Earth (Part-1)

EARTH

Contents

1.EARTH

1.1  ORIGIN OF THE EARTH

1.1.1        The Big Bang Theory

1.1.2        The Star Formation

1.1.3        Formation of Planets

1.1.4        Formation of the Moon

1.1.5        Our Solar System

1.2  EVOLUTION OF THE EARTH

1.2.1        Evolution of Lithosphere

1.2.2        Evolution of Atmosphere

1.2.3        Evolution of Hydrosphere

1.2.4        Origin of Life

1.3  DISTRIBUTION OF OCEANS AND CONTINENTS

1.3.1        Continental Drift Theory

1.3.2        Post-Drift Studies

1.3.2.1  Convectional Current Theory

1.3.2.2  Mapping of the Ocean Floor

1.3.2.3  Concept of Sea Floor Spreading

1.3.3        Plate Tectonics

1. EARTH

The earth is the only planet that can sustain life. It is one of the planets in our solar system. It revolves around the sun in an elliptical fashion through which we can experience different seasons on the planet earth.  It also rotates on its own axis through which day and night phenomena happen. The shape of the earth is geoid. The earth is also known as a blue planet because the majority of the earth’s surface is covered with water. Earth is composed of different concentric layers that originated over millions and billions of years. These concentric layers of the atmosphere and lithosphere help in sustaining life on the earth. The miniature form of the earth is a globe, which is used to study the dimension of the earth accurately.

1.1 ORIGIN OF THE EARTH

It is believed that the earth is formed 4.6 billion years ago. The existence of the earth and other planets came into account because of the big bang explosion. It is a highly accepted theory by astronomers and cosmologists.

The universe consists of millions and billions of galaxies. The galaxies incorporate millions of stars. The single star (like the sun) and planets, asteroids, and comets that are revolving in a unique elliptical orbit around the sun are known as our solar system.

1.1.1 BIG BANG THEORY

The big-bang theory was first propounded by Belgian astronomer-priest, Abbe Georges Lemaitre. He is also known as the father of big-bang theory. This theory is also known as the evolutionary theory. He stated that cosmic matter was in an extremely compressed state and expansion was started by the primordial explosion.

Evidence that supported the big-bang theory is as follows-

Edwin Hubble, “Stated that all the detectable galaxies are moving apart from our Milky way galaxy”

The farther galaxies are moving at a fast pace, which clearly shows that the universe is still expanding and these galaxies were close together at one time. This is also called Hubble’s law.

The following stages in big-bang theory are as follows

 

STAGE 1: SINGULARITY-

All the matter and energy in a universe was once a “tiny ball” singular atom with unimaginable temperate, size, and density.

STAGE 2: EXPLOSION-

The singular atom exploded destructively. Some energy is converted into the matter at the time of the explosion. It expanded within a few fractions of a second. This led to a huge expansion of the universe. As per the astronomers, it is believed that this explosion took place 13.7 billion years ago.

 STAGE 3: INFLATION-

The period of singularity was followed by the period of very rapid expansion and cooling known as ‘Inflation’. In the third stage, the temperature dropped to 4500k (Kelvin) and gave rise to atomic matter. The universe becomes transparent. The distance between the galaxies is increasing slower than in stage two.

Alternative theory

  • Steady State Theory- It is an alternative to the big-bang theory which is propounded by Hoyle. It is considered that the universe is the same at any point in time. In other words, the steady-state universe has no beginning or ending in time.
  • Some other theories are pulsating universe theory and the anti-universe etc.                         

1.1.2 STAR FORMATION

In the initial universe, the distribution of matter and energy was not uniform. The density differences gave rise to differences in gravitational forces and it causes matter to attract together. These are the bases on which galaxies formed. The formation of galaxies started with an accumulation of hydrogen gas in the form of a very large cloud known as a nebula. Finally, the nebula starts growing and develops localized clumps of gases. These clumps continue to grow into even denser gaseous bodies. Hence, a star is formed. The formation of stars is believed to occur between 5-6 billion years ago.

NOTE-

  • A galaxy spread across thousands of light years. The diameter of each galaxy ranges from 80,000-150,000 light years.
  • Light year – The light year is a distance covered by light in a year at the speed of 300,000 km/second. It measures distance only.

                            

1.1.3 FORMATION OF PLANETS

Planets are gravitationally bound objects that orbit around the sun (star). The formation of planets depends on the formation of the sun (star) and a disc of gases and dust. These occur in the following stages.

STAGE 1: FORMATION OF SUN

The nebula contains lumps of gases known as stars (Sun). These lumps of gases formed due to the powerful gravitational force. It attracts the molecules of hydrogen into the center. These hydrogen atoms fused with each other and formed helium. Therefore, the sun is formed. The gas core (Sun) has an unmeasurable density and gravitational pull that develops a huge rotating disc of gas and dust around it.

