Chapter 2: The Origin & Evolution of the Earth
1. Early Theories (Origin of Earth)
- Nebular Hypothesis: Proposed by Immanuel Kant, revised by Laplace (1796).
- Planets formed from a rotating cloud of material (nebula) around a youthful sun.
- Later revised by Schmidt (Russia) & Weizascar (Germany).
2. Modern Theories (Origin of Universe)
Big Bang Theory:
Also called "Expanding Universe Hypothesis".
Proposed by Edwin Hubble (1920).
Timeline: Occurred 13.7 Billion years ago.
Balloon Experiment: Mark points (galaxies) on a balloon. Inflate it -> Points move away from each other = Expanding Universe.
3. Star Formation
- Galaxies contain large clouds of H2 gas called Nebula.
- Clumps of gas condense -> form Stars (5-6 Billion yrs ago).
Light Year:
It is a measure of DISTANCE, not time!
Speed of light = 300,000 km/s.
4. Formation of Planets (Stages)
- Gas lumps -> Gravitational force -> Core forms.
- Condensation -> Planetesimals (small rounded objects).
- Accretion (sticking together) -> Large Planets.
Evolution of the Earth
A. Lithosphere (Layered Structure)
Earth was volatile & hot. Through Differentiation, heavy materials (Iron) sank to center, lighter ones moved up.
Result: Crust, Mantle, Core.
B. Atmosphere & Hydrosphere
Three stages of evolution:
- 1. Loss of primordial atmosphere (Solar winds).
- 2. Degassing: Hot interior released gases (H2O, CO2, N2) -> Rains -> Oceans.
- 3. Modification by life (Photosynthesis).
C. Origin of Life
Life began ~3,800 Million years ago.
Blue Algae fossils found (3000 mya).