WORLD PHYSICAL

GEOGRAPHY

Unit-1: Geomorphology Origin of the Earth

Internal Structure of Earth

Formation of Continents & Ocean Basins

Rocks

Forces Affecting the Landforms

Mountain Building

Earthquakes & Tsunamis

Volcanism

Plains & Plateaus

Lakes & Swamps

Weathering & Erosion

Landforms & Their Evolution

Unit-2: Oceanography Ocean Bottom Relief Temperature & Salinity of Ocean WatersOcean Currents Tides Coral Reefs

Unit-3: Climatology Earth’s Atmosphere Weather & Climate Air Mass, Fronts & Cyclones Classifi cation of Climatic Regions Climate Change Impacts

Unit-4: Bio-Geography Bio-Geography TO

PICS

MP1-G-WPG-19

13www.iasscore.in

CHAPTER

WORLD PHYSICAL GEOGRAPHY

2

Introduction to Geomorphology The word has been derived from Greek words - “geo” meaning earth, “morphe” meaning form and “logos” meaning discourse. Thus “Geomorphology is a systematic and organised description and analysis of the surface landforms, processes and landscape evolution of the Earth.

Scope of Geomorphology Geomorphology contains in itself various disciplinary study and aspects of study, giving rise to many subdivisions within it. The main subdivisions within geomorphology can be -

Structure - a result of Endogenetic forces. Process - a result of Exogenetic forces Evolution of Landforms

Origin of the Earth Human life is largely infl uenced by the physiography of the region. The physiography of the surface of the earth is largely a product of the processes operating in the interior of the earth. A proper understanding of the physiographic character of a region remains incomplete if the effects of endogenic processes are ignored. Therefore, it is necessary that one gets acquainted with the forces that infl uence landscape development.

Sources of Information about the Interior of the Earth The earth’s radius is 6,370 km. No one can reach the centre of the earth and make observations or collect samples of the material. Under such conditions most of the 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. Volcanic eruption forms another source of direct information. As and when the molten material (magma) is thrown onto the surface of the earth, it becomes available for laboratory analysis. However, it is diffi cult to ascertain the depth of the source of such magma.

INTERNAL STRUCTURE OF EARTH

WORLD PHYSICAL GEOGRAPHY

14 www.iasscore.in

Indirect Sources

Analysis of properties of matter provides indirect information about the interior of the Earth. We know through the mining activity that temperature and pressure increases with the increasing depth from the surface. Moreover, it is also known that the density of the material also increases with depth. Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths. Another source of information is the meteors that at times reach the earth. However, it may be noted that the material that becomes available for analysis from meteors, is not from the interior of the earth. The other indirect sources include:

Gravitation Magnetic fi eld, and Seismic activity

The gravitation force (g) is not the same at different latitudes on the surface. It is greater near the poles and less at the equator. This is because of the distance from the centre at the equator being greater than that at the poles. The gravity values also differ according to the mass of material. The uneven distribution of mass of material within the earth infl uences this value. The reading of the gravity at different places is infl uenced by many others factors. Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part. Seismic Activity: Some of the indirect evidences of seismic activity are:

Study of ancient rocks and parts of interior now exposed to surface due to erosive activity Study of lava erupted from volcanism from the interiors of the earth.

Seismological Evidences

The most authenticate source of knowledge about earth’s interior is through detailed study of earthquake waves. The seismic waves can be classifi ed into two categories:

Surface waves: These waves travel through the surface of the earth. Due to their amplitude, they are most destructive waves causing extensive damage on the surface of the earth.Types of Surface Waves:

Love waves (L-waves) - It is fastest surface wave and move on ground in snake like motion. It is confi ned to surface of the crust. Rayleigh waves - Rayleigh waves rolls along the ground just like a wave roll across a lake or an ocean. Stoneley waves - Stoneley waves propagate along a solid - fl uid boundary and also along solid - solid boundary. Standing waves - Standing waves are the result of free oscillation of earth. When two waves moving opposite to each other interfere standing waves are produced.