STAGE 2: FORMATION OF PLANETESIMALS

The process of cohesion occurs because the dust around the sun contains carbon and iron. These elements have binding properties. Dust particles crash into one another to form a “clump”. These clumps accumulate and condense to form round objects. The clumping of objects makes the objects larger and larger in size. Thus, Dust becomes pebbles, pebbles become rocks, rocks become blocks, and these rocks blend and stick together with gases to form planetesimals.

NOTE- These building blocks of planets are known as planetesimals.

STAGE 3: FORMATION OF PLANETS

These small planetesimals come together to form large bodies called planets. The process of condensation helps in forming inner planets rocky and outer planets majorly in gaseous form.

1.1.4 FORMATION OF THE MOON

The moon is the only natural satellite that orbited around the planet earth. The early attempts to explain the origin of the moon were initiated by Sir George Darwin. According to his theory, the earth and moon were single rotating bodies that rotated rapidly. Then, the whole mass became a dumbbell-shaped body, and eventually, it broke. This hypothesis also suggested that the material forming the moon was separated from the present-day Pacific Ocean.

However, the present scientists do not accept the dumbbell-shaped hypothesis. It is generally believed that the formation of the moon is an outcome of a giant impact i.e., explained in “the big splash hypothesis”. According to this theory, an object (Theia) size of one to three times mars collided with the earth after the formation of the earth. With this impact, a large part of the earth was blasted into space. This portion consisted of large particles (Disks of debris), that started orbiting around the earth. and eventually, debris accumulated and formed a major clump known as the present-day moon. This theory suggested that the moon formed about 4.44 billion years ago.

 

1.1.5 OUR SOLAR SYSTEM

The solar system refers to the collection of massive bodies that revolve around the sun. it consists of a star i.e., the sun, and other massive bodies that travel around it like planets, asteroids, comets, etc. It is believed that the nebula from which our solar system is formed, started collapsing and forming a core sometime between 5-5.6 billion years from the present, and planets are formed 4.6 billion years ago. The solar system consists of eight planets that are classified as inner and outer planets based on the asteroid belt. The planets near to sun are Mercury, Venus, Earth, and Mars. All these planets are largely made up of iron and rock. These planets are also known as Terrestrial Planets. The outer planets are Jupiter, Saturn, Uranus, and Neptune, largely composed of hydrogen, helium, ammonia, and methane. These planets are known as Jovian or Giant planets.

ASTEROIDS- these objects are made up of rock and are much smaller than planets although asteroids revolve around the sun like planets. These asteroids are found between the orbits of Mars and Jupiter.

 

 

THE PLANETS

MEAN DISTANCE FROM THE SUN (MILLION KM)

PERIOD OF REVOLUTION [DAYS (D), YEARS (Y)]

PERIOD OF ROTATION

[DAYS (D), HOURS (H), MINUTES(M), SECONDS (S)]

DENSITY IN gm/cm3

SATELLITES

INNER PLANETS

MERCURY

59.9

88 D

58.646 D

5.44

0

VENUS

108.2

224.7 D

+243 D

5.245

0

EARTH

149.6

365.26 D

23H56M4S

5.517

1

MARS

227.9

687 D

24H37M6S

3.945

2

THE ASTEROIDS

OUTER PLANETS

JUPITER

778.3

11.86 Y

9H50M30S

1.33

about 53

SATURN

1427

29.46 Y

40H14M

0.70

about 53

URANUS

2869.6

84.01 Y

10H45M

1.17

27

NEPTUNE

4496.6

164.8 Y

15H48M

1.66

13

 

1.2 EVOLUTION OF THE EARTH

Since, 4.6 billion years, Earth has evolved from being a barren, rocky, and hot object having an atmosphere of hydrogen and helium to its present form of a layered structure in increasing order of density towards the core. These layers are made up of different zones with materials having different characteristics. The layers of the atmosphere have the least density whereas the core has the highest density.

1.2.1 EVOLUTION OF LITHOSPHERE

Earth in the primordial stage was very volatile. Gradually, the density increased as the materials started to separate and get organized. Therefore, heavier metals sink towards the center (like iron) whereas, lighter ones moved towards the surface. As time passes materials solidify and condensed into smaller sizes. This outermost layer is known as the crust. This gave multiple layering in the interior of the earth i.e., Crust, Mantle, and Core.

CRUST- The crust is the outermost solid part of the earth. The crust is divided into two parts i.e., continental crust and oceanic crust. The thickness of the crust varies as the oceanic crust is thinner than the continental crust. The mean thickness of the oceanic crust is 5 Km whereas the mean thickness of the continental crust is 30 Km and it can reach up to 70 Km in the Himalayan Mountain region. The mean density of continental crust is 2.7 g/cm3 and it is majorly made up of granite rock whereas the oceanic crust is made up of basalt rock with a mean density of 3 g/cm3.

