• Earth is not a perfect sphere; it is slightly flattened at the North and South Poles and bulges in the middle as it rotates around its axis, an imaginary line from West to East.

The origin of Earth

• The origin of Earth dates back approximately 4.54 billion years. Initially, Earth was a barren, rocky, and hot object with a thin atmosphere composed of hydrogen and helium. The present composition of Earth's atmosphere includes nitrogen and oxygen.
• Various scientists, philosophers, and scholars have played a significant role in shaping early ideologies about the origin of Earth:
• Thales: He presented a scientific explanation, moving away from mythological beliefs. According to his monotheistic theory, all things originate from water, and eventually, they dissolve in water.
• Anaximenes: He proposed that everything in the world originated from air. According to his belief, air is infinite, transforms into fire, becomes liquid, solidifies into water, and ultimately turns into Earth.
• Pythagoras: He suggested that all matter in the world is limited. Despite their differences, all substances represent one form, and no single substance is considered the sole cause of the world's origin.
• Heraclitus: According to him, fire (agni) is the fundamental cause of the world's origin. Water and Earth emerged from fire, and these elements are dynamic and transient, with nothing permanent in the world.

Theories of Origin of Earth

• Various hypotheses/theories related to the origin of Earth were presented by various modern scientists. Some of the important hypotheses have been described below.

Monistic Concept (Parental Hypothesis)

• Advocates of the Monistic Concept propose that all planets originated from the same celestial object or body.
• Various scholars have presented diverse ideas about the solar system's origin, suggesting a single-star origin. Key hypotheses associated with the Monistic Concept include:

Comet Hypothesis

• Proposed by Georges Louis Leclerc in 1749, this hypothesis suggests that a comet collided with the Sun, resulting in fragments that later formed the planets.
• Consider the initial version of the cataclysmic hypothesis for the solar system's origin.
• Georges Louis Leclerc mistakenly assumed comets could be nearly as large as stars.

Gaseous Hypothesis

• Introduced by German philosopher Immanuel Kant in 1755, this theory posits the existence of divinely created matter particles in the universe.
• These particles, initially hard, cold, and motionless, collided due to gravity, generating heat and motion.
• Small particles fused into larger bodies, ultimately forming a giant gaseous body called Nebula.
• The high rotational speed of Nebula led to increased heat, causing the centrifugal force to surpass the centripetal force.
• Circular rings formed, cooled down, and transformed into planets, while the remaining nebula evolved into the Sun over millions of years.

Nebular Hypothesis

• Described by French scholar Laplace in 1796, the Nebular Hypothesis presents a hot, slow-moving superbody in the universe known as a nebula.
• Over time, the nebula continuously cooled, contracting and reducing in size and volume.
• Increased rotational speed led to a balance shift between centrifugal and gravitational forces.
• A ring detached from the nebula, breaking into smaller rings that, upon cooling, evolved into planets and satellites.
• Later, the French scholar Ross made modifications to Laplace's hypothesis. According to Ross, multiple rings gradually separated from the nebula, and each ring condensed to give rise to a planet. The continuous movement in a sequence led to the origin of all planets.

Meteoritic Hypothesis

• Proposed by Russian scientist Otto Schmidt in 1944, this hypothesis suggests that the Earth and other solar system planets formed from interstellar matter clusters captured by the Sun during its passage near the galactic center.
• Schmidt substantiated the regularity in the distance between planets, their mass, and density distribution.

Dualistic Hypothesis

• The proponents of this ideology believed that the planets have originated from the combination of two stars. Therefore, this concept is also known as Bi-parental concept. Theories developed under this concept are given below

Fission Theory

• George Howard Darwin proposed the fission theory in 1878, suggesting that the Moon was once part of the Earth, separating early in the solar system's history.
• The present Pacific Ocean basin is considered a likely site for the part of the Earth from which the Moon originated.

Planetesimal Hypothesis

• Proposed by Chamberlin and Moulton in 1905, this hypothesis suggests that the solar system's planets formed from a collision between the Sun and another star.
• Small particles, known as granules, emerged from the Sun's surface due to gravitational forces, revolving around the Sun and later condensing into planets.

