Class 11 Geography Chapter 2: Evolution of Earth Notes

Early Theories

Early theories sought to explain the formation of the Earth and the universe. Before modern scientific understanding, people proposed a variety of ideas to explain the origin of the Earth:

• Nebular Hypothesis: This theory, proposed by Immanuel Kant in 1755 and further modified by Pierre-Simon Laplace in 1796, suggests that the Sun and planets formed from a rotating cloud of gas and dust, or nebula. As this nebula cooled, it began to collapse due to gravity, spinning faster and flattening into a disk. Over time, the central mass became the Sun, while the remaining material condensed to form the planets, including Earth.
• Otto Schmidt and Carl Weizsäcker’s Revision: In the 1950s, scientists such as Otto Schmidt and Carl Weizsäcker revised the nebular hypothesis, suggesting that the solar system developed from a solar nebula filled with hydrogen, helium, and dust. Through a process of accretion, small particles began to clump together to form planetesimals, which eventually grew into planets.

These early theories laid the groundwork for modern understanding but were incomplete, especially regarding how planets formed in detail.

Modern Theories: The Big Bang Theory

The Big Bang Theory is the most widely accepted explanation for the origin of the universe and the subsequent formation of the Earth:

• The Big Bang Event: The universe is believed to have begun about 13.7 billion years ago from an incredibly dense and hot state, known as a singularity. This singularity exploded in an event called the Big Bang, causing rapid expansion and the creation of space, time, and matter.
• Expansion and Cooling: After the Big Bang, the universe began to expand rapidly. As it expanded, it cooled down, allowing energy to convert into matter. Over time, simple atoms like hydrogen and helium began to form, leading to the creation of stars and galaxies.
• Formation of Atoms: Around 300,000 years after the Big Bang, the temperature dropped enough to allow the first atoms (primarily hydrogen and helium) to form. This cooling process also allowed light to travel through space, making the universe transparent.

Formation of Stars and Galaxies

The next step in the evolution of the universe involved the formation of stars and galaxies:

• Nebulae and Star Formation: Nebulae are massive clouds of gas (mainly hydrogen) and dust. Under the influence of gravity, parts of these nebulae began to collapse, forming dense regions that eventually became stars.
• Star Clusters and Galaxies: Stars are not evenly distributed but tend to cluster into galaxies, which are vast collections of billions of stars, gas, and dust. Our solar system is located in the Milky Way Galaxy.

Formation of Planets

The formation of planets, including the Earth, occurred in several stages:

• Core and Disk Formation: In the early solar system, a disk of gas and dust surrounded the newly formed Sun. This disk was known as the protoplanetary disk. As particles in the disk collided, they began to stick together, forming larger clumps known as planetesimals.
• Planetesimal Growth: Over time, these planetesimals continued to collide and merge, gradually forming larger objects called protoplanets. Through repeated collisions, these protoplanets gained mass and formed the planets we know today.
• Accretion and Differentiation: The process of accretion (where smaller particles combine to form larger ones) allowed Earth to grow in size. Once it reached a certain size, the heavier elements, such as iron, sank to the center, forming the core, while lighter elements rose to the surface, forming the mantle and crust.

Evolution of the Earth

The Earth’s evolution is marked by significant changes from its initial molten state to its current form:

• Formation of the Moon: The most widely accepted theory for the Moon’s formation is the Giant Impact Hypothesis. According to this theory, a Mars-sized body called Theia collided with the early Earth. The debris from this collision coalesced to form the Moon. This impact also caused the Earth to heat up and melt again, resetting its surface.
• Cooling and Solidification: After the collision, the Earth gradually cooled, allowing its surface to solidify into a crust. The differentiation of the Earth’s layers (core, mantle, crust) occurred as heavier materials like iron sank to the center while lighter materials formed the outer layers.

Evolution of the Lithosphere

The lithosphere is the Earth’s rigid outer shell, composed of the crust and the uppermost mantle. Its evolution involved:

• Differentiation of the Earth: As the Earth cooled, it underwent differentiation, meaning the denser materials (like iron and nickel) sank to the core, while the lighter materials rose to form the crust and mantle. This process resulted in the Earth’s layered structure.
• Lithosphere Formation: The lithosphere formed as the Earth’s surface cooled and solidified. Over time, tectonic forces caused the lithosphere to break into plates, giving rise to the theory of plate tectonics.

Evolution of the Atmosphere and Hydrosphere

The Earth’s atmosphere and hydrosphere developed through several stages:

• Primordial Atmosphere: The Earth’s original atmosphere, composed mainly of hydrogen and helium, was lost due to solar winds.
• Degassing: Volcanic activity released gases trapped in the Earth’s interior, contributing to the formation of a new atmosphere composed of water vapor, carbon dioxide, nitrogen, and small amounts of other gases. This process, called degassing, was crucial in forming the Earth’s secondary atmosphere.
• Formation of Oceans: As the Earth cooled, the water vapor in the atmosphere condensed, leading to rainfall and the formation of oceans. This process occurred about 4 billion years ago.
• Photosynthesis and Oxygenation: The first life forms, cyanobacteria, appeared in the oceans and began to produce oxygen through photosynthesis. This oxygen gradually accumulated in the atmosphere, leading to the Great Oxygenation Event around 2 billion years ago, which transformed the atmosphere from one lacking oxygen to one rich in oxygen.

Origin of Life

Life on Earth is believed to have originated around 3.8 billion years ago through a process known as chemical evolution:

• Formation of Organic Molecules: Early Earth’s oceans provided the ideal environment for the formation of simple organic molecules. Over time, these molecules combined to form more complex structures, eventually leading to the first protocells capable of replication.
• Fossil Evidence: The earliest known life forms were simple, unicellular organisms such as bacteria and cyanobacteria. Fossils of these organisms have been found in rocks dating back more than 3 billion years.
• Evolution of Multicellular Life: As oxygen levels increased in the atmosphere, more complex life forms began to evolve. The development of multicellular organisms led to a greater diversity of life on Earth.