Book: Fundamentals of Physical Geography | Chapter 2
The Origin and Evolution of the Earth
1. Learning Objectives
After reading these notes, you will be able to:
1
Explain the early and modern theories about origin of the earth and universe.2
Describe the Big Bang Theory and the stages in formation of stars and planets.3
Understand how the Earth evolved its layered structure (lithosphere, atmosphere, hydrosphere).4
Know the timeline of major events — from Big Bang to origin of life on Earth.5
Define key terms: nebula, planetesimals, accretion, degassing, differentiation, photosynthesis.2. Early Theories — Origin of the Earth
Many philosophers and scientists gave hypotheses about the origin of the earth. The most well-known early theory was the Nebular Hypothesis.
- Original argument by Immanuel Kant (German philosopher), later revised by mathematician Laplace in 1796 → called the Nebular Hypothesis.
- Nebular Hypothesis: Planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.
- In 1950, Otto Schmidt (Russia) and Carl Weizascar (Germany) revised the nebular hypothesis — they considered that the sun was surrounded by a solar nebula containing mostly hydrogen, helium and dust.
- The friction and collision of particles led to a disk-shaped cloud, and planets formed through the process of accretion.
📌 Key Term — Accretion
Accretion means the gradual growth of planets by accumulation and collision of smaller particles (planetesimals) sticking together due to gravitational force.3. Modern Theory — The Big Bang Theory
The most popular argument about the origin of the universe is the Big Bang Theory, also called the Expanding Universe Hypothesis. Edwin Hubble (1920) provided evidence that the universe is expanding — galaxies keep moving further and further apart.
Big Bang Theory
Also called: Expanding Universe Hypothesis
Edwin Hubble
Provided evidence for expanding universe (1920)
13.7 Billion yrs
Age of the universe (when Big Bang occurred)
4,500 K
Temperature after 300,000 yrs → atomic matter formed
💥 Stages of Big Bang Theory
i
The Tiny Ball (Singularity)
In the beginning, all matter existed in one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density.ii
The Explosion & Expansion
The tiny ball exploded violently — this is the Big Bang. It led to a huge expansion. The event occurred 13.7 billion years ago. Expansion continues even today. Within the first 3 minutes, the first atom began to form. Some energy was converted into matter.iii
Cooling and Atomic Matter
Within 300,000 years from the Big Bang, temperature dropped to 4,500 K (Kelvin), giving rise to atomic matter. The universe became transparent.🔭 Hoyle’s Steady State Theory — The Alternative
Hoyle’s concept of Steady State considered the universe to be roughly the same at any point of time — it did NOT expand. However, with greater evidence for the expanding universe, the scientific community now favours the Big Bang / Expanding Universe argument.💡 Light Year — Important Fact
A light year is a measure of DISTANCE, not time. Light travels at 300,000 km/second. Distance light travels in 1 year = 9.461 × 10¹² km. The mean distance between the Sun and Earth = 149,598,000 km = 8.311 light minutes.4. Formation of Stars and Planets
⭐ How Stars Formed
- Uneven distribution of matter and energy in the early universe caused differences in gravitational forces.
- Matter got drawn together → formed galaxies. A galaxy contains a large number of stars.
- Galaxies spread over vast distances measured in thousands of light years. Diameter of individual galaxies = 80,000–1,50,000 light years.
- A galaxy forms by accumulation of hydrogen gas in a very large cloud called nebula.
- Growing nebula develops localised clumps of gas → these grow into denser gaseous bodies → stars.
- Star formation is believed to have taken place some 5–6 billion years ago.