Tidal Hypothesis

• Sir James Jeans proposed this hypothesis in 1919 to explain Earth's origin.
• Harold Jeffreys modified the tidal hypothesis in 1929 for increased relevance. According to this hypothesis, the Sun was once a gaseous mass, and an intruding star, larger than the Sun, came close.
• Tidal forces damaged the intruding star on the primitive Sun's surface, ejecting matter that became the building material for future planets.

Binary Star Hypothesis

• Russell and Littleton proposed the binary star hypothesis in 1937, suggesting that the Sun was not initially the only star; two others were present.
• The Sun and its companion star revolved around the same center, and a third star's approach increased the gravitational force and attraction between them.
• Material from the companion star circled the giant stars, forming planets and later satellites when solidified.

Supernova Hypothesis

• Fred Hoyle and Littleton presented this hypothesis in 1939, stating that planets originated from the explosion of a companion star of the Sun involving three stars.
• The explosion of the companion star led to the scattering of dust particles and gasses, which eventually coalesced around the Sun, forming the planets.

Interstellar Dust Hypothesis

• Otto Schmidt proposed this hypothesis in 1943. According to the theory, when the Sun passed near the Milky Way, gaseous clouds and dust particles were attracted by the Sun's gravitational force, orbiting around it.
• Initially, gas and dust particles moved separately in a disorganized manner. The dust, present in larger quantities, condensed into a flat plate, forming planets and satellites through multiple stages of condensation.
• The hypothesis suggests that dark matter in the universe exists in the form of gas and dust clouds, possibly originating from meteors and stars.

Modern Hypothesis

•  Modern theories seek to understand the formation of the universe, going beyond the evolution of Earth and planets. Various modern hypotheses are discussed below.

Big Bang Theory

• Georges Lemaitre proposed the Big Bang theory in 1931, suggesting that all matter in the universe was once concentrated in a tiny space with high temperature.
• Robert Wagoner elaborated on this theory in 1967, following Edwin Hubble's evidence of the expanding universe. The Big Bang theory posits that the universe began about 13.7 billion years ago in a massive expansion.
• The creation of substances after the explosion formed various objects, leading to the formation of galaxies over time. Fission in galaxies may have produced stars, and fission in stars may have produced planets.
• The Steady-State theory, an alternative to the Big Bang, was developed by Fred Hoyle in 1948, suggesting a continuous creation of matter to maintain the density of the expanding universe. However, it didn't gain widespread acceptance.

Cosmic Inflation Theory

• Alan Harvey Guth proposed the Cosmic Inflation theory in 1980, suggesting that the early universe experienced rapid exponential expansion for a brief period after the Big Bang.
• This theory gained support with the discovery of gravitational waves, reinforcing the inflation model of cosmology.

Evolution of Earth

• With an estimated age of around 4.54 billion years, the Earth's origin was first hypothesized by the French scientist Comte De Buffon in 1749 AD.
• Initially, Earth existed as a barren, rocky, and hot entity with a thin atmosphere of hydrogen and helium. Oxygen emerged significantly later with the advent of floral species engaging in photosynthesis.
• Over time, the Earth transformed into a vibrant planet with abundant water and a hospitable atmosphere conducive to life.
• The Earth exhibits a layered structure, with materials varying in density from the center to the upper atmosphere.
• The lithosphere represents the Earth's land portion, while the hydrosphere encompasses the water elements.
• Life is believed to have originated around 3,800 million years ago, primarily confined to the oceans. The initial phase witnessed the evolution of microscopic organisms similar to blue-green algae.
• Subsequent developments included the emergence of fishes, amphibians, reptiles, flowering plants, mammals, and, finally, Homo sapiens.

Geological Time Scale

• The Geological Time Scale serves as a chronological measurement system that correlates stratigraphy with time. It is employed by geologists, paleontologists, and other Earth scientists to delineate the timing and relationships between significant events in Earth's history.
• The estimation of Earth's age, based on the study of radioactive elements, is considered the most reliable method.

