The earth is the only known planet with developed life in the universe. Like most of the celestial bodies, the earth is spherical in shape. You also know that hot water and molten lava eject out from the earth’s interior. This indicates that the temperature below the earth’s surface is very high. World’s deepest mining is limited only to the depth of less than 5 kilometers. These activities can be explained by getting a better understanding of Earth’s interior. As we know that the land features seldom retain any fixed form. Their shape is constantly changing. One group of exogenetic forces includes those which weaken and disintegrate the rocks at their original location. The second group consists of indogenetic forces which remove the disintegrated rocks from high lands and deposit them in the Low lands. These two processes have been responsible for disintegrating rocks and shaping new landforms. They are also partly responsible for the formation of soil, which is very important for us.  

In this lesson, we will study about the earth’s interior and the materials that form the upper portion of the earth’s crust. You will also learn about, weathering and its types, the process of gradation and the significance and formation of soils.

OBJECTIVES

After seen this video, you will be able to:

• explain the limitations of direct observations of the earth’s interior;
• compare the different layers of the earth’s interior with reference to thickness,
• temperature, density and pressure;
• distinguish between rock and mineral;
• classify rocks according to their mode of formation;
• describe the economic significance of rocks;
• explain the term weathering and describe its types with suitable examples;
• explain the various gradational processes changing the face of the land;
• differentiate between degradation and aggradations;
• relate weathering with soil formation and
• explain the various factors contributing to soil formation;

Earth’s interior and its material (Part-1st)

EARTH’S INTERIOR

It is not possible to know about the earth’s interior by direct observations because of its huge size and the changing nature of its internal composition. Through mining and drilling operations we have been able to observe the earth’s interior directly only up to a depth of few kilometers. The rapid increase in temperature below the earth’s surface is mainly responsible for setting a limit to direct observation inside the earth. The temperature in the earth’s interior is so high that it can even melt any tool used for drilling. This fact also restricts deep drilling, thus causing hindrance to direct observation of the materials of the earth’s interior.

STRUCTURE OF THE EARTH’S INTERIOR

The above diagram (see fig. 2.1) shows the concentric layers of the earth’s interior. The innermost layer surrounding the earth’s centre is called core, which is about 3500 kms in radius. Core is the most dense layer of the earth with its density range from 9.5 to 14.5 and sometimes even higher. It is composed mainly of the iron and nickel thus commonly known as Nife. (Nickel+Ferrum). Core consists of two sub layers. The inner one is solid (C2 of fig. 2.1) and the outer one is semiliquid (C1 of fig. 2.1). The layer surrounding the core is known as mantle, a rock shell about 2900 kms thick and is composed of basic silicates. Major constituent elements of mantle are magnesium and silicon, hence, this layer is termed as Sima (Silica+Magnesium).

TEMPERATURE, PRESSURE AND DENSITY OF THE EARTH’S INTERIOR

Temperature

Rise in temperature with increase in depth is observed in mines and deep wells. These evidences along with molten lava erupted from the earth’s interior, support that temperature increases towards the centre of the earth. The different observations show that the rate of increase of temperature is not uniform from the surface towards the earth’s centre

It is faster at some places than at others. In the beginning this increase is at an average rate of 10C for every 32 metres increase in depth. At such a constant rate of increase in temperature, at 10 km depth, the temperature will be approximately 3000C and at 40 km depth it will be 12000C. At this rate, earth’s interior should be in a molten state. Yet it is not so because the rocks buried under the pressure of several km thickness of overlying rocks melt at higher temperature than similar rocks at the surface. A basaltic lava rock which melts at 12500C at the surface will melt at 14000C at 32 km depth. The extra heat required for melting is produced by radioactivity. It is the result of breakdown of atomic nuclei of minerals emitting radiant energy in the form of heat from the rocks.

Pressure

The pressure also increases from the surface towards the centre of the earth due to huge weight of the overlying rocks. Therefore in deeper portions, the pressure is tremendously high. The pressure near the centre is considered to be 3 to 4 million times the pressure of atmosphere at sea level. At high temperature, the material beneath will melt towards the central part of the earth. This molten material under tremendous pressure conditions acquires the property of a solid and is probably in a plastic state.

Density

Due to increase in pressure and presence of heavier materials towards the earth’s centers, the density of earth’s layers also goes on increasing. Obviously the materials of the innermost part of the earth are very dense as already stated.

MATERIALS OF THE EARTH’S CRUST

The outermost part of lithosphere is called crust. This is the most significant part of the earth because it is occupied by humans. The material of the crust is made up of rocks. The rocks are of different types. They are hard like granite, soft like clay and loose like gravel. Rocks have a great variety of colour, weight and hardness.

