Origin & Evolution of Earth's Crust | Geography Optional for UPSC (Notes) PDF Download

Introduction

• The Earth's crust is the thinnest and most fundamental layer of our planet, serving as home to all living organisms that have ever existed. Despite being a relatively simple structure, the crust is highly dynamic and forms a key part of the Earth's various layers.
• Characterized by its distinct chemical composition, the crust can be divided into two main types: oceanic and continental. These differences arise due to plate tectonics, which involve the movement of plates above the asthenosphere. This movement drives lithospheric processes that result in natural phenomena such as earthquakes and ridges. As one of the Earth's five chemical layers, the crust showcases the unique chemical properties found at each level.
• In conjunction with the upper mantle, the Earth's crust plays a crucial role in the ongoing creation and destruction of the surface where all living beings thrive. This section will explore various aspects of the crust, delving into its origin, structure, and composition, as well as shedding light on its continuous evolution up to the present day.

Origin of Earth's Crust

• The Earth's crust is believed to have formed approximately 4.5 billion years ago, after the late stages of planetary accretion. There are three main theories that have been proposed to explain the formation of the crust: the inhomogeneous accretion model, the impact model, and the terrestrial model.
• The inhomogeneous accretion model suggests that the Earth's crust was formed during the accretion of the planet, with lighter and volatile elements forming a thin layer on the primitive planet, which eventually became the crust. However, this model has been criticized for not accounting for the presence of non-volatile elements, such as uranium and thorium, in the Earth's crust.
• The impact model proposes that the Earth's crust was formed through the melting and subsequent solidification of materials brought to Earth by asteroids and other celestial bodies. While this theory could potentially explain the composition of the oceanic crust, which is primarily composed of basalt, it does not account for the fact that many impact events on Earth occurred after the oceanic crust had already formed. Additionally, the amount of basalt produced by impact events is considered too small to have formed the crust.
• The terrestrial model is currently the most widely accepted explanation for the formation of the Earth's crust. This model posits that the crust originated from the Earth's internal processes. After the late accretion of the Earth, the heat retained by the planet caused the upper mantle to melt, creating a magma ocean that covered the Earth's surface. As the Earth cooled, the magma ocean crystallized, forming a widespread crust. Alternatively, the melted upper mantle could have risen to the surface and solidified, forming the crust.
• The terrestrial model is considered the most likely explanation for the formation of the Earth's crust, as it can account for the uniform composition of the crust as well as its layered structure. The cooling of the magma ocean over time would have resulted in the formation of the different layers observed in the Earth's crust today.

Nebular Hypothesis Theory

• The Nebular Hypothesis Theory was initially proposed by German philosopher Immanuel Kant and later revised by mathematician Laplace in 1776. According to the theory, the Sun was encircled by a solar nebula, primarily composed of hydrogen and helium, along with dust particles. The friction and collision of these particles led to the creation of a disk-shaped cloud. Gradually, through the process of accretion, planets were formed from the material surrounding the young Sun.
• However, the Nebular Hypothesis Theory faced several criticisms. For instance, the theory fails to explain the origin of the nebula itself. It also does not account for the sources of heat and motion within the nebula. Furthermore, the hypothesis suggests that since the nebula was gaseous, the planets should also be in a gaseous state, which is not the case as planets are solid. The transition from a gaseous nebula to solid planets is not adequately explained by the theory.
• In addition, the Nebular Hypothesis Theory predicts that all planets and their satellites should rotate in the same direction as the rotation of the nebula. However, this is not observed in reality, as Venus and Uranus rotate in the opposite direction compared to other planets in our solar system.

Planetesimal Hypothesis

The Planetesimal Hypothesis, proposed by Chamberlain and Moulton in 1900, offers an explanation for the origin of Earth and the formation of planets. According to this theory, before the formation of planets, there was only a sun, which was cold at that time. Another sun passed close to our sun, and due to their gravitational attraction, a cigar-shaped bulge of material was pulled from our sun. This bulge eventually separated from the solar surface and formed smaller bodies called planetesimals. These planetesimals then came together and combined to form larger celestial bodies, which we now know as planets.

However, the Planetesimal Hypothesis has been criticized for several reasons:

• The theory was unable to explain why there are only nine planets in our solar system (now recognized as eight, with Pluto being reclassified as a dwarf planet).
• It failed to account for the various sizes of planets in our solar system.
• The distance between the two suns was immense, and it is unlikely that the gravitational pull between them would have been strong enough to extract such significant amounts of material from our sun.

The Big Bang Theory 

The Big Bang Theory is a modern scientific explanation that seeks to address various questions about the origin and evolution of the universe, the solar system, the Earth's crust, and the atmosphere. This theory, also known as the expanding universe hypothesis, has significantly advanced our understanding of the cosmos.