Earth Geological History

• The geological history of the Earth is categorized into several eras, each further divided into epochs and various periods.

 Pre Palaeozoic Era

• In this era, rocks lacked fossils, and soft-skinned invertebrates originated in warm seas.
• The Hadean Eon, dating from 4.5-4.0 billion years ago, is named after the Greek God Hades.
• Evidence of the late heavy bombardment (4100 to 3800 million years ago) is present.
• Archean Eon reveals evidence of Prokaryotic life, such as bacteria and blue-green algae.

Palaeozoic Era

• Cambrian Period: Main Caledonian mountain-building occurred, and seas intruded into land areas. Flora and fauna originated, including invertebrates.
• Devonian Period: Earth's climate was suitable for sea animals, marking the Fish Age. Amphibians, ferns, and insects emerged.
• Ordovician Period: The Age of Invertebrates with flourishing trilobites and straight cephalopods. Fossils of sea grasses and reptiles are recorded.
• Carboniferous Period: Known as the Age of Tall Trees, witnessing primary reptiles, early amphibians, insects, and hundred feet tall trees. Gondwana rocks formed, with large coal deposits.
• Silurian Period: The Age of Vertebrates, witnessing the origin of vertebrates and fish. Leafless plants developed on land, and coral originated.
• Permian Period: Hercynian mountain-building episodes occurred, witnessing diverse plant and animal species on land. Potash reserves formed, and various mountain ranges were created.

Mesozoic Era

• Triassic Period: Known as the Age of Reptiles, large reptiles evolved, and mammals emerged from reptiles on land.
• Coniferous trees and ferns developed in the Northern hemisphere. Pangaea began to break apart during this period.
• Pangaea initiated its breakup in the mid-Triassic, leading to the formation of Gondwana in the South and Laurasia in the North.
• Jurassic Period Jura mountains emerged, marking the widespread appearance of dinosaurs. Flowering plants evolved during this period.
• Cretaceous Period Angiosperm plants developed, and giant tortoises evolved. Lava eruption in India's Peninsular plateau created the Deccan plateau and Black soil. Extensive sedimentary rock deposition occurred, offering vital resources.

Cenozoic Era

• Alpine orogeny shaped mountain ranges like the Alps, Himalayas, Rockies, and Andes.

Different Periods of Cenozoic Era

• Paleocene Period Alpine orogeny began, and mammals covered land areas extensively.
• Eocene Period Reptiles vanished from land, and modern mammalian primates, such as elephants, horses, rhinoceros, and pigs, evolved.
• Oligocene Period Apes, ancestors of man, originated. Ancestors of modern cats, dogs, and bears evolved. Main period for the Greater Himalayas' formation.
• Miocene Period The present elephant developed. Maximum shark growth occurred. Aquatic birds and elephants migrated from Africa to Europe, Asia, and North America.
• Pliocene Period Ancestors of man and modern mammals developed. Shiwalik mountain formed, and sediment deposition led to the formation of the Great Plains of North India.

Neozoic Era

• Pleistocene Period Dynamic environments changed dramatically in response to climatic cycles. This epoch includes the last ice age, with widespread glacier coverage.
• The Holocene Period Started 10,000 years ago, marking the end of glaciation and the current stage of the world. Aquatic life reached its present stage.
• Anthropocene Period An unofficial geologic unit reflecting the period when human activities significantly impacted the planet's climate and ecosystem, e.g., global warming and habitat loss.

Interior of the Earth

• The formation of Earth around 4,500 million years ago is not precisely known, but evidence from volcanic eruptions, earthquake waves, deep-mines, and crustal boring indicates a distribution of Earth-forming materials into concentric layers with distinct characteristics.

Sources of the Earth

• Sources providing knowledge about Earth's interior are classified into two categories: Direct sources and Indirect sources.

