🧫 Origin of the Earth

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

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Origin of earth

Earth is one of the 9 planets (8 excluding Pluto) orbiting the SUN in the Solar

System. The Universe is composed of several Galaxies. Our solar system is part of Milky

way galaxy which is disk shaped with about 1, 00,000 million stars of varying sizes. Our

solar system consists of 9 planets and 31 satellites, a belt of asteroids.

Theories about origin of Earth

Nebular hypothesis (Kant and Laplace, 1755)

This hypothesis suggests that the solar system formed through the condensation of a

nebula which once encircled the Sun. The outer planets formed first, followed by Mars, the

Earth, Venus, and Mercury. This hypothesis suggests a sequential origin from outermost

planet to innermost. As per this hypothesis, Mars evolved earlier than the Earth. This

hypothesis is widely accepted to explain origin of different planets.

During the past, the entire solar systems existed as a hot gaseous mass called

nebula rotating in space. With time, the gaseous mass (nebula) cooled and contracted. Due

to contraction, there developed a bulge at the equatorial region. This bulge subsequently

separated into several rings. The ring coalesces in the form of a globe and continues to

revolve around the nebula. In similar manner, ten rings were formed of which nine of them

gave rise to nine planets.

One broke down into smaller fragments to form the group of planetoids, while the

remnant of the pre-existing nebula formed the central incandescent mass of the solar

system and is known as the Sun . The planets were originally gaseous but were

subsequently cooled down into liquid and ultimately to the solid state.

Planetismal hypothesis (Chamberlain and Moulton, 1905)

Planets were formed as a result of mutual interaction between the sun and another

star of suitable size. This is the theory of biparental origin of the solar system. This theory

postulates that due to the near approach of a larger star, tidal distortions were raised upon

the surface of the sun and these in conjunction with the eruptive force prevalent in the sun

(known as the solar prominence) brought about a description of the mass of the sun and a

number of gaseous bulbs were shot forth, in space, to great distances.

These gaseous solar materials thus ejected in space were subjected to immediate

chilling, resulting in the formation of a number of minute solid particles known as

planetisimals

These planetisimals continued to rotate round the sun in highly elliptical orbits. The

orbits must have happened to intersect one another and at points of intersection, they must

have collided whereby the small planetisimals continued to coalesce gradually giving rise to

the planets. During collision and coalescence of the planetisimals, large quantities of heat

must have been generated and were dissipated in space before the next collision could

occur and accordingly the planets must have been solid all the time during the growth.

This theory accepts that the collision must have taken place in quick succession and

accumulation of heat might have caused a melting of the masses of the planets.

Gaseous tidal hypothesis (Jeans and Jeffreys, 1925)

This also accepts the idea of biparental origin like that of planetisimal hypothesis but

refuses to consider the disruptive forces in the sun (i.e. the solar prominence) had anything

to do with the formation of the planets.

According to Jeans and Jeffreys, during the ancient past, an extremely large star, while

moving in space chanced to approach the sun. Due to progressive and nearer approach of

the star, a tidal pull was raised on the surface of the sun and this increased in size with the

nearer approach of the passing star. At the stage, where the passing star began to recede,

the tidal pull on the surface of the sun, thus formed, was detached from the body of the sun

in the shape of a spindle bulging near the centre and tapering at both ends. This very large

mass of gas, thus ejected in space, was naturally extremely unstable and was immediately

broken down into a number of small fragments. In all, ten such pieces were formed, nine

giving rise to the planets and one, which was broken down into pieces, to the group of

planetoid. These fragments formed into globular masses revolving round the sun along

definite orbits and cooled down gradually from the gaseous to the liquid and ultimately to

the solid state.

This hypothesis is the most popular one due to do simplicity and capability of

explaining the co-planar placement of the planets and the features relating to the

distribution of mass and density in them and the density stratification that exists within the

earth.

