Chapter – 2

The Origin and Evolution of the Earth

In this post we have given the detailed notes of class 11 Geography Book 1 Chapter 2 (The Origin and Evolution of the Earth) in English. These notes are useful for the students who are going to appear in class 11 board exams.

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Chapter 2: The Origin and Evolution of the Earth

Introduction

• This chapter explores the formation of stars, the origin of the Earth, and its subsequent evolution.  

Early Theories on the Origin of the Earth

• Nebular Hypothesis:
  • Proposed by German philosopher Immanuel Kant and later revised by mathematician Laplace in 1796.  
  • It posited that planets formed from a cloud of material associated with a young, slowly rotating sun.  
• Revised Nebular Hypothesis:
  • Revised by Otto Schmidt (Russia) and Carl Weizascar (Germany) in 1950, with some differences in details.  
  • They proposed that the sun was surrounded by a solar nebula composed mainly of hydrogen, helium, and dust.  
  • Friction and collision of particles within the nebula led to the formation of a disk-shaped cloud.  
  • Planets formed through the process of accretion, where particles gradually clumped together due to gravity.  

Modern Theories on the Origin of the Universe

Big Bang Theory:

• Also known as the expanding universe hypothesis.  
• It gained support from evidence provided by Edwin Hubble in 1920, showing that the universe is expanding.  
• Key Stages:
  • Initial State: All matter forming the universe existed in one place as a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature, and infinite density.  
  • The Big Bang: This “tiny ball” exploded violently 13.7 billion years ago, leading to a massive expansion that continues to this day.  
  • Energy Conversion: As the universe expanded, some energy was converted into matter. The expansion was particularly rapid within fractions of a second after the Big Bang, but has since slowed down.  
  • Formation of Matter: Within the first three minutes from the Big Bang, the first atoms began to form. Within 300,000 years, the temperature dropped to 4,500 K (Kelvin), allowing atomic matter to form, and the universe became transparent.  
• Evidence: The increasing distance between galaxies supports the idea of an expanding universe.  
• Alternative Theory: Hoyle’s concept of a steady state universe proposed that the universe remains roughly the same at any point in time. However, the scientific community currently favors the Big Bang theory due to the evidence supporting an expanding universe.  

Star Formation

• The distribution of matter and energy in the early universe was uneven.  
• These initial density differences led to variations in gravitational forces, causing matter to be drawn together.  
• This process formed the bases for the development of galaxies.  
• Galaxies:
  • Contain a large number of stars.  
  • Spread over vast distances, measured in thousands of light-years.  
  • Diameters range from 80,000 to 150,000 light-years.  
• Nebula:
  • A very large cloud of hydrogen gas that accumulates and eventually forms localized clumps of gas.  
  • These clumps continue to grow denser, ultimately giving rise to stars.  
• Star formation is believed to have taken place around 5-6 billion years ago.  

Formation of Planets

• Stages:
  • Core Formation: The gravitational force within localized lumps of gas in a nebula leads to the formation of a core. A huge rotating disk of gas and dust develops around this core.  
  • Planetesimal Formation: The gas cloud condenses, and the matter around the core forms small-rounded objects called planetesimals.  
  • Planet Formation: Planetesimals accrete, colliding and sticking together due to gravitational attraction, eventually forming a fewer number of large bodies in the form of planets.  

Evolution of the Earth

• Early Earth: The planet Earth initially was a barren, rocky, and hot object with a thin atmosphere of hydrogen and helium.  
• Evolution of Lithosphere:
  • Primordial Stage: The Earth was mostly in a volatile state during its early stage.  
  • Differentiation: Gradual increase in density led to a rise in internal temperature, causing materials to separate based on their densities. Heavier materials like iron sank towards the center, while lighter ones moved towards the surface.  
  • Crust Formation: The Earth cooled further, solidified, and condensed into a smaller size. This led to the development of the outer surface in the form of a crust.  
  • Giant Impact: The giant impact that is believed to have formed the moon further heated the Earth.  
  • Layered Structure: Through differentiation, the Earth’s forming material got separated into distinct layers: crust, mantle, outer core, and inner core. Density increases from the crust to the core.  
• Evolution of Atmosphere and Hydrosphere:
  • Present Atmosphere: The present composition of Earth’s atmosphere is primarily nitrogen and oxygen.  
  • Three Stages:
    • Loss of Primordial Atmosphere: The early atmosphere, composed of hydrogen and helium, was stripped away by solar winds. This occurred not only for Earth but also for all terrestrial planets.  
    • Degassing: As the Earth cooled, gases and water vapor were released from the interior. This process, called degassing, contributed to the evolution of the present atmosphere. The early atmosphere mainly consisted of water vapor, nitrogen, carbon dioxide, methane, ammonia, and very little free oxygen. Continuous volcanic eruptions also added water vapor and gases to the atmosphere.  
    • Formation of Oceans and Photosynthesis: As the Earth cooled further, water vapor condensed and formed rain. Carbon dioxide dissolved in rainwater, causing temperatures to drop, leading to more condensation and rain. Rainwater collected in depressions, forming oceans. Earth’s oceans were formed within 500 million years from the Earth’s formation, making them about 4 billion years old.  
    • Around 3.8 billion years ago, life began to evolve.  
    • Around 2.5-3 billion years ago, photosynthesis evolved, initially confined to oceans.  
    • Photosynthesis gradually increased oxygen levels in the oceans.  
    • Around 2 billion years ago, the oceans became saturated with oxygen, and it began to flood the atmosphere. 

Origin of Life

• Early Conditions: The early Earth and its atmosphere were not initially conducive to the development of life.  
• Chemical Evolution: Modern scientists believe that the origin of life was a kind of chemical reaction that first generated complex organic molecules and then assembled them in a way that allowed them to duplicate themselves, converting inanimate matter into living substance.  
• Fossil Evidence: The record of life that existed on Earth in different periods is found in rocks in the form of fossils. Microscopic structures closely related to the present form of blue-green algae have been found in geological formations over 3 billion years old.  
• Timeline:
  • Life is estimated to have begun evolving around 3.8 billion years ago.  
  • The Geological Time Scale provides a summary of the evolution of life from unicellular bacteria to modern humans.  

Key Points:

• Light-year: A measure of distance, not time, equal to the distance light travels in one year (9.461 x 10^12 km).  
• Photosynthesis: The process by which plants and other organisms use sunlight to convert carbon dioxide and water into oxygen and energy.  
• Degassing: The outpouring of gases from the Earth’s interior.  
• Differentiation: The process of Earth’s material separating into layers based on density.  
• Solar winds: Streams of charged particles ejected from the Sun’s upper atmosphere.  

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