Direct Sources

• Earth's Surface Surface rocks offer information about materials up to a certain depth.
• Volcanoes Volcanic materials post-eruption provide direct observation, offering insights into material quality from significant depths.
• Material from Mining and Drilling In the exploration of mining and drilling sites, numerous materials are extracted for analysis. These materials offer insights into the characteristics present at specific depths.
• When mining, it becomes evident that both pressure and temperature intensify as we move from the surface towards the Earth's interior. Additionally, material density escalates from the upper surface to the deeper interior.

Indirect Sources

Increase in Pressure and Temperature with Depth

• As we delve deeper into the Earth's interior, there is a concurrent increase in temperature, pressure, and material density. Scientists have estimated these values at different depths by considering the Earth's characteristics and thickness.

Meteorites

• Studying meteorites, formed similarly to Earth and of nearly the same age, provides valuable information. Both Earth and meteorites originate from the same nebular cloud, suggesting a likely similarity in internal structure. When meteoroids fall to Earth, extreme friction burns their outer layer, exposing the inner core.

Gravity

• Analyzing gravity anomalies offers insights into the distribution of material mass in the Earth's crust. Earth's gravity values vary based on material mass. The uneven distribution of material mass within the Earth influences gravity values, leading to anomalies.

Magnetic Field

• Mapping the magnetic field provides information about the distribution of magnetic materials in the Earth's crust, revealing details about material distribution.

Seismic Activities

• Seismic waves, generated by the release of energy from the Earth's interior, serve as crucial sources of information. The study of seismic waves, known as seismology, involves waves that travel in all directions.
• A seismograph records waves reaching the surface, presenting earthquake curves in three distinct sections, each representing different wave patterns.

Earthquake Waves

• Earthquake waves come in three types: P-waves or Primary waves, S-waves or Secondary waves, and L-waves or Surface waves. P and S waves, collectively called Body waves, traverse through the Earth's body, revealing various facts about the interior.
• P-waves: Longitudinal or compressional waves, similar to sound waves, travel through gaseous, liquid, and solid materials, providing information about the Earth's core.
• S-waves: Transverse waves, akin to light waves, vibrate perpendicular to the direction of propagation. S-waves can only travel through solid materials, not through liquids, making them unable to pass through the mantle.
• L-waves: These waves aid in determining the Earth's interior as they get reflected and refracted while passing through the homogeneous composition of the Earth's interior due to varying density zones.

Structure of the Earth's Interior

• According to the researchers of International Union of Geodesy and Geo-physics (IUCG) and the study of the Earth's interior, it is now confirmed that Earth has three broad concentric zones based on chemical compositions and differential characteristics.
• These three broad zones are crust, mantle and core.

Crust

• The crust refers to the outermost shell of a terrestrial planet, with thickness varying along oceanic and continental regions, totaling approximately 100 km.
• Oceanic crust is thinner, averaging 5 km, and can vary from 10 to 12 km, while continental crust averages around 30 km, reaching up to 70 km in major mountain systems like the Himalayas.
• The upper continental crust, known as sial, is predominantly composed of silica and aluminum. Granites, found in the sial layer, float on the sima layer, rich in silica and magnesium.
• Sima, with a density of about 2.95 g/cm³, has a composition similar to common black volcanic lava, basalt, while sial has a density of about 2.7 g/cm³.
• Oceanic crust consists of igneous rocks like basalt and peridotite, rich in iron and magnesium, with an average density of about 2.7 g/cm³.

Mantle

• Beyond the crust lies the mantle, extending up to a depth of 2,900 km, with a density of approximately 3.4 g/cm³.
• The upper mantle, known as the asthenosphere or weak layer, extends up to 400 km and is a major source of magma eruptions that reach the surface as lava.
• The lower mantle, called the mesosphere, extends beyond the asthenosphere and remains in a solid state. It is also termed the zone of low velocity, as S-waves cannot travel through the liquid mantle layer, and other seismic waves slow down.
• Large convective cells in the mantle circulate heat and may drive plate-tectonic processes. The mantle contains rock materials rich in magnesium and iron silicates, along with substantial quantities of sulphides in the upper mantle and nickel and iron in the lower mantle, forming a peridotite shell.