Tectonic plate theory

Tectonic plates are layers of rock inside the earth. This crust and upper mantle form

lithosphere. Under the lithosphere lies a fluid rock layer called asthenosphere. The rocks in

Tectonic plates are able to float upon the asthenosphere. There are seven large and several

small plates. The largest plates include the pacific, North American, Eurasian, Antarctic and

African plates.

Continental drift theory

The huge land masses of the earth are in constant movement. The continents and

the oceans rest upon rock plates. They move relative to each other at rates of few

centimeters a year. Until 200 million years ago, the continents were connected into one

single land mass called Pangaea. This Pangaea splits and pieces drifted apart creating 2 new

continents160 million years ago. The land mass that was to become India separated and

moved north ward (140 million years ago) and hit another landmass leading to the eruption

of the Himalayas. Australia separated from Antarctica (100 million years ago). The two

super continents then pulled apart east and west opening the Atlantic oceans.

The continents are still moving even today. The continental drift makes the Pacific

Ocean smaller, the Atlantic Ocean larger and the Himalayan Mountains taller. The Atlantic

oceans become wider, Mediterranean Sea collapses and Australia reaches equator in 60

million years.

Age of the earth

So far scientists have not found a way to determine the exact age of the Earth

directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed.

Nevertheless, scientists have been able to determine the probable age of the Solar System

and to calculate an age for the Earth by assuming that the Earth and the rest of the solid

bodies in the Solar System formed at the same time and are, therefore, of the same age.

The ages of Earth and Moon rocks and of meteorites are measured by the decay of

long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and

that decay with half lives of 700 million to more than 100 billion years to stable isotopes of

other elements. These dating techniques, which are firmly grounded in physics and are

known collectively as radiometric dating, are used to measure the last time that the rock

being dated was either melted or disturbed sufficiently to rehomogenize its radioactive

elements.

These ancient rocks have been dated by a number of radiometric dating methods

and the consistency of the results give scientists confidence that the ages are correct to

within a few percent. An interesting feature of these ancient rocks is that they are not from

any sort of ‘primordial crust’ but are lava flows and sediments deposited in shallow water,

an indication that Earth history began well before these rocks were deposited.

In Western Australia, single zircon crystals found in younger sedimentary rocks have

radiometric ages of as much as 4.3 billion years, making these tiny crystals the oldest

materials to be found on Earth so far. The source rocks for these zircon crystals have not

yet been found. The ages measured for Earth's oldest rocks and oldest crystals show that

the Earth is at least 4.3 billion years in age but do not reveal the exact age of Earth's

formation.

The best age for the Earth (4.54 Ga) is based on old mineral grains (zircon) with U

Pb ages of 4.4 Ga have recently been reported from sedimentary rocks in west-central

Australia. The oldest dated moon rocks, however, have ages between 4.4 and 4.5 billion

years and provide a minimum age for the formation of our nearest planetary neighbour. The

age of 4.54 billion years found for the Solar System and Earth is consistent with current

calculations of 11 to 13 billion years for the age of the Milky Way Galaxy (based on the

stage of evolution of globular cluster stars) and the age of 10 to 15 billion years for the age

of the Universe.

Geological Time Scale of Earth – Development of Life

Era	Period	Age (m.Yr)	Organism
Archean	Archean	> 3500	Lifeless
Precambrian	Precambrian	-do-	Soft bodied plant & animals
Palaeozoic	Cambrian	500	Algae and shell bearing molluscus
Palaeozoic	Ordovician	400	Molluscus & sea weeds
Palaeozoic	Silurian	360	Land plants & breathing land animals
Palaeozoic	Devonian	320	True fishes and ampibians
Palaeozoic	Carboniferous	280	Coal forming materials , non flowering plants
Palaeozoic	Permian	246	Reptiles
Mesozoic	Triassic	235	Marine life & reptiles
Mesozoic	Jurassic	185	Dinosours
Mesozoic	Cretaceous	139	Many plants & fishes
Cenozoic	Tertiary Eocine	20	Mammals

Col1	Myocine	29	Mammals
	Pliocine	10	Mammals
	Quartanary
	Plistocine	01	Mammals
	Recent time		Mammals

