Chapter 2: Notes
1. Origin of Earth
- Early Theory: Nebular Hypothesis (Kant & Laplace). Planets formed from a rotating cloud (Nebula) around the sun.
- Modern Theory: Big Bang Theory (Edwin Hubble, 1920).
-> Universe is expanding (Galaxies moving apart).
-> Occurred 13.7 Billion years ago.
2. Evolution of Earth (Layers)
A. Lithosphere (Land)
Earth was initially volatile & hot. Through Differentiation, material separated by density.
Heavy (Iron) -> Core (Sank)
Light (Silica) -> Crust (Floated)
B. Atmosphere (Air)
Formed in 3 Stages:
- Loss of Primordial Atmosphere (H & He stripped by Solar Winds).
- Degassing: Hot interior released water vapor, CO2, NH3. (Created Oceans).
- Modification: Photosynthesis by early life added Oxygen.
C. Origin of Life
Life began 3.8 Billion years ago (Chemical evolution -> Organic Molecules).
NCERT Solutions
Q1. Multiple Choice Questions
(i) Which figure represents the age of the earth?
Ans: (c) 4.6 billion years
(ii) Which is NOT related to atmosphere formation?
Ans: (b) Differentiation (It is related to Lithosphere).
(iii) Life appeared around?
Ans: (d) 3.8 billion years (before present).
Q2. Short Answers (30 words)
(i) Process of Differentiation?
It is the process where earth forming materials got separated based on density. Heavier materials (like iron) sank to the centre, while lighter materials moved to the surface.
(ii) Nature of Earth initially?
Initially, the earth was a barren, rocky, and hot object with a thin atmosphere of hydrogen and helium. It was very different from the present day.
(iii) Gases in initial atmosphere?
The early atmosphere largely contained Hydrogen and Helium. Later, degassing added water vapour, nitrogen, carbon dioxide, methane, and ammonia.
Long Answers & Project
Q3. Long Answers (150 words)
(i) Big Bang Theory
Also known as the Expanding Universe Hypothesis. Proposed by Edwin Hubble in 1920.
Stages:
1. Originally, all matter was in a "tiny ball" (Singularity) with infinite temperature & density.
2. Around 13.7 billion years ago, it exploded violently (The Bang).
3. This led to huge expansion which continues today. Energy converted to matter, forming the first atoms.
(ii) Stages in Evolution of Earth
1. Planetesimals: Gas cloud condensed into small bodies.
2. Accretion: Small bodies collided to form Earth. It was hot & barren.
3. Differentiation: Earth melted; heavy layers sank (Core), light layers floated (Crust).
4. Atmosphere: Solar winds removed H/He. Volcanoes released gases (Degassing). Rain formed Oceans.
5. Life: Life evolved 3800 mya and added Oxygen via photosynthesis.
Project Work: "Stardust"
(i) Agency: NASA (National Aeronautics and Space Administration).
(ii) Why Collect Stardust? To study the chemical makeup of the early solar system and understand how stars/planets formed.
(iii) Collected From? It was collected from the coma (tail) of Comet Wild 2.