Core

• The innermost layer, the core, spans from 2,900 km to 6,370 km.
• The core is divided into two layers with different characteristics. The outer core, in a liquid state, is twice as thick as the inner core, which remains solid.
• The inner core has a density of about 14 g/cm³, while the outer core has a density of about 11 g/cm³.

The core primarily consists of a dense composition of nickel and iron, forming the nife layer, also referred to as the centrosphere.

The core experiences high temperatures, reaching up to 4000°C at the inner core, and extreme pressure, resulting in a solid inner core. The Earth's magnetic field is influenced by the liquid outer core.

The outer core expands from 2900 km to 5100 km, existing in a liquid state due to the inability of S-Waves to penetrate this region.

The inner core expands from 5100 km to 6378 km, existing in a solid or plastic state, with a density of approximately 13 grams per cubic cm. The speed of P-waves in the core increases to 11.23 km/sec.

Discontinuities

Discontinuities serve as boundaries between different layers of the Earth, indicating variations in physical and chemical properties. Key discontinuities include:

• Conrad Discontinuity: Marks the boundary between Sial and Sima layers.
• Mohorovicic or Moho Discontinuity: Marks the boundary between the upper mantle and the lower crust.
• Repetti Discontinuity: Marks the boundary between the upper mantle and lower mantle.
• Weichert-Gutenberg Discontinuity: Marks the boundary between the lower mantle and the outer core.
• Lehmann Discontinuity: Marks the boundary between the outer core and the inner core.

Prelims Facts

• The Nebular hypothesis of origin of Earth was given by
  • - Laplace (UPPSC (Pre) 2012]
• The scientist who first discovered that Earth revolves around the Sun was
  • - Copernicus (CGPSC (Pre) 2016)
• The period from 1650 AD to 1870 AD has generally been dered to be the
  • Little Ice Age (UPPSC (Pre) 2015]
• Wic method is used to determine the age of the Earth?
  • - Uranium dating (UPPSC (Pre) 2006]
• A direct source of information about the interior of the Earth's
  • -Surface rock from mining areas [APSC (Pre) 2017)
• The discontinuity appearing between the Earth's crust and the mantle is known as
  • - Mohorovicic (MPSC (Pre) 2012]
• The structure of core of interior of the Earth is made up of
  • - Nife (nickel and iron) (UPPSC (Mains) 2013]
• The area of sudden change in Seismic waves velocity is known as
  • - Moho discontinuity [MPSC (Pre) 2013]
• The approximate mean velocity with which the Earth moves around the Sun in its orbit
  • -30 km/sec [IAS (Pre) 2006)
• The positions in space where gravitational forces of Sun and Earth produce regions of mutual attraction is known as
  • - Lagrange points (UPPSC (Pre) 2005]
• The largest satellite of Jupiter planet as well as of the solar system is known as
  • - Ganymede (BPSC (Pre) 2009)
• The very first logical Hypothesis for the origin of the Earth was given by the French scientist
  • -Comte-de-Buffon [BPSC (Pre) 2005
• The small celestial particles detached from the proto Sun due to massive gravitational pull exerted by the giant intruding star are called
  • - Planetesimals [IAS (Pre) 2016]
• In the structure of planet Earth, below the mantle, the core is mainly made up of which element? Sun and the Earth? What is the average distance (approximate) between the
  • -Silicon [IAS (Pre) 2009)
• Who was the first person to state that the Earth was spherical?
  • -150 x 10 km [IAS (Pre) 2007]
• The most abundant element in Earth's crust is Oxygen (JPSC (Pre) 2011)
  • - Aristotle [IAS (Pre) 2001

Self Check

1. Which of the following scholars has suggested that the Earth has originated from the gasses and the dust particles? IAS (Pre) 1999