Radius, distance from sun and revolution period

S.No	Planet	Radius (km)	Distance from sun(m.k)	Revolution period (day)
1	Mercury	2439	57.9	88
2	Venus	6052	108.2	245
3	Earth	6378	149.4	365.25
4	Mars	3397	227.9	687
5	Jupiter	71398	778.3	4333
6	Saturn	60000	1427	10743
7	Uranus	23620	2870	30700
8	Neptune	24300	4496.5	60280
9	Pluto	1150	5970	90130

Present condition of the earth

Our earth is supposed to have been formed in similar manner. It must have been

hot and plastic when it was first formed. At present the outside of the earth is quite cool

and solid but the interior is very hot and in a fluid condition. The outer cool layer, which we

know as the earth’s crust rests upon a denser molten substratum in which various gases are

dissolved at high pressure

Interior of Earth

The Earth Ball consists of 3 concentric rings namely Crust, Mantle and Core.

Crust: 5 to 56 km on the surface of Earth. Density of rocks is 2.6 to 3.0 g cc [-1] . Distance: 5

to 11 km in oceans and 35 to 56 km in the continents. The crust has been divided into two

sub-zones called Sial and Sima. The Sial is a heterogeneous mixture of rocks. The Sima is a

homogenous plastic or semi-plastic concentric layer that behaves like a solid. The Sial floats

on the Sima, which in turn floats on the lower concentric layer called the Mantle.

Sial contains about 65-75 % silica. Aluminium is the next important element in the

Sial, represented by the most common rocks like granite, and rhyolite. Silica decreases to

about 50-60 percent in the Sima where aluminium has largely been replaced by magnesium

with minor quantities of iron. Sima is represented by the most common rocks like basalt and

gabbro at the upper level and by olivine rich rocks at the lower level.

Mantle

A massive solid to semi solid layer below the crust; 2900 km in thickness; comprises

mixed metals and silicate and basic rocks with density of 3.0 to 4.5 g cc [-1] .

Core

Innermost portion of Earth, 3500 km in thickness, contains molten metals like Nickel,

iron; average density: 9.0 to 12.0 g cc [-1] .

Exterior of Earth

Solid lithosphere, Liquid hydrosphere and gaseous atmosphere. The atmosphere is of

320 km above the lithosphere / hydrosphere. (70% of Earths surface is covered by water

( Hydrosphere ).

Composition of atmospheric air

By volume (%) By weight (%)

N2 78.08 76.5

O2 20.9 23.1

CO2 0.033 0.04

Other gases 0.93 1.36

(H, NH3, H2S, SO2, O3, He, Argon, Neon, Krypton, Xenon)

Hydrosphere

It is the layer of water surrounding the lithosphere. It is present in the form of seas

and oceans. It covers 70% of the earth leaving only about 30% above sea level. The

surface of the waters of the various seas is in one level in contrast with the surface of the

land. This surface is known as the sea level.

The seawater has a higher specific gravity than terrestrial water due to the salts it

contains in solution. The average density is 1.026, but it varies slightly from place to place.

Sea water contains 3.5% salts (minerals) It is least dense at the places where river enters

the sea and very heavy at places where evaporation is high.

Lithosphere

It is the inner most body within the gaseous and watery envelops. That portion of

the lithosphere, which rises above the seawater, is visible to us and is known as land. The

land is only about ¼ [th] of the total surface of the earth. Most of this land is situated in the

northern hemisphere. The lithosphere consists of two portions, viz.,

1. The upper or outer cool solid surface.

2. The inner hot and molten mass.

It is the heaviest of the three spheres. Its mean density is 5.5 compared to that of

water as one. The outer crust has a density of about 2.5 to 3.0, while the inside core,

consists of much heavier materials.

