📘 CHAPTER 2 — THE EARTH

1. THE EARTH – INTRODUCTION

• The Earth is the only known planet where life exists.
• Its unique combination of atmosphere, water, landforms, and temperature makes it habitable.
• Understanding the Earth involves exploring its origin, internal structure, evolution of land, water, air, and life.
• The Earth has undergone continuous physical and biological changes since its formation around 4.6 billion years ago.
• Scientists use various methods such as geological records, fossils, radiometric dating, and cosmological theories to understand its past.
• This chapter explains how Earth originated, how planets were formed, and how the Earth’s spheres—atmosphere, hydrosphere, lithosphere, and biosphere—evolved.

2. ORIGIN OF THE EARTH

2.1 Meaning of Origin

• The origin of the Earth refers to how the Earth was formed in the universe.
• It includes the formation of the Solar System and the separation of Earth from the Sun or the nebular clouds.
• Many theories have been proposed over time:
  • Early religious myths
  • Scientific theories based on physics, astronomy, and chemistry

2.2 The Big Bang Theory (General Cosmic Origin)

• The most accepted theory of the universe’s beginning.
• Universe began 13.7 billion years ago from a massive explosion known as the Big Bang.
• After the explosion:
  • Matter expanded outward
  • Temperature dropped
  • Atoms and galaxies began forming
• Solar System formed much later, nearly 4.6 billion years ago.

2.3 Theories Explaining Earth’s Formation

There are two major categories:

A. Early Theories

2.3.1 Nebular Hypothesis (Immanuel Kant & Laplace)

• Sun and planets formed from gaseous nebulae.
• A rotating gas cloud condensed due to gravitational collapse.
• As the cloud contracted, it began spinning faster and flattened into a disk.
• Rings of gas detached and condensed to form planets.
• This was the earliest scientific explanation but later revised.

2.3.2 Tidal Theory (Chamberlain & Moulton)

• A passing star came close to the Sun.
• This caused a massive tidal pull, drawing out gases from the Sun.
• These gases later condensed into planets.
• Rejected due to lack of evidence of such close star encounters.

B. Modern Theories

2.3.3 Modern Nebular Theory

• Supported by modern astrophysics.
• Solar system formed from a rotating nebular cloud of gas and dust (solar nebula).
• Steps:
  1. Solar nebula began collapsing under gravity.
  2. It spun faster and flattened into a disk.
  3. The Sun formed at the center.
  4. Dust particles stuck together, forming planetesimals.
  5. Planetesimals merged to form planets.
• Earth formed as a rocky (terrestrial) planet from heavy elements.

2.3.4 Planetesimal Hypothesis

• George Buffon and later scientists developed this concept.
• Solid particles stuck due to electrostatic and gravitational attraction.
• Gradually became large bodies → planetesimals → planets.

3. FORMATION OF PLANETS

3.1 Solar System Formation

• The solar system originated from a rotating gas-dust cloud called the solar nebula.
• Gravity led to condensation of heavier materials towards the center forming the Sun.
• Lighter materials moved outward and formed the outer planets.

3.2 Planet Formation Stages

Stage 1: Accretion

• Dust particles collided and stuck together.
• Formed small, irregular bodies called planetesimals.

Stage 2: Differentiation

• As the planetesimals grew larger:
  • Heat increased (due to radioactive decay and collisions).
  • Heavy materials sank inward.
  • Lighter materials rose toward the surface.

Stage 3: Formation of Terrestrial and Jovian Planets

• Terrestrial planets (Mercury, Venus, Earth, Mars):
  • Formed from heavier elements (iron, silicates).
  • Dense and rocky.
• Jovian planets (Jupiter, Saturn, Uranus, Neptune):
  • Contain hydrogen, helium, and light gases.
  • Low density, large size.

3.3 Earth’s Early State

• Hot molten mass.
• Frequent meteorite bombardment.
• No water, no atmosphere, no life.
• Gradual cooling led to crust formation.

4. EVOLUTION OF ATMOSPHERE AND HYDROSPHERE

4.1 Evolution of Atmosphere

Stage 1 — Early Atmosphere (Primary Atmosphere)

• Formed from solar nebula.
• Mainly hydrogen and helium.
• Lost quickly because:
  • Earth had low gravity
  • Strong solar wind blew gases away

Stage 2 — Secondary Atmosphere

• Gases came from volcanic eruptions (outgassing).
• Rich in:
  • Water vapour
  • Carbon dioxide
  • Nitrogen
  • Methane
  • Ammonia
• No free oxygen at this stage.