Rocks are composed of minerals. They are aggregates or physical mixture of one or more minerals. Minerals on the other hand are made up of two or more elements in a definite ratio. They have a definite chemical composition. Crust is made up of more than 2000 minerals, but out of these, 6 are the most abundant and contribute the maximum to this uppermost part of the earth. These are feldspar, quartz, pyroxenes, amphiboles, mica and olivine.

Granite is a rock and its constituent minerals bound together are quartz, feldspar and mica which make it a hard rock. Change in the ratio of these minerals give rise to granites of different colours and hardness. The minerals containing metals are called metallic minerals. Haematite, a major iron ore is a metallic mineral. Ores are metallic minerals which can be profitably mined. Rocks are of immense economic importance to us.

TYPES OF ROCKS

Rocks differ in their properties, size of particles and mode of formation. On the basis of mode of formation rocks may be grouped into three types:

(a) Igneous
(b) Sedimentary and
(c) Metamorphic

Earth’s interior and its material (Part-2nd)

Igneous Rocks

The word igneous is derived from the Latin word ‘ignis’ meaning fire. Igneous rocks are formed by the cooling of highly heated molten fluid material, known as magma. The word magma is derived from a Greek word which means ‘dough’. It requires a greater quantity of heat to melt the rocks under overlying pressure than
at the surface. We do not know the exact depths at which magma forms but probably it is formed at different depths not exceeding 40 km. Molten rocks produce an increase in volume which is responsible for causing fractures or cracks in the crust. The overlying pressure gets weakened along these openings, thus forcing out the magma through them. Otherwise it can’t escape due to great overlying pressure.

On the basis of their mode of occurrence, igneous rocks can be classified as : extrusive or volcanic rocks and intrusive rocks.

• Extrusive igneous rocks are formed by cooling of lava on the earth’s surface. As lava cools very rapidly on coming out of the hot interior of the earth, the mineral crystals forming these rocks are very fine. These rocks are also called volcanic rocks. Gabbro and basalt are very common examples of such rocks. These rocks are found in volcanic areas. Deccan plateau’s regur soil in India is derived from lava.
• Intrusive igneous rocks are formed when magma solidifies below the earth’s surface. The rate of cooling below the earth’ s surface is very slow which gives rise to formation of large crystals in the rocks. Deep seated intrusive rocks are termed as plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal. Granite and dolerite are common examples of intrusive rocks

(a) Plutonic

(b) Hypabyssal

(c) Volcanic rockmasses

They generally form the core of the major mountains, as shown in this diagram. Their irregular dome shaped roofs sometimes appear on the surface after erosion of millions of years. Sill is the horizontal intrusion of solidified magma
between the layers of pre-existing rocks. Dyke is similarly a more or less vertical formation from few metres to several kilometers in length and from few centimeter to hundreds of metres in thickness.

Sedimentary Rocks

These rocks are formed by successive deposition of sediments. These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic or old sedimentary rock. Sedimentary rocks have layered or stratified structure. The thickness of strata varies from few millimeters to several metres. So these rocks are also called stratified rocks. Generally, these rocks have some type of fossil between their strata. Fossil is the solid part or an impression of a prehistoric animal or plant embedded in strata of sedimentary rocks. Sedimentary rocks are widely spread on the earth surface but to a shallow depth.

This type of formation of consolidated material is termed as mechanically formed sedimentary rock. The consolidation of organic matter derived from plants and animals forms sedimentary rocks of organic origin. Coal and limestone are organic sedimentary rocks. The sediments may also result from chemical reaction. Direct precipitation of minerals from their solution in water may give rise to sedimentary rocks of chemical origin. Gypsum, rock salt and nitre are examples of such sedimentary rocks.

Metamorphic Rocks

Most rocks in mountainous regions show an evidence of change. All these in course of time become metamorphic or changed forms of rocks. Metamorphic rocks are formed under the influence of heat or pressure on sedimentary or igneous rocks. Tremendous pressure and high temperature change the colour, hardness, structure and composition of all types of pre-existing rocks. The process which bring about the change is known as Metamorphism and the ultimate products, formed due to operation of such processes are defined as the Metamrphic rocks.

Temperature, pressure stress and access of chemically reactive substances are the main agents, which are responsible for metamorphism. Heat causes the minerals to recrystallise in the rock. The process of change by heat is called thermal or contact metamorphism. When molten magma or lava comes in contact with surrounding rocks, it bakes them and changes them into metamorphic rocks. Similarly the formation of metamorphic rocks due to tremendous pressure is known as dynamic or regional metamorphism.

