneous rocks
  • Magma erupts onto the surface, solidifying rapidly. Fine-grained textures due to rapid cooling (e.g., basalt and obsidian).
Intrusive igneous rocks
  • Magma solidifies within the crust, cooling slowly. Coarse-grained textures (e.g., granite).
Sedimentary Rocks
  • Made of sediments of weathered rocks, shells, or remains of living things.
  • Sediments are transported by water, wind, or ice and deposited in layers.
  • Compacted or cemented together.
Types of Sedimentary Rocks
  • Clastic, chemical, bioclastic, and organic sedimentary rocks.
Clastic sedimentary rocks
  • Composed of fragments of weathered rocks (clasts), transported, deposited, and cemented.
  • Make up about 85% of all sedimentary rocks (e.g., sandstone, siltstone, and shale).
Chemical sedimentary rocks
  • Formed by direct precipitation from minerals in water (e.g., gypsum, halite (rock/table salt), and potash).
Bioclastic sedimentary rocks
  • Composed of broken shell fragments and remains of living organisms (e.g., limestone and chalk).
Organic sedimentary rocks
  • Consist of solidified remains of plants or animals (e.g., coal).
Metamorphic Rocks
  • Result from changes in other kinds of rock, through heat and pressure inside the earth’s crust.
  • Igneous, sedimentary, and even other metamorphic rocks can be transformed.
  • Limestone and sandstone change to marble and meta-quartzite, respectively.
  • Mudstone and shale change to schist or quartzite, respectively, under heat and pressure.
  • They change to slate when subjected only to pressure.

Distribution of Major Rocks in Ethiopia

  • Igneous, sedimentary, and metamorphic rocks are found exposed at the surface.
  • Igneous rocks cover large areas in the central part of the country.
  • Eastern Ethiopia is mainly covered with sedimentary rocks.
  • Metamorphic rocks are found in the northern, western, and southern edges of the country.

Soil

Composition

  • Complex mixture of inorganic minerals, decaying organic matter, water, air, and living organisms.
  • Inorganic materials (minerals): 45%.
  • Air: 25%.
  • Water: 25%.
  • Organic material: 5% (humus, roots, and other living organisms).

Formation

  • Takes a long time (hundreds or thousands of years).
  • Basic components: inorganic (rock particles, air, water, and mineral salts) and organic (humus and living organisms).

Types

  • Vary in their content of clay, silt, and sand.
  • Porosity controls the amount of water and air the soil can hold.
Sandy soils
  • Large pores, little tendency to clump together.
  • Good aeration, easy to work.
  • Retain little water, dry up quickly, poor in plant nutrients.
Clay soils
  • Small particles, easily packed together.
  • Low porosity, retain water, rich in nutrients.
  • Poorly drained, poorly aerated, difficult to cultivate, prone to waterlogging.
Silt soils
  • Particles smaller than sand.Less gritty, somewhat sticky.
  • Accepts water moreslowly than sandy soil, but more quickly than clay. It also retains water longer than sandy soil, but it dries faster than clay.
Loam
  • Ideal soil, mixture of sand, clay, and silt with plenty of humus.
  • Fertile, properly aerated, contains adequate mineral and water.

Major Soil Types of Ethiopia

  • FAO has identified about 18 major soil associations.
  • Ten of the soil associations cover more than 87.4% of the land area.

Leptosols

  • Cover about 29.8% of the total land area.
  • Developed on recent lava and young quaternary sediments.
  • Mostly found in a rugged topography and steep slopes.

Nitosols

  • Cover about 12.5% of the area.
  • Develop on gently sloping ground.
  • Associated with high rainfall and, originally, forest-covered areas.
  • Now widely found on cultivated areas and mountain grasslands.

Vertisols

  • Cover about 10% of the land area.
  • Mostly develop on volcanic plateau.
  • Develop on sedimentary rocks and alluvial plains.
  • Occur in moderate climatic regions and gently slopping grounds.

Luvisols

  • Cover about 7.8% of Ethiopia.
  • Developed mainly in areas where pronounced wet and dry seasons occur.

Fluvisols

  • Formed through erosion from the lava plateau and deposited in depressions, lower valleys, and lowlands.
  • Associated with fluvial (river), marine (sea), and lacustrine (lake) deposits.

