NCERT Summary - Our Solar System

Category : UPSC


Our Solar System


Our Solar system consists. Of eight planets. The nine planet 2003 UB313 has also been recently sighted. The nebula from which our Solar system is supposed to have been formed, started; its collapse and core formation sometimes 5-5.6 billion ago and the planets formed about 4.6 billion years ago. Our solar system consists of the sun (the star), 8 planets 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grams and gases.


A light year is a measure of distance and not of time, Light travels at a speed of 300, 00 km/ second. Considering ‘this, the distances the light will travel in' one year is taken to be one light year this equals to 9.461x1012 km; the mean distance between the sun and the earth is 149,598,000 km in terms of light years, it is 8.311 minutes of a year. Out of the eight planets, mercury, Venus, earth and mars are called as the inner planets as the lie between the sun and the belt of asteroids the other five planets are called the outer planets. Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high.' densities. The rest five are called Jovian or Gas Giant planets. Jovian means Jupiter-like. Most of them are much larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen. All the planets were formed in the same period sometime about 4.6 billion years ago. Some data regarding our solar system are given in the box below.


The Solar System











































About 18

About 17


* Distance from the sun is astronomical unit i.e. average mean distance of the earth is 149,598,000 km= 1

@ Density in gm /

# Radius : Equatorial radius 6378.137 km=1  


The Moon

The moon is the only natural satellite of the earth. Like the origin of the earth, there have been attempts to explain how the moon was formed. In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly rotating body. The whole mass became a dumb-bell-shaped body and eventually it broke. It was also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean. However, the present scientists do not accept either of the explanations. It is now generally believed that the formation of moon, as a satellite of the earth, is an outcome of 'giant impact' or what is described as "the big splat". A body of the size of one to three times that of mars collided into the- earth sometime shortly after the earth was formed. It blasted a large part of the earth into space. This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.


Geological Time Scale



Period Quaternary

Epoch Holocene Plestocene 

Are/Year 0- 10.000 10.000-2 million

Life/Major Extents


Catnozoic (from 65 million year to the presents time)


Pitocene Miocene Oligocene

2-5 million

5-24 million and trees

24-37 Ma

37-68 million

57-65 million

Early Human Ancestor Ape : Flowering Plants Anthropoid Ape Rabbits and Hare Small Mammals: Rats- Mice


Mesozoic 65-245 million Mammals

Creataceous Jurassic Triassic


65-144 million

144-208 million

208-245 million

Extinction of Dinosaurs Age of Dinosaurs frogs and turtles 


Palaeozotic 245-570 million

Permtan Carboniferous Devonian Stlurtan Ordovtctan Cambratan


245-286 million 286-360 million

360-408 million

408-438 million

Repttle dominate-replace amphibtans first trace of life on land: Plants first fish No terrestrial life: Marine Invertebrate


Proterozoic Archean Pre Cambratan 4,800 million



3,800-4,800 million

570 million

570,-2,500 million Soft-bodied arthropods 2,500-3,800 million blue green Algae: Untcellular bacteria Oceans and Continents from-Ocean and Atmosphere are rich in Carbon dioxide


Origin of




5,000 million Origin of


Development of Lithosphere

The earth was mostly in a volatile state during its primordial stage. Due to gradual increase in density the temperature inside has increased. As a result the material inside started getting separated depending on their densities. This allowed heavier materials (like iron) to sink towards the center of the earth and the lighter ones   to   move towards the surface. With passage of time it cooled further and solidified and condensed into a smaller size. This later led to the development of the outer surface in the form of a crust. During the formation of the moon, due to the giant impact, the earth was further heated up. It is through the process of differentiation that   the   earth forming material got separated    into defilement layers. Starting from the surface to the central parts, we have layers like the crust, muffle, outer core and inner core. From the crust to the-core, the density of the material increases.


Minerals & Rocks

The earth is composed of various kinds of elements. These elements are in solid form in the outer layer of the earth and in hot and molten form in the interior. About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminum, ion, calcium, sodium, potassium and magnesium, and the rest is constituted by titanium hydrogen/ phosphorus, manganese, sulphur, carbon, nickel and other elements.