The outer solid layer, called as the earth’s crust is estimated to be about 10 to 20

miles thick. It consists of the various rocks together with a more or less thin mantle of soil

enveloping them. It is on this crust that life, both animal and plant sustains. The inner

mass, which forms the interior of the earth, is in molten condition. According to one belief,

the whole of the inner core is a molten mass of materials, upto the centre. According to

another view, the interior of the earth consists of a molten magma, about 50 to 100 miles

thick, surrounding a gaseous centre. A gradation exists from the central gaseous nucleus,

through the intermediate molten mass, to the outer solid crust.

The interior of the earth is intensely hot and is at the same time under enormous

pressure. There is progressive increase in temperature with depth. Though, the increase is

not uniform at all depths and all places, there is a rise of 1 [o] F for every 64 feet on an

average. Assuming the temperature to increase at this rate, at a depth of 25 to 50 miles, it

should be sufficiently high to melt all substances known to us. This indicates that the

interior of the earth is in molten condition.

Composition of the earth’s crust

The Earth’s crust is principally compassed of mineral matter. This mineral matter is

made up of various elements combined together to form compounds. Some elements exist

as such without forming compounds. Almost all the elements known to us at present,

except the inert gases, are present in the earth’s crust. The elements do not exist in the

earth’s crust as such. Each element is in combination with one or more other elements to

form definite chemical compounds known as minerals . Many of these minerals in turn

combine together to form aggregates, which we know as rocks . Almost all the mineral

mater is present in the form of rocks in the earth’s crust. Rock is composed of elements,

which in turn are made up of atoms. Out of 106 elements knows, 8 are sufficiently

abundant as to constitute about 99 percent by weight of the Earth’s Crust (upto 16 Km).

The elements are geochemically distributed into five main groups based on their bonding

characters.

 Lithophile elements - which ionize readily or form stable oxyanions, viz. O,Si, Ti,

Fe, Mn, A1, H, Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, B, Ga, Ge, Sn, Sc, Y, F,C1, Br, I, C,

HF,Th, P,V, Nb, Ta, Cr,W, U, Zr, (Mo), (Cu), (Zn), (Pb), (T1), (As), (Sb), (Bi), (S), (Se),

(Te), (Ni), (Co) and rare earths.

 Chalcophile element - which tend to form covalent bonds with sulphide, viz, S,Se,

Te, (Fe), Ni, Co, Cu,Zn, Pb, Mo, Ag,Sb,(Sn), Cd, In,T1,Pb, As,Bi, Re, (Mn), (Ga) and Ge).

 Siderophile elements - which readily form metallic bonds, viz, Fe, Ni, Co,Ru,

Rh,Pd, Ir, Os and Au.

 Atmosphile elements - which tend to remain in atmospheric gases, viz. N, (O) He,

Ne, Ar, Kr, Xe.

 Biophile elements - which tend to be associated with living organisms, viz. C,H,O,N,

P,S,C1, I, B, Ca, Mg, K, Na, Mn, Fe, Zn, Cu, Ag, Mo, Co, Ni, Au, Be, Cd, Se, T1,Sn, Pb, As

and V.

Composition of Earth crust (% by weight)

Non - metallic	Oxygen	O - 2	46.60%	74.32% ( ¾th)
Non - metallic	Silica	Si4+	27.72%	27.72%
Metallic	Aluminium	Al3+	8.13%	¼th of the total
Metallic	Iron	Fe2+	5.00%	5.00%
Metallic	Calcium	Ca2+	3.63%	3.63%
Metallic	Sodium	Na+	2.83%	2.83%
Metallic	Potassium	K+	2.59%	2.59%
Metallic	Magnesium	Mg2+	2.09%	2.09%
Metallic	Others	-	1.41%	1.41%

Eight elements are abundant – 98.6%

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Summary Cheat Sheet

Quick Recall Points

• Nebular hypothesis explains early planetary formation.
• Geologic time scale is essential to understand long-term soil-forming processes.
• Earth's crust development sets parent material conditions for soils.

Exam Traps

• Do not confuse Earth's formation timeline with later soil profile development.
• Parent material origin is geologic; soil properties emerge after pedogenic processes.

References

2 sources • [1] [2]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

FAO Soils Portal

Website