Different types of metamorphic rocks are found all over the world. In India, marble is found in Rajasthan, Bihar and Madhya Pradesh, whereas slates are available in plenty in Orissa, Andhra Pradesh and Haryana. In Kangra and Kumaun regions of Himalaya, slates of different colours are found.

ECONOMIC SIGNIFICANCE OF ROCKS

(a) Soils: Soils are derived from rocks. Soils provide suitability for that agricultural products that provide food for mention and provide raw material for many industries.

(b) Building Material: Rocks are the source of types of building material directly or indirectly. Granite, gneiss, sandstone, marble and slates are extensively used in the construction of buildings. Tajmahal is made of white marble, Red Forts of Delhi and Agra, are made of red sandstone. Slates are used for roof purposes in different parts of India.

(c) Mineral Source: Minerals are the foundation of the modern civilization. Metallic minerals provide all metals ranging from very precious gold, platinum, silver, copper to aluminium and iron. These metals are obtained from different rocks.

(d) Raw Material: Certain rocks and minerals are used as raw material for many industries. In cement industry and limestone kilns different type of rocks and minerals are used for production of finished goods. Graphite is used in crucible and pencil manufacturing as raw materials

(e) Precious Stones: Precious stones and metals are obtained from different metamorphic or igneous rocks. Diamond is a precious stone used in jewelry and is a metamorphic rock. Similarly other precious stones like gems, rubies and sapphires are obtained from different type of rocks.

(f) Fuel: Fuel in the form of coal, petroleum, natural gas and nuclear minerals are derived from different rocks.

(g) Fertilizer: Fertilizers are also derived from some rocks. Phosphatic fertilizers are obtained from phosphorite mineral found in abundance in some parts of the world.

WHAT IS WEATHERING?

Weathering is the general term applied to the combined action of all processes that cause rock to disintegrate physically and decompose chemically because of exposure near the Earth’s surface through the elements of weather. Among these elements temperature, rainfall, frost, fog and ice are the important ones. Weathering begins as soon as rocks come in contact with one or more than one elements of weather on the surface of the earth. In nature, generally both the disintegration and decomposition act together at the sametime and assist each other.

Earth’s interior and its material (Part-3rd)

TYPES OF WEATHERING

We can recognize three types of weathering?

1. Physical Weathering
2. Chemcial weathering
3. Biotic weathering

PHYSICAL WEATHERING

(a) Block disintegration – We all know that the successive heating and cooling causes expansion and contraction of the rocks. In hot desert regions, day temperatures are very high while nights are very cold. This high diurnal range of temperature causes successive expansion and contraction of the rocks which tend to enlarge the joints. Finally the rocks disintegrate into smaller blocks. This process is known as block disintegration.

(b) Exfoliation – Rocks are generally poor conductors of heat. As a result of intense heating the outer layers of the rock expand rapidly while the inner layers remain almost unaffected by heat. Due to successive expansion and contraction, the outer layer of the rock subsequently peels off from the main mass of the rock in the form of concentric shells.

(c) Frost Action – One of the most important physical weathering processes in cold climates is frost action, the alternate freezing and melting of water inside the joints of the rocks, splits them into fragments. This is because conversion of water into ice increases the volume of water by 10 percent. In cold regions rocks are disintegrated into small particles through this process. It is called frost action.

CHEMICAL WEATHERING

(a) Oxidation – This is the process in which atmospheric oxygen reacts with the rock to produce oxides. The process is called oxidation. Greatest impact of this process is observed on ferrous minerals. Oxygen present in humid air reacts
with iron grains in the rocks to form a yellow or red oxide of iron. This is called rusting of the iron. Rust decomposes rocks completely with passage of time.

(b) Carbonation – This is the process by which various types of carbonates are formed. Some
of these carbonates are soluble in water. For example, when rain water containing carbon dioxide passes through pervious limestone rocks, the rock joints enlarge due to the action of carbonic acid. The joints enlarge in size and lime is removed in solution. This type of breakdown of rocks is called carbonation.

(c) Hydration – This is the process by which water is absorbed by the minerals of the rock. Due to the absorption of water by the rock, its volume increases and the grains lose their shape. Feldspar, for example, is changed into kaolin through hydration. Kaolin on Vindhyan Hills near Jabalpur has been formed in this manner.

(d) Solution – This is the process in which some of the minerals get dissolved in water. They are therefore removed in solution. Rock salt and gypsum are removed by this process.

BIOTIC WEATHERING

(a) Plants – Plants contribute to both mechanical and chemical weathering. The roots of the plants penetrate into the joints of the rocks. They grow longer and thicker. In this manner they exert pressure on the rocks and the rock joints are thereby enlarged and break into smaller fragments.