These substances are recognized as minerals. Thus, a mineral is a naturally occurring inorganic substance, tidying an .orderly, atomic structure and a definite chemical composition arid physical properties. A mineral is composed of two or more elements. But, sometimes single element minerals like sulphur, copper, silver/gold, graphite etc. are found. 


The elements in the earth's crust are rarely found exclusively but are usually combined with other elements to make various substances.


Though the number of elements making up-the lithosphere are limited they are combined in many different ways to make up many varieties of minerals. There are at least 2^000 minerals that have been named and identified in the earth crust; but almost all commonly occurring ones are related to six major mineral groups that are known as major rock forming minerals.

The basic source of all minerals is the hot magma in the interior of the earth. When magma cools, crystals of minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks.


Minerals such as coal, petroleum and natural gas are organic substances found in solid, liquid and gaseous forms respectively.


Besides these main minerals, other minerals like chlorite, calcite, magnetic, hematite, bauxite and barite are also present in some quantities in the rocks.



The Major Elements of the Earth's Crust


Sr. No.


By Weight (%)





























Metallic Minerals.

These minerals contain metal content and can be sub-divided into three types:

(i) Precious,- metals: gold, silver, platinum etc.

(ii) Ferrous metals: iron and. other metals often mixed with iron to form various kinds of steel.

(iii) Non-ferrous metals: include metals like copper, lead/zinc, tin aluminum etc.


Hardness- relative resistance being scratched; ten minerals are selected to measure the degree of hardness from 1-10.They are: 1. talc; 2. Gypsum; 3. calcite; 4.fluorite; 5. apatite; 6. felspar; 7. quartz; 8. topaz; 9. corundum; 10. Diamond. Compared to this for example/ a fingernail is 2.5 and glass or knife blade is 5.5.


Non-Metallic Minerals

These minerals Do not contain metal content. Sulphur/ phosphates and nitrates are examples of non-metallic minerals. Cement is a mixture of non-metallic minerals.



The earth's crust is composed for rocks. A rock is an aggregate of one or more minerals. Rock may be hard or soft and in varied colors. For example/ granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white. Rocks do not have definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.

As there is a close relation between rocks and landforms, rocks and soils/ a geographer requires basic knowledge of rocks. There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation. They are: (i) Igneous Rocks- solidified from magma and lava; (ii) Sedimentary Rocks- the result of deposition of fragments of rocks by exogenous processes; (iii) Metamorphic Rocks- formed out of existing rocks undergoing recrystallization.

Igneous Rocks

As igneous rocks form out of magma and lava from the interior of the earth they are known as primary rocks. The igneous rocks (Ignis- in Latin means 'Fire') are formed when magma cools and solidifies. You already know that magma is. When magma in its upward movement cools and turns into solid form it is igneous rock. The process of cooling and solidification can happen in the earth's crust or on the surface of the earth.


Igneous rocks are classified based on texture. Texture depends upon size and arrangement of grains or other physical conditions of the materials. If molten material is cooled slowly at great depths/ mineral grains may be very large. Sudden cooling (at the surface) results in small and smooth grains. Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks. Granite, gabbro, pegmatite/ basalt/ volcanic breccia’s and tuff are some of the examples of igneous rocks.


Sedimentary Rocks

The word 'sedimentary' is derived from the Latin word sediment which means settling. Rocks (igneous/ sedimentary and metamorphic) of the earth's surface are exposed to denudation agents and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction turn into rocks. This process is called lithification. In many sedimentary rocks/ the layers of deposits retain their characteristics even after lithification. Hence/ we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.


Depending upon the mode of formation, sedimentary rocks are into three major groups: (i) mechanically formed- sandstone, conglomerate, limestone, shale, loess etc. are examples; (ii) organically formed- geyserite’s, chalk, limestone, coal etc. are some examples; (iii) chemically formed- chart, limestone, halite, potash etc. are some examples.


Metamorphic Rocks

The word metamorphic means 'change of form these rocks form under the action of pressure, volume and temperature (PVT) changes. Metamorphism. Occurs when rocks are forced down to lower levels by tectonic processes/or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected 'to great amounts of pressure by overlying rocks, Metamorphism is process by which already consolidated rocks undergo recrystallization arid reorganization of materials within original rocks.