MANTLE- Crust, and Mantle separated from Moho’s discontinuity. The mantle can extend to a depth of 2900km with a mean density of 3.4 g/cm3. The upper part of the mantle is known as the asthenosphere. The word “astheno” means weak. It is a kind of liquid form on which plates move. It extends up to 400 Km. It is also the main source of magma that comes out from a vent during a volcanic eruption.

The crust and the asthenosphere combined are known as the lithosphere. The lower mantle is in a solid state.

CORE- The core of the earth is divided into two parts outer and inner core. The outer core is in the liquid state while the inner core is in the solid state. This evidence is acquired from the waves of earthquakes. The core is made up of nickel and iron. Thus, the mean density of the core is very high (5 g/cm3). 

NOTE- The density of the materials increases from the crust to the core.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.2.2 EVOLUTION OF ATMOSPHERE

Earth’s initial atmosphere was composed of hydrogen and helium, the passage of time got replaced by Nitrogen and Oxygen due to the impact of solar winds. This process gets completed through three stages.

STAGE 1- MODIFICATION OF ATMOSPHERE THROUGH SOLAR WINDS

The first stage of the formation of the Atmosphere is known as the loss of primitive atmosphere. Initially, the earth’s atmosphere is composed of hydrogen and helium. All the terrestrial planets including earth lost their primordial atmosphere due to solar winds.

NOTE- The solar winds refer to the stream of charged particles coming from the outer atmosphere of the sun and traveling in the entire solar system.

STAGE 2- MODIFICATION OF ATMOSPHERE THROUGH EARTH’S INTERIOR

In the second stage, the heat coming from the interior of the earth change and evolve the atmosphere. When the earth started cooling various gases and water vapor was released from the earth’s core. At this time atmosphere consists of water vapor, nitrogen, methane, carbon dioxide, ammonia, and very little oxygen. These gases and water vapor are coming out with the help of volcanic eruptions.

NOTE- Degassing is the process through which the gases were outpoured from the interior of the earth.

STAGE 3- MODIFICATION OF ATMOSPHERE THROUGH PHOTOSYNTHESIS

Further chances occurred in the atmosphere due to the living organism through photosynthesis. Living organisms were dominate only oceans for a long period. It helps in flooding the atmosphere with oxygen. Thus, nitrogen and oxygen dominate the earth’s atmosphere.

1.2.3 EVOLUTION OF HYDROSPHERE

Hydrosphere refers to the total water available on earth. Oceans, seas, rivers, lakes, and glaciers are part of the hydrosphere. The water on our planet is presented in three forms solid, liquid, and gas.

The evolution of the hydrosphere has the following stages-

STAGE 1- AGE OF DEGASSING

The degassing accumulates water vapors and gases into the atmosphere. It is believed that these gases and water vapors are released from within the interior of the earth through volcanic eruptions, hot springs, and geysers.

STAGE 2- AGE OF CONDENSATION AND PRECIPITATION

As the earth cooled, the water vapor starts to condense, and carbon dioxide mixed with rainwater further reduces the temperature causing more precipitation in the form of rain, snow, hail, etc.

STAGE 3- AGE OF RAINWATER HARVESTING

The rainwater collected in depressions gives rise to the present form of the world’s oceans. It takes 500 million years to form oceans and they are 4000 million years old.

Continents and oceans are considered “relief features of the first order”. About 71% of the total surface area of the earth is dominated by oceans. The oceans contain 97% of the total volume of water. Water dominates the southern hemisphere. Thus, the southern hemisphere is known as the “Water Hemisphere”. The ocean of the world can further be divided into two parts i.e., the aqua sphere and cryosphere. The word aqua sphere refers to the liquid form of water and the word cryosphere refers to the solid form of water (Ice), respectively.

WORLD OCEAN

Oceans

Area (106km2)

Average depth (m)

The area of the earth’s surface (%)

PACIFIC OCEAN

181.34

3,940

35.5

ATLANTIC OCEAN

94.31

3,844

18.4

INDIAN OCEAN

74.11

3,840

14.5

ARCTIC OCEAN

12.25

1,117

2.4

 

1.2.4 ORIGIN OF LIFE

       

Scientists found microscopic fossils of blue algae which were traced back from Pre- Cambrian era i.e.,3000 million years old. Thus, it is believed that the origin of life on the planet earth started with unicellular bacteria sometime 3800 million years ago. Since then, it starts evolving into a modern-day man. The evolution from unicellular bacteria to modern-day man is depicted in the above flow chart.