Stage 3 — Modern Atmosphere

• As Earth cooled:
  • Water vapour condensed → rain → oceans formed
• Plants released oxygen through photosynthesis.
• Oxygen accumulated gradually, forming the ozone layer.
• Ozone protected Earth from harmful UV radiation.

4.2 Evolution of Hydrosphere

4.2.1 Origin of Water

• Water vapour released through volcanic outgassing.
• As Earth cooled, water condensed and fell as continuous rain.
• Rainwater accumulated in low-lying areas → oceans.

4.2.2 Stabilisation of Hydrosphere

• Oceans formed nearly 4 billion years ago.
• Water cycle (evaporation-condensation-rainfall) established.
• Oceans played a major role in:
  • Supporting life
  • Regulating temperature
  • Weather and climate formation

5. EVOLUTION OF LITHOSPHERE

5.1 Cooling of the Earth

• Initially Earth was molten.
• Outer surface cooled faster, forming the early crust.
• Heavy elements like iron and nickel sank → core formation.
• Silicates formed the mantle and crust.

5.2 Major Steps in Lithosphere Development

Step 1 — Differentiation

• Heavy materials → core
• Lighter materials → crust

Step 2 — Crust Formation

• Initially thin and unstable.
• Meteorite impacts frequently broke it.
• Gradually crust thickened.

Step 3 — Formation of Continents and Ocean Basins

• Light materials (sial) formed continents.
• Dense materials (sima) formed ocean floors.

Step 4 — Plate Tectonics Begins

• Heat inside the Earth caused movement of plates.
• Continents drifted and mountains formed.

6. ORIGIN OF LIFE

6.1 Conditions Needed

Life required:

• Liquid water
• Suitable temperature
• Protection from radiation (ozone layer)
• Chemical elements like carbon, hydrogen, nitrogen, oxygen

6.2 Theories of Life Origin

6.2.1 Special Creation Theory

• Life was created by divine forces.
• Not scientific.

6.2.2 Spontaneous Generation

• Life emerged from non-living matter.
• Disproved scientifically.

6.2.3 Biogenesis

• Life originates from pre-existing life.
• Does not explain first life.

6.2.4 Modern Scientific Theory – Oparin & Haldane

• Early oceans contained chemical ‘soup’ (amino acids).
• Energy from lightning, UV radiation helped form simple molecules.
• Simple molecules evolved into complex organic compounds.
• Eventually led to formation of first cells.

6.3 First Life Forms

• Emerged around 3.5 billion years ago.
• Simple single-celled organisms.
• Anaerobic (no oxygen).
• Over time:
  • Photosynthetic organisms developed
  • Released oxygen
  • Enabled evolution of aerobic organisms

7. EVOLUTION OF THE EARTH

7.1 Geological Time Scale

Earth’s history divided into:

• Eons → largest span
• Eras
• Periods
• Epochs

7.2 Major Stages

Stage 1 — Hadean Eon

• 4.6 to 4 billion years ago
• Earth molten, unstable
• Formation of crust, oceans

Stage 2 — Archean Eon

• 4 to 2.5 billion years ago
• First life appeared
• Primitive atmosphere

Stage 3 — Proterozoic Eon

• 2.5 billion to 540 million years ago
• Oxygen increases
• Ozone layer forms
• Simple multicellular organisms appear

Stage 4 — Phanerozoic Eon

• Present eon
• Complex plants and animals
• Major events:
  • Cambrian explosion
  • Evolution of fish, amphibians, reptiles
  • Dinosaurs dominate → become extinct
  • Evolution of mammals and humans

8. SUMMARY OF EARTH’S EVOLUTION

8.1 Key Processes

• Accretion of materials → planet formation
• Differentiation → core, mantle, crust
• Outgassing → atmosphere
• Cooling → hydrosphere
• Chemical evolution → origin of life
• Biological evolution → complex life
• Geological evolution → continents, mountains, oceans

9. CONCLUSION

• The Earth’s origin and evolution is a continuous process shaped by cosmic events, geological changes, atmospheric development, and biological evolution.
• From a hot molten sphere, Earth transformed into a planet rich with water, air, landforms, and life.
• The atmosphere and hydrosphere developed through volcanic outgassing and condensation.
• Lithosphere evolved through cooling, tectonic activity, and formation of continents and oceans.
• Life originated from chemical evolution and gradually diversified.
• Earth continues to change, and understanding its past helps predict its future.