Interior of the Earth

The earth's radius, is 6,370km. No one can reach the center of the earth and make observations of collect samples of material. "Under such conditions, you may wonder how scientists tell us about me earth's interior and the type of materials that exist at such depth. Most of our knowledge about the interior of the earth is largely based on estimates and inferences. Yet, a part of the information; is obtained through direct observations and analysis of materials.


Direct Sources

The most easily available solid earth material is surface rock or the rocks we get from mining areas. Gold mines in South Africa are as-deep as 3-4 km. going beyond this depth is not possible as it is very hot at mis depth; besides mining, scientists have. Taken up a number of Projects to penetrate deeper depths to explore the conditions in the crustal portions scientist world over are working on two major project” such as “Deep Ocean Drilling Project" and "integrated Ocean Drilling project". The deepest drill at Kola, in Arctic Ocean, has so far reached a .depth of 12 km. This and many deep drilling projects have provided large volume of information through the analysis of materials collected at different depths.


Volcanic eruption forms another source of obtaining direct information. As and when the molten material (magma) is thrown onto the surface of the earth/during volcanic eruption it becomes available for laboratory analysis. However, it is difficult to ascertain the depth, of the source of such magma.



Indirect Sources

Analysis of properties of matter indirectly provides information about the interior. We know through the mining activity that temperature and pressure increase with the increasing distance from the surface towards the interior in deeper depths. Another source of information are the meteors that at time reach the earth. The other indirect sources include gravitation, magnetic field, and seismic activity.




The study of seismic waves provides a complete picture of the layered interior. An earthquake in simple words in shaking of the earth. This is a natural event. It is caused due to release of energy, which generates waves that ravel in all directions.


Why does the earth shake?

The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along ‘a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together. However their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly. This causes a release of energy, and the energy wave’s travel in all directions. The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocenter. The energy waves travelling in different directions reach the surface. The point on the surface, nearest to the focus, is called ‘epicenter. It is the first one to experience the waves. It is a point directly above the focus.



Earthquake Waves

All natural earthquakes take place in the lithosphere. It is sufficient to note here that the lithosphere refers to the portion of depth up to 200 km from the surface of the earth.  An instrument called 'seismograph' records the waves reaching the surface. Note that the curve shows three distinct sections each representing different types of wave patterns. Earthquake waves are basically of two types- body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves. The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.


There are two types of body waves. They are called P and S-waves. P-waves move faster and are the first to arrive at the surface. These are also called 'primary waves'. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials. S-waves arrive at the surface with some time lag.        

These are called secondary waves. An important fact about S-waves is that they can travel only through solid materials. This characteristic of the S-waves is quite important.


It has helped scientists to understand the structure of the interior of the earth. Reflection causes waves to rebound whereas refraction makes waves move in different directions. The variations in the direction of waves are inferred with the help of their record on seismograph. The surface waves are the last to report on seismograph. These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.

Propagation of Earthquake Waves

Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass. P- Waves vibrate parallel to the direction of the wave.


This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material. Other three waves vibrate perpendicular to the direction of propagation. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass. Surface waves are considered to be the most damaging waves.


Emergence of Shadow Zone

Earthquake waves get recorded in seismographs located at far off locations. However, there exist some specific areas where the waves are not reported such a zone is called the ‘shadow zone’. The study of different events reveals that for each earthquake, there exists an. altogether different shadow zone.


It was observed that seismographs located at any distance within  from the epicenter, recorded the arrival of both P and S-waves. However, me seismographs located beyond from epicenter, record the arrival of P-waves, but not that of S-waves. Thus, a zone between 105° and 145° from epicenter was identified as the shadow zone for both the types of waves the entire zone beyond does not receive S-waves. The shadow zone of S-wave is much larger than that of the P-waves. The, shadow zone of P- waves appears as a band around the earth between and 145° away from' the epicenter. The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface.


Measuring earthquakes

The earthquake events are scaled either according to the magnitude or intensity of the shock. The magnitude scale is known as the Richter scale. The magnitude related to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10.The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.


Though the actual quake activity lasts for a few seconds. Its effects are devastating provided the magnitude of the quake is more than 5 on the Richter scale.


Structure of the Earth The Crust

It is the outermost solid part of the earth. It is brittle in nature. The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km whereas that of the continental is around 30 km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region.

