Class 11 Geography Chapter 2 Notes | The Origin and Evolution of the Earth Important topics
This article will provide you with the Class 11 Geography Chapter 2 Notes. Geography is a very important topic for students in class 11. They need to study it properly. Make sure to complete your studies before exams, so that you can get good grades. I also used to do most of the study before the exam.
Chapter 2: The Origin and Evolution of the Earth
Class 11 Geography Chapter 2 Notes
- The Earth is around 4.6 billion years old.
- To understand its origin and evolution, we need to study:
- Formation of Universe
- Formation of Stars & Solar System
- Formation of Earth’s layers
- Evolution of Atmosphere & Hydrosphere
- Origin of Life
Early Theories on Origin of Earth
Nebular Hypothesis (Immanuel Kant, revised by Laplace – 1796)
- Sun and planets formed from a rotating cloud of gas and dust (solar nebula).
- Material condensed due to rotation → planets formed.
Revised Nebular Hypothesis (Otto Schmidt – 1950, Carl Weizascar)
- Sun was surrounded by solar nebula (mainly hydrogen, helium, dust).
- Friction & collisions made the nebula flatten into a disk-shaped cloud.
- Planets formed through accretion (gradual sticking of small particles).
Modern Theories
Origin of the Universe – Big Bang Theory
- Proposed by Edwin Hubble (1920s) → Universe is expanding.
- Universe started from a tiny, hot, dense point (Singularity).
- About 13.7 billion years ago, a massive explosion occurred.
Stages of Big Bang:
- Initially – all matter compressed in a tiny ball, infinite density & temperature.
- At Big Bang → violent explosion → rapid expansion.
- Within 3 minutes → first atoms formed.
- Within 300,000 years → temperature dropped → matter condensed → universe became transparent.
- Expansion continues till today.
Alternative Theory:
- Steady State Hypothesis (Hoyle) → Universe is eternal, always same → rejected due to stronger proof of expansion.
Formation of Stars
- Uneven distribution of matter → gravity pulled matter → galaxies formed.
- Galaxies contain billions of stars.
- Stars formed when hydrogen gas in nebula clumped under gravity → formed dense hot cores → nuclear fusion began.
- Star formation: 5–6 billion years ago.
Light Year: Distance light travels in one year = 9.461 × 10¹² km.
Formation of Solar System & Planets
Stages:
- Stars = lumps of gas in nebula.
- Gas cloud condensed → huge rotating disc around star core.
- Small rounded bodies formed = Planetesimals.
- Planetesimals collided & stuck → protoplanets.
- Some grew larger → became planets.
Our Solar System formed ~4.6 billion years ago.
Evolution of the Earth
- Early Earth → hot, barren, rocky, thin atmosphere of H & He.
- Gradual changes transformed it into present Earth.
Differentiation Process
- Heavy elements (Iron, Nickel) sank → Core.
- Lighter elements (Silica, Aluminium) rose → Crust.
- Middle → Mantle.
- Earth cooled → formed layered structure:
- Crust → Mantle → Outer Core → Inner Core.
Evolution of Lithosphere
- Initially Earth was molten.
- Cooling → solidification → crust formed.
- Collision with a large body → formation of Moon → reheated Earth.
- Gradual differentiation created layers of different densities.
Evolution of Atmosphere
Stages:
- Primordial Atmosphere → Mostly Hydrogen & Helium → blown away by solar winds.
- Degassing Stage → Volcanic eruptions released water vapour, CO₂, methane, ammonia, nitrogen → formed secondary atmosphere.
- Biological Stage → Oxygen introduced by photosynthesis (blue-green algae, ~2.5–3 billion years ago).
Evolution of Hydrosphere
- Cooling of Earth → water vapour condensed → heavy rains.
- Rainwater accumulated in depressions → oceans formed (~4 billion years ago).
- CO₂ dissolved in rainwater → reduced greenhouse effect → more cooling.
Origin of Life
- Early Earth → not suitable for life.
- Life began ~3.8 billion years ago in oceans.
- Origin explained as chemical evolution:
- Simple inorganic compounds → organic molecules → complex molecules → self-replicating structures → life.
- Earliest evidence = blue-green algae fossils (~3 billion years old).
Table Summary
| Stage | Time (Years Ago) | Key Features |
|---|---|---|
| Big Bang | 13.7 billion | Universe formed, expansion started |
| Solar System Formation | 4.6 billion | Sun + planets (by accretion) |
| Earth Formation | 4.6 billion | Hot, barren, thin atmosphere |
| Oceans formed | 4.0 billion | Water condensed, oceans collected |
| Life origin | 3.8 billion | First life in oceans |
| Oxygen Atmosphere | 2.5–3.0 billion | Photosynthesis added free O₂ |
Key Points to Remember
- Earth Age: ~4.6 billion years.
- Universe Age: ~13.7 billion years.
- Big Bang Theory: Expansion started with singularity explosion.
- Earth’s Layers formed by Differentiation.
- Atmosphere evolved in 3 stages: primordial, degassing, photosynthesis.
- Oceans formed within 500 million years of Earth’s origin.
- Life originated ~3.8 billion years ago in oceans.
- Photosynthesis added oxygen → atmosphere changed.
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