• According to the Big Bang Theory, all the matter that makes up the universe was initially concentrated in a single point called the singularity. This point had an incredibly small volume, infinite temperature, and infinite density. Around 13.7 billion years ago, the singularity exploded in a massive event known as the Big Bang. Within the first three minutes after the explosion, the initial atoms began to form as energy was converted into matter. Approximately 300,000 years after the Big Bang, the universe became transparent.
• The formation of stars, as explained by the Big Bang Theory, began about 5 billion years ago. In the early universe, the distribution of matter and energy was uneven. This uneven distribution led to differences in density, which in turn created gravitational forces that caused matter to come together. The universe is composed of a vast number of galaxies, each containing a large number of stars.

A galaxy is a massive collection of stars that formed from nebulae, which are clouds of hydrogen and helium gas. The formation of planets takes place in several stages:

• Stage 1: Within the nebula, stars begin as lumps of gas. Gravitational forces within these lumps lead to the formation of a core, surrounded by a huge rotating disc of gas and dust.
• Stage 2: The matter around the core starts to condense into smaller, rounded objects called planetesimals. These planetesimals collide with one another, forming larger bodies in the process.
• Stage 3: The smaller terrestrial bodies, or planetesimals, continue to combine and form larger planetary bodies.

In summary, the Big Bang Theory is a crucial modern theory that delves into the origin and evolution of the universe, the solar system, and the Earth. This theory provides a comprehensive explanation for the formation of stars, galaxies, and planets, greatly expanding our knowledge of the cosmos.

Evolution of earth

• The evolution of Earth can be traced back to when it was initially a barren, rocky, and hot object with a thin atmosphere composed of hydrogen and helium. Over time, Earth developed a layered structure, which is not uniform, due to the gradual increase in density and temperature within the planet. This caused materials to separate based on their density, with heavier elements like iron sinking towards the center and lighter ones moving towards the surface. The formation of the Moon further heated the Earth, leading to the development of three major layers: the crust, mantle, and core.
• The atmosphere and hydrosphere of Earth evolved through three main stages. In the first stage, primordial gases like helium and hydrogen were lost as they were blown away by solar winds. In the second stage, the hot interior of Earth contributed to the development of the atmosphere. As the planet cooled, gases and water vapor were released from its interior into the atmosphere, leading to a composition consisting mainly of water vapor, nitrogen, carbon dioxide, methane, ammonia, and very little free oxygen. This process is known as degassing.
• In the third stage, living organisms played a significant role in modifying the atmosphere's composition. Through the process of photosynthesis, oxygen began to accumulate in the atmosphere. Meanwhile, carbon dioxide levels decreased as it dissolved in rainwater, leading to more condensation and rainfall.
• The origin of life on Earth is believed to be a result of chemical reactions that transformed inorganic materials into living organisms. Around 3.8 billion years ago, life began to evolve, starting with simple organisms like blue-green algae.

Conclusion

The Earth's crust, atmosphere, and hydrosphere have undergone significant transformations since their formation billions of years ago. The terrestrial model, the Big Bang Theory, and the evolution of Earth's atmosphere and hydrosphere provide insights into the processes that have shaped our planet and its dynamic crust. From the formation of stars, galaxies, and planets to the emergence of life, these theories have greatly expanded our understanding of the cosmos, the origins of Earth, and the complex interactions between its various layers and components.

Frequently Asked Questions (FAQs) of Origin & Evolution of Earth's Crust

What are the two main types of Earth's crust?

The Earth's crust can be divided into two main types: oceanic and continental. These differences arise due to plate tectonics, which involve the movement of plates above the asthenosphere.

What is the most widely accepted explanation for the formation of the Earth's crust?

The terrestrial model is currently the most widely accepted explanation for the formation of the Earth's crust. This model posits that the crust originated from the Earth's internal processes, involving the melting and solidification of the upper mantle.

What are the main criticisms of the Nebular Hypothesis Theory?

The Nebular Hypothesis Theory faces several criticisms, including its inability to explain the origin of the nebula itself, the sources of heat and motion within the nebula, and the transition from a gaseous nebula to solid planets. It also does not account for the opposite rotation of Venus and Uranus compared to other planets in our solar system.

How does the Big Bang Theory contribute to our understanding of the formation of the Earth and its crust?

The Big Bang Theory provides a comprehensive explanation for the formation of stars, galaxies, and planets, greatly expanding our knowledge of the cosmos. This theory delves into the origin and evolution of the universe, the solar system, and the Earth, shedding light on the processes that led to the formation of the Earth's crust.

How did life on Earth influence the evolution of the atmosphere?

Living organisms, particularly through the process of photosynthesis, played a significant role in modifying the atmosphere's composition. Oxygen began to accumulate in the atmosphere, while carbon dioxide levels decreased as it dissolved in rainwater. This led to more condensation and rainfall, further shaping the Earth's atmosphere and hydrosphere.