(b) Animals – Burrowing animals like earthworms, rats, rabbits, termites and ants breakdown the rocks. These disintegrated rocks can easily be eroded or removed by wind etc. Hooves of animals break the soil and thus assist soil erosion. The role of earthworms and termites is of special significance. According to scientists, there is a possibility of occurrence of about 1,50,000 earthworms in an acre and they can convert 10 to 15 tonnes of rock mass into good soil and bring it to the surface.

(c) Man – Human beings play a very important role in weathering of various rocks. Man breaks a large amount of rocks in the course of his activities, like agriculture, construction of houses, roads etc. He quarries for mining minerals, thus helps in weathering by breaking, weakening and loosening the rocks.

WEATHERING AND SOIL

We have studied the process of weathering and have learnt how different types of land features are produced in areas of different types of climate through this process. Weathering also plays an important role in formation of soil which provides basis for agriculture and world’s food supply.

Mechanical weathering of the surface rocks disintegrates the rock and converts it into a fine powder. These small particles are deposited in layers with the help of water. biotic weathering produces humus. This organic matter is formed through the action of plants and animals which helps in the formation of soil. Various processes of weathering help in giving different colours and properties of soil.

GRADATION

Exogenetic forces are constantly working to bring about leveling or the gradation of land. They attempt to achieve a condition of balance between erosion and deposition which mean a graded position. The above forces operate through the process called the process of gradation. Agents of gradation like rivers, glaciers winds, sea waves and underground water perform their task with the help of the triple action of weathering, erosion and deposition.

(a) Degradation

(b) Aggradation

SOIL AND ITS FORMATION

(A) FACTORS OF SOIL FORMATION

The five factors, which control the formation of soil are parent rock, relief, time, climate and plant and animal organisms. The former three are called the passive factors while the later two are the active factors. The parent material and climate are the most important because these two affect the other factors.

(B) SOIL HORIZONS

Four main horizons are important – A,E,B and C. The A horizon is the upper most horizon and rich in organic matter. Next is the E horizon. Clay particles and oxides of aluminum and iron are removed from the E horizon by downward seeping water, leaving behind pure grains of sand or coarse silt. The B horizon receives the clay particles, aluminum and iron oxides, as well as organic matter washed down from the A and E horizons. Beneath the B horizon is the C horizon, which is not considered part of the soil. If consists of the parent mineral matter of the soil.

SOIL EROSION

(a) Type of Soil Erosion – Soil erosion is of four types: wind erosion, sheet erosion, rill erosion and gully erosion.

(i) Wind Erosion

(ii) Sheet Erosion

(iii) Rill Erosion

(iv) Gully Erosion

SOIL CONSERVATION

Soil is one of the most important natural resources, which sustains different types of lives directly or indirectly. Moreover, soil forming is a slow natural process. The process of soil erosion not only destroys this wonderful gift of nature in a shorter span of time, It creates new problems like floods, damage to roads and rail bridges, hydro electric projects, water supply and pumping stations.

(a) Protection of forests : Indiscriminate felling of trees in the forests has been one of the major causes of soil erosion. Since roots of the trees hold the soil material together, it is desirable to protect these trees from such felling. This has led governments to declare forests as reserved in which felling of trees has been banned. This method of soil conservation is most suited to all types of landscapes. Forests are also harbinger of rain which increases the process of soil formation.

(b) Afforestation : Planting of trees along river courses, waste lands and mountainous slopes is another method of soil conservation. It reduces excessive erosion taking place in these regions. Afforestation is also effective in controlling wind erosion along the desert regions. Tree plantation along desert boundary stops swallowing of agriculture land by desert sands. In our country large scale planting of trees is being carried out in Rajasthan, Haryana, Gujrat and Punjab to control the extension of Thar Desert.

(c) Flood Control : During rainy season, the amount of water in rivers, increases exceedingly which in turn increases soil erosion. Dams are being constructed to control floods and consequently the soil erosion. This can
also be done by diverting river water to dry regions through canals and by other well planned methods of water conservation.

(d) Planned Grazing : Over grazing on hill slopes has helped loosening and washing away of soils in these areas. If grazing is carried out in a planned way it will reduce soil erosion by protecting vegetation cover in these areas which are comparatively more prone to soil erosion.

(e) Bunding: Construction of bunds or obstruction is applied in lands affected by gully erosion. This method is not only helpful in controlling soil erosion but also in maintaining soil fertility, conserving water resources and levelling of sloping lands.

(f) Terracing: To conserve poorly developed thin soils on mountain slopes, terracing is another method. Terracing refers to the construction of terraces across the slope in a mountainous region. This helps in controlling soil erosion and using water resources of these areas economically and effectively for growing different crops on these terraces.