It is made up of heavier rocks having 'density of 3 g/cm3. This type of rock found in the oceanic crust is basalt. The mean density of material in ‘oceanic crust is 2.7 g/ cm3.


The Mantle

The portion of the interior beyond the crust s called the mantle. The mantle extends from Moho's discontinuity to a depth of 2,900 km. The upper portion of the mantle is called asthenosphere. The word astheno means weak. It is considered to be extending up to 400 km. It is the main source of magma that finds its way to the surface during volcanic eruptions. It has s density higher than the crusts (3.4 g/cm3). The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lower mantle extends beyond the asthenosphere. It is in solid state.


The Core

As indicated earlier, the earthquake wave velocities helped in understanding the existence of the core of the earth. The core mantle boundary is located at the depth of 2,900 km; the outer core is in liquid state while the inner core is in solid state. The density of material at the mantle core boundary is around 5 g/cm3 and at the center of the earth at 6,300 km. the density value is around 13 g/cm3. The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the knife layer.   


Volcanoes and Volcanic landforms

A volcano is a place where gases, ashes and or molten rock material-lava- escape to the ground. A volcano is called an active volcano if the materials mentioned are being released or have been released out in the recent past. The layer below the solid crust is mantle. It has higher density than that of the crust. The mantle contains a weaker zone called asthenosphere. It is from this mat the molten rock materials find their way to the surface. The material in the upper mantle portion is called magma. Once it starts moving towards the crust or it reaches the surface, it is referred to as lava. The material that reaches the ground includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon.



Volcanoes are classified on the basis of nature of eruption and the form developed at the surface. Major types of volcanoes are as follows:


Shield Volcanoes

Barring the basalt flows, the shield volcanoes the largest of all the volcanoes on the earth. The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason, these volcanoes are not steep. They become explosive if somehow water gets into the vent; otherwise, they are characterized by low-exclusivity. The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into cinder cone.


Composite Volcanoes

These volcanoes are characterized by eruptions of cooler and mokre viscous lavas than basalt. These volcanoes often result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground. This material accumulates in the vicinity of the vent openings leading, to formation of layers, and this makes the mounts appear as composite volcanoes.



These are the most explosive of the earth's volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying fhe lava is not only huge but is also in close vicinity.


Flood Basalt Provinces

These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km. of thick basalt lava flows. There can be a series of flows with some flows attaining thickness of more than 50 m. Individual flows may extend for hundreds of km. The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much large flood basalt province. It is believed that initially the trap formations covered a much larger area than the present. Mid-Ocean Ridge Volcanoes: These volcanoes occur in the oceanic areas. There is a system of midocean ridges more than 70,000 km long that stretches through all the ocean basins. The central portion of this ridge experiences frequent eruptions.



Volcanic Landforms

Intrusive Forms: The lava that is released during volcanic eruptions on cooling develops into igneous rocks. The cooling may take place either on reaching the surface or also while the lava is still in the crustal portion; Depending on the location of the cooling of the lava, igneous rocks are classified as volcanic-rocks (cooling at the surface) and plutonic rocks (cooling in the crust) the leva that  cools within the crustal portions assumes different forms. These forms are called intrusive forms.


Batholiths: A large body of magmatic material that cools in the; deeper depth of the crust develops in 'the form of large domes. They appear on the surface only after the denudation processes remove the overlying materials. They cover large areas and at times, assume depth that may be several km. These are granitic bodies. Batholiths are the cooled portion of magma chambers.


Laccoliths: These are large dome- shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It resembles the surface volcanic domes of composite volcano, only these are located at deeper depths. It can be regarded as the localized source of lava that finds its way to the surface. The Karnataka plateau is spotted with domal hills of granite rocks. Most of these/ now exfoliated, are examples of laccoliths or batholiths.


Lapolith, Phacolith and Sills

As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called lapolith. A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticline in folded igneous country. Such wavy materials have a definite conduit to source beneath in the form of magma chambers (subsequently developed as batholiths). These are called the phacoliths.


The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.

Dykes: When the lava makes its way through cracks and -the fissures developed in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes. These are the most commonly found intrusive forms in the western Maharashtra area. These are considered the feeders for the eruptions that led to the development of the Deccan traps.


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