🪐 How Planets Formed (3 Stages)
i
Nebula → Rotating Disc
Stars are localised lumps of gas within a nebula. Gravitational force within the lumps leads to formation of a core. A huge rotating disc of gas and dust develops around this gas core.ii
Gas Cloud → Planetesimals
Gas cloud condenses → matter around core develops into small rounded objects → by the process of cohesion, these form planetesimals. Planetesimals are a large number of smaller bodies. Larger bodies form by collision and gravitational attraction.iii
Planetesimals → Planets
Large number of small planetesimals accrete to form fewer, large bodies in the form of planets. This is the process of accretion.🗺️ Mind Map — From Nebula to Planet
Nebula (Hydrogen Gas Cloud)⭐ Clumps of Gas
Localised clumps form
Localised clumps form
🌀 Rotating Disc
Gas core + dust disc
Gas core + dust disc
🪨 Planetesimals
Cohesion of rounded objects
Cohesion of rounded objects
💥 Collision
Gravitational attraction
Gravitational attraction
🌍 Planets
Accretion of planetesimals
Accretion of planetesimals
5. Evolution of the Earth
Initially, the earth was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium — very different from today. Between 4,600 million years ago and the present, several events led to the evolution of life on Earth.
🏔️ Evolution of Lithosphere (Layered Structure)
- Earth was mostly in a volatile state during its primordial (earliest) stage.
- Gradual increase in density → temperature inside increased.
- Material started separating based on density → this process is called Differentiation.
- Heavier materials (like iron) sank towards the centre; lighter materials moved to the surface.
- Earth cooled, solidified and condensed → outer surface developed as crust.
- During formation of the moon, a giant impact further heated up the earth.
- Final layered structure (surface → centre): Crust → Mantle → Outer Core → Inner Core. Density increases from crust to core.
🔑 Key Term — Differentiation
Differentiation is the process by which the earth-forming material got separated into different layers based on their density. Heavier materials sank (iron → core), lighter materials rose (silicates → crust).Crust
Outermost layer — lightest material
Mantle
Below crust — denser than crust
Outer Core
Liquid layer — denser than mantle
Inner Core
Densest — mainly iron; solid
🌤️ Evolution of Atmosphere — 3 Stages
1
Loss of Primordial Atmosphere
The early atmosphere of hydrogen and helium was stripped away by solar winds. This happened to all terrestrial planets.2
Hot Interior Contributes New Atmosphere
During cooling of earth, gases and water vapour were released from the interior — called degassing. Continuous volcanic eruptions contributed water vapour, nitrogen, CO₂, methane, ammonia, and very little free oxygen.3
Living World Modifies Atmosphere
Composition of atmosphere was modified by the living world through the process of photosynthesis. Oceans began contributing oxygen → 2,000 million years ago, oxygen began to flood the atmosphere.💧 Evolution of Hydrosphere (Oceans)
As earth cooled → water vapour condensed → CO₂ dissolved in rainwater → temperature decreased further → more rain. Rainwater collected in depressions → oceans formed within 500 million years of earth’s formation → oceans are 4,000 million years old.Around 2,500–3,000 million years ago → process of photosynthesis evolved → oceans saturated with oxygen → 2,000 million years ago → oxygen flooded the atmosphere.
6. Origin of Life
The last phase in the evolution of the earth relates to the origin and evolution of life. Initially, neither the earth nor its atmosphere was conducive for life.
- Modern scientists refer to origin of life as a kind of chemical reaction — it first generated complex organic molecules and assembled them.
- This assemblage could duplicate itself — converting inanimate matter into living substance.
- Records of life in different periods are found in rocks in the form of fossils.
- Microscopic structures closely related to blue algae have been found in geological formations older than 3,000 million years.
- Life began to evolve sometime 3,800 million years ago.