Soil Degradation in Ethiopia

Soil Degradation

  • Change in the state of soil due to increased erosion and leaching.
Erosion
  • Wearing away of soil by natural agents (running water, wind, ice, wave action, and corrosion).
  • Human activities frequently aggravate this process.
Causes of Soil Erosion in Ethiopia
  • Physical causes: steepness of slope, rainfall intensity, duration, and seasonality, soil type, and vegetation cover.
  • Human causes: deforestation, overgrazing, and bad farming practices.

Types of Soil Erosion

Sheet erosion
  • Uniform removal of soil from every part of a slope.
Rill erosion
  • Small depressions develop into shallow channels (rills).
Gully erosion
  • Channels enlarge, causing deep cuts in the ground.
Stream Erosion
  • Rivers cut the surface of the earth laterally and vertically and transport eroded materials.
Wind erosion
  • Most common in dry and treeless areas where the soil is exposed.

Soil Conservation in Ethiopia

  • Protection of resources from destructive influences.
Soil Conservation Measures
  • Contour ploughing
  • Terracing
  • Strip cultivation
  • Windbreaks
  • Afforestation and reforestation
  • Avoiding cultivation of unsuitable land
  • Plugging gullies
  • Limiting grazing
  • Fallowing

Climate

Classification of the Climates of the World

Classification by Ancient Greeks
  • Divided the world into three climatic regions based on temperature and sunshine distribution.
  • Tropical (torrid) zone: winterless region between 23½°N and 23½°S.
  • Middle-latitude temperate zone: between 23½° and 66½°N and S, distinct summer and winter seasons.
  • High-latitude polar (frigid) zone: above 66½°N and S, cold all year round.
  • Simplistic method as it does not consider precipitation.
Köppen System of Climate Classification
  • Based on seasonal variations in temperature and precipitation.
  • Defines five principal climatic groups.
  • Tropical moist climate (A): All months have an average temperature above 18ºC.
  • Dry climates (B): Precipitation is deficient most of the year.
  • Moist mid-latitude climates with mild winters (C): Warm-to-hot summers with mild winters.
  • Moist mid-latitude climates with severe winters (D): Distinct summer and winter seasons.
  • Polar climates (E): Extremely cold winters and summers.
  • Highland climate (H): Produce tundra and polar conditions due to altitude.

Climate Change

  • Long-term change in weather patterns.
  • May be limited to a region or occur across the whole Earth.
Causes of Climate Change
Natural causes
  • Continental drift
  • Volcanoes
  • Ocean currents
Human Causes
  • Industrial revolution leading to the large-scale use of fossil fuels.
  • Deforestation
  • Population growth
Greenhouse gases and their sources
  • Carbon dioxide (CO2): from land use pattern changes, deforestation, land clearing, agriculture, and other activities
  • Methane (CH4): from domesticated animals and rice fields.
  • Nitrous oxide (N2O): from fertilizer application.
Greenhouse effect
  • Thermal radiation from the planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions towards the surface.
  • As a result, the temperature there is higher than it would be if direct heating by solar radiation were the only warming mechanism.
Consequences of climate change
Global warming
  • Increase in the average temperature of Earth’s near-surface air and oceans.
Desertification
  • Desert-like conditions may spread over human habitats, crop lands and wet lands.
Drought
  • Likely to occur more frequently.
Rise in the sea-level
Shift of the direction of global winds
Shift of Tropical Zone
Expansion of Tropical Diseases
Loss of Biodiversity

Climate of Ethiopia

Temperature Distribution

  • Determined primarily by altitude and latitude.
  • Highlands have non-tropical temperatures.
  • Tropical climate occurs in lowlands.
  • Temperature decreases from the peripheries towards the interior.
  • Daily maximum temperature varies from >37°C in lowlands to 10-15°C in highlands.
  • March, April, and May are generally the hottest months.
  • Mean annual temperature varies from 10°C in highlands to 35°C at the country’s northeastern edges.

Distribution of Rainfall

  • Mean annual rainfall ranges from >2200 mm in southwestern highlands to <400 mm in the northeast and southeastern lowlands.
  • Southwestern Ethiopia is the wettest part of the country.

Seasonal Pattern of Rainfall

  • Summer Rainfall (Keremt)
    *Rainfall in Ethiopia is seasonal, varying in amount over space and time. There is the long and heavy summer rain, which is normally referredto as “the big rain” or keremt. There are also short and moderate rains in autumn (Tebi), winter (Bega) and spring (Belg). They are collectivelycalled “the little rains”.

  • Winter Rainfall
    In winter the ITCZ shifts farthest south. Most of Ethiopia comes under theinfluence of North East Trade winds, which originate from west Asian highpressure centers.