The Origin and Evolution of the Earth Class 11 Geography

Early Theories

Origin of the Earth

• A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth.
• One of the earlier and popular arguments was by German philosopher Immanuel Kant.
• Mathematician Laplace revised it in 1796.
• It is known as Nebular Hypothesis.
• The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.
• In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany somewhat revised the 'nebular hypothesis', though differing in details.
• They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust.
• The friction and collision of particles led to formation of a disk-shaped cloud and the planets were formed through the process of accretion.
• However, scientists in later period took up the problems of origin of universe rather than that of just the earth or the planets.

Modern Theories

• The Big Bang Theory stands as a contemporary explanation for the origin and evolution of the Earth.
• Edwin Hubble introduced this theory in 1920, and it is also recognized as the Expanding Universe Hypothesis.
• This theory posits that the universe is in a constant state of expansion.
• According to the theory, all the matter in the universe was concentrated at its center, occupying a minute space with incredibly high temperatures and densities.
• Approximately 13.7 billion years ago, a significant explosion occurred, leading to the formation of atoms, and over time, the conversion of energy into matter took place.

Stages of the Big Bang Theory

1. Singularity:

The Star Formation

Formation of Planets

Formation of Planets

Formation of Core and Rotating Disc

• Stars form as localized lumps of gas within a nebula.
• Gravitational forces create a core in the gas cloud.
• A rotating disc of gas and dust develops around this core.

Condensation and Planetesimals Formation

• The gas cloud condenses, forming small-rounded objects around the core.
• These small objects, through cohesion, become planetesimals.
• Larger bodies form through collisions and gravitational attraction, sticking together.

Accretion into Planets

• Numerous small planetesimals accrete to form fewer, larger bodies.
• These larger bodies become planets.

Evolution of Earth

Initially, Earth existed in a scorching state, devoid of life, and characterized by rocky desolation. Its atmosphere primarily comprised hydrogen and helium. Over time, the processes of lithosphere, atmosphere, and hydrosphere formation gradually transformed the planet.

Evolution of Lithosphere

Evolution of Lithosphere

• During its primordial stage, the Earth was mostly in a volatile state with increasing internal temperature.
• Due to the gradual increase in density and temperature, materials inside the Earth began to separate based on their densities. Heavier materials, such as iron, sank towards the center, while lighter materials moved towards the surface.
• As time passed, the Earth cooled, solidified, and condensed into a smaller size. This cooling process led to the development of the outer surface in the form of a crust.
• During the formation of the moon, a giant impact further heated the Earth.
• Through the process of differentiation, the Earth-forming materials separated into different layers. These layers, from the surface to the center, include the crust, mantle, outer core, and inner core. The density of the materials increases from the crust to the core.

Evolution of Atmosphere and Hydrosphere

Three Stages of Atmosphere Evolution

1. The early atmosphere, composed of hydrogen and helium, was stripped off by solar winds. This phenomenon occurred not only on Earth but also on other terrestrial planets.
2. During the cooling of the Earth, gases and water vapor were released from the interior solid Earth, initiating the evolution of the present atmosphere. The early atmosphere largely contained water vapor, nitrogen, carbon dioxide, methane, ammonia, and very little free oxygen. Continuous volcanic eruptions contributed water vapor and gases to the atmosphere.
3. The composition of the atmosphere was significantly modified by the process of photosynthesis. This process, carried out by early life forms, increased the oxygen levels in the atmosphere.

Formation of Oceans

• As the Earth cooled, the water vapor in the atmosphere began to condense, leading to rain.
• The rainwater collected in depressions on the Earth's surface, giving rise to oceans.
• The Earth's oceans were formed within 500 million years from its formation, making them approximately 4,000 million years old.

Origin of The Life

• Life emerged on Earth approximately 3.8 billion years ago, with blue algae representing the earliest life form.
• These life forms evolved from chemical reactions that gave rise to molecules capable of replication and transformation into living entities.
• Two primary theories include the Nebular Hypothesis and the modern Big Bang Theory.
• Earth's early state was characterized by solid rockiness and desolation, eventually leading to the formation of its distinct layers: the crust, mantle, and core.
• Initially, Earth's atmosphere lacked oxygen, but the emergence of life and the process of photosynthesis subsequently raised oxygen levels while reducing carbon dioxide.
• The evolution of life on Earth can be traced back to the presence of blue algae.