(a) James Jeans

(b) H Alfven

(c) F Hoyal

(d) O Schmidt

2. Which of the following views was supported by Carl Weizsacker and Otto Schmidt about the Origin of Earth? 

(a) Meteoritic Hypothesis

(b) Fission Theory

(c) Tidal Hypothesis

(d) Planetesimal Hypothesis

3. Arrange the following period of geological timescale in chronological order.

(a) Era, Period, Eon, Epoch

(b) Period, Epoch, Eor, Era

(c) Eon, Era, Period, Epoch

(d) Ean, Epoch, Era, Period

4. Which one of the following has the longest duration?

(a) Eons

(c) Era

(b) Period

(d) Epoch

5. Which one of the following experts gave the collision hypothesis?

(a) Immunal Kant

(b) Lalpace

(c) Chamberlain

(d) Sir James and Harold Jeffrey

6. Big-Bang Theory was postulated by

(a) Georges Lemaître

(b) Edwin Hubble

(c) Schmidt

(d) Wegener

7. Match the following lists correctly and choose the correct code.

Codes

(a) 1 2 4 3

(c) 3 1 2 4

(b) 2 3 4 1

(d) 2 4 1 3

8.Match the following lists correctly and choose the correct code.

Codes

(a) 1 2 4 3

(b) 2 3 4 1

(c) 3 1 2 4

(d) 2 4 1 3

9. Consider the following events. UPPSC (Pre) 2018

1. First reptiles
2. First insects 
3. Shelled animals
4. First mammals

Arrange the above events in chronological order of their origin on the Earth and select the correct answer by using the codes given below.

(a) 2, 1, 3, 4

(b) 2, 3, 1. 4

(c) 3, 2, 1, 4

(d) 2, 4, 1. 3

10. Which of the following is/are the direct sources of information about the interior of the Earth?

1. Earthquake wave
2. Volcano
3. Gravitational force
4. Earth magnetism

Select the correct answer by using the codes given Below.

( a) 1 and 2

(b) Only 2

(c) 3 and 4

(d) All of these

11. Which of the following statements is correct about the structure of core or interior of the Earth?

MPSC (Pre) 2012, UPPSC (Mains) 2013

a) The core of the interior of the Earth is made up of sial.

(b) The core of the interior of the Earth is made up of sima.

(c) The core of the interior of the Earth is made up of knives.

 (d) The core of the interior of the Earth is made up of hard rocks.

12. Match List I with List II and select the correct answer by using the codes given below the lists

Codes

(a) 1 2 3 4

(b) 2 1 3 4

(c) 4 1 2 3

(d) 2 3 1 4

13. The stages of formation of the Earth are given in the options given below. Arrange the stages in a correct sequence and choose the correct answer using the codes given below.

1. Formation of ridges and basins
2. Swirling ball of dust and clouds
3. Floating of gases to from atmosphere
4. Formation of oceans

Codes

(a) 2, 1, 3, 5, 4

(b) 2, 3, 1, 4, 5

(c) 1, 2, 4, 5,3

(d) 5, 1, 3, 4, 2

14. In the interior of the Earth, BPSC (Pre) 2017

(a) the temperature falls with increasing depth.

(b) the pressure falls with increasing depth.

(c) the temperature rises with increasing depth.

(d) both temperature and pressure fall with increasing depth.

15. Major contents and their proportions in the composition of the Earth's crust upto 17 km depth).

Codes

(a) 1 3 5 4 2

(b) 3 1 2 5 4

(c) 12 5 4 3 

(d) 3 2 1 5 4 

16. Three zones of varying properties have been identified in the Earth i.e., crust, mantle and core.

MPSC (Pre) 2013

1. The area of change in density between sial and sim is known as 'Conrad discontinuity'.
2. The area of sudden change in seismic waves velocity is known as 'Moho discontinuity'.
3. The mantle-core boundary is determined by 'Gutenberg discontinuity'.

Which of the statement(s) given above is/are correct?

(a) Only 1

(b) 2 and 3

(c) 1 and 3

(d) All of these

Know Right A

1 (d)

2 (a)

3 (c)

4 (a)

5 (d)

6 (a)

7 (b)

8 (d) 

9 (c)

10 (b)

11 ( c)

12 (b) 

13 ( b)

14 ( c)

15 (c)

16 (c)