🌊 Life was Confined to Oceans First
For a long time, life existed only in oceans. Through photosynthesis, oceans gained oxygen → oceans became saturated → oxygen then entered the atmosphere, eventually allowing life to evolve on land.7. Complete Timeline — From Big Bang to Life
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13.7 Billion Years Ago
Big Bang — tiny ball exploded. Rapid expansion of universe began.2
Within 3 Minutes of Big Bang
First atoms began to form.3
300,000 Years After Big Bang
Temperature dropped to 4,500 K → atomic matter formed → universe became transparent.4
5–6 Billion Years Ago
Stars formed from nebulae. Our Sun formed.5
4,600 Million (4.6 Billion) Years Ago
Earth formed — barren, rocky, hot, with thin H₂ + He atmosphere.6
4,000 Million Years Ago
Oceans formed (within 500 million years of Earth’s formation). Degassing → atmosphere evolved.7
3,800 Million Years Ago
Life began to evolve — complex organic molecules assembled. Earliest life forms appeared.8
2,500–3,000 Million Years Ago
Process of photosynthesis evolved. Life confined to oceans. Oceans began gaining oxygen.9
2,000 Million Years Ago
Oceans saturated with oxygen → oxygen began to flood the atmosphere. Conditions for land life developing.Summary — Quick Revision
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Nebular Hypothesis — by Kant, revised by Laplace (1796). Planets formed from a cloud of material around a rotating sun. Accretion process.
2
Big Bang Theory = Expanding Universe Hypothesis. Edwin Hubble (1920) proved universe is expanding. Big Bang occurred 13.7 billion years ago.
3
Big Bang Stages: Tiny ball (singularity) → explosion → 1st atom in 3 minutes → atomic matter at 300,000 yrs when temp = 4,500 K.
4
Stars formed 5–6 billion years ago from nebula (hydrogen gas cloud) → clumps → denser bodies → stars.
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Planet formation: Nebula → rotating disc → planetesimals (cohesion) → planets (accretion). Earth formed 4,600 million years ago.
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Differentiation = separation of earth’s material by density → Crust (lightest) → Mantle → Outer Core → Inner Core (densest/iron).
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Atmosphere evolved in 3 stages: Solar winds stripped H₂+He → Degassing (volcanic eruptions) → Photosynthesis added oxygen.
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Degassing = gases outpoured from earth’s interior through volcanic eruptions → early atmosphere had N₂, CO₂, CH₄, NH₃, H₂O vapour, very little O₂.
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Oceans formed within 500 million years of Earth’s formation → 4,000 million years old. Life began 3,800 million years ago.
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Photosynthesis evolved 2,500–3,000 million years ago → oceans saturated with O₂ → 2,000 million years ago oxygen flooded atmosphere.
Important Terms to Remember
- Nebular Hypothesis: Early theory by Kant and Laplace — planets formed from a cloud of material (nebula) associated with the rotating sun.
- Nebula: A very large cloud of hydrogen gas in space from which stars and galaxies form.
- Accretion: Process by which planets formed — small planetesimals collided and stuck together (due to gravity) to form large planetary bodies.
- Planetesimals: Small rounded objects formed during the early stages of planet formation through cohesion of gas and dust particles.
- Big Bang Theory: Most accepted theory for origin of the universe — a tiny ball of infinite density exploded 13.7 billion years ago, leading to the expanding universe.
- Expanding Universe: Galaxies are continuously moving farther apart — the space between them is increasing. Proved by Edwin Hubble (1920).
- Steady State Theory: Hoyle’s alternative — universe remains the same at all times. Now rejected in favour of Big Bang.
- Light Year: A unit of DISTANCE (not time). Distance light travels in one year = 9.461 × 10¹² km.
- Differentiation: Process by which Earth’s interior separated into layers based on density — heavy material (iron) sank to core, lighter material (silicates) rose to surface.
- Degassing: Process by which gases were outpoured from Earth’s hot interior through volcanic eruptions, contributing to the evolution of the atmosphere.
- Photosynthesis: Process by which plants/algae use sunlight + CO₂ + water to produce food and oxygen — responsible for adding oxygen to Earth’s atmosphere.
- Primordial Atmosphere: Earth’s earliest atmosphere of hydrogen and helium — stripped away by solar winds.
- Fossil: Preserved record of past life forms found in rocks — evidence of the history of life on Earth.
- Solar Winds: Stream of charged particles from the sun that stripped away the early (primordial) atmospheres of terrestrial planets.
- Galaxy: A massive system of stars, gas, and dust held together by gravity. Diameters range from 80,000–1,50,000 light years.