Body waves: These waves travel through the interiors of the earth. While travelling through interiors, their characteristics such as velocity and wavelength changes according to the density of the medium in which they are travelling. The body waves are recorded at different seismograph stations located at different places throughout the surface of earth.Body waves can be further categorized into

Primary Waves: Also known as P-waves. These are longitudinal or compressive in nature. These waves can pass through solid as well as liquid medium. The velocity of these waves increases with increasing density and rigidity of the medium. (They travel faster in solid than in liquids) Secondary waves: Also known as S-waves. These are transverse or distortional in nature. These waves cannot pass through liquid medium. Their velocity also increases with increasing rigidity of the medium.

WORLD PHYSICAL GEOGRAPHY

15www.iasscore.in

Nature of Body Waves These waves (both P and S waves) travel faster in rigid medium. Among P and S waves the velocity of P waves is more. These waves while passing from one medium to another medium of different density experiences refraction (bending from original path) similar to the light waves.

Observations from the study of body waves The velocity of body waves initially increases continuously denoting the increasing density of material with increasing depth in the part of outer layer of earth crust. After around 100 km of depth, the velocity of both the waves shows a drastic decrease which denotes the less rigidity of the layer. This layer was named as asthenosphere and is made of plastic material. The body waves velocity increases, again denoting the increasing density with depth in mantle. After certain depth, the S-waves disappear and again re-emerges at surface of earth at an angle of 105°. The area where S-waves are not received is known as S-waves shadow region and lies between 105° on both sides. This concludes the presence of a liquid layer which forms outer core. The P-waves continue its journey and its velocity increases drastically representing very dense material in inner core. Due to high degree of refraction the P-waves are not recorded between 140° and 105°, and hence the region is known as P-waves shadow region. The velocity and wavelength of waves in different regions give a concrete evidence of composition of different layers of interior of earth.

Earth’s Interior Based on all the evidences from Seismic data and their analysis, the earth’s interior has been divided into three layers

WORLD PHYSICAL GEOGRAPHY

16 www.iasscore.in

Crust: This is the outermost layer of the earth. Its depth varies from 16 km – 40 km. It is more thick at continents (30km – 40 km) and its thickness underneath the ocean basin varies from 5-10 kms. At continental crust, the uppermost part is mainly sedimentary rocks followed by granite and gneisses rocks which overlies the basaltic rocks. The oceanic crust however is devoid of sedimentary or granitic cover and mainly consists of basaltic rocks. Thus, continental and oceanic crusts differ in nature where continental crust is mainly granitic while oceanic crust is mainly basaltic in composition. Mantle: This is the intermediate layer below crust. It extends upto 2900 km depth. It is composed of dense and rigid rocks having predominance of minerals like magnesium and iron. These rocks are similar to peridotite. The Mantle can be further divided into upper mantle, intermediate and lower mantle.

Upper Mantle: This is in plastic state and is in a partially molten condition and is known as asthenosphere. The velocity of earthquake decreases in the asthenosphere and hence, this layer is also known as “Low Velocity zone”. This layer is mainly made of peridotite rocks which is composed of olivine mineral. Since the crust rests above this partially molten layer, this has an effect on movement of lithosphere plates. Intermediate Mantle: It extends from 400 km depth to 670 km depth from sea level. Below the partially molten asthenosphere, this is a solid layer constituting intermediate mantle. The rigidity, density and basicity of this layer is more than the upper mantle. This is also known as “mantle transition zone”. This is also the maximum depth, below which no earthquake have been ever recorded. Lower Mantle: It extends from 670 km depth to 2900 km depth from sea level. Density increases further to which reaches to around 9. This is the lowest depth from which a magma can be sourced from and that too in rare cases of hotspot eruption. Since, the source is so deep, the magma sourced is very fl uid in nature.

Core: It is the innermost layer of earth. It is divided into outer core and inner core. Outer Core: It extends from 2900 km to 5100 km depth from sea level. This is primarily made of iron with a small proportion of nickel, which is in liquid condition. At this depth, the S-waves suddenly disappear. Also, the velocity of P-waves abruptly decreases. Despite such a high pressure, outer core is in liquid form because of high temperature and the presence of silicon which decreases the melting point of iron. Inner Core: It lies beyond 5100 km depth. The average density increases to 13. This is mainly composed of pure iron and nickel in solid state. The outer liquid core moving around solid inner core of iron acts as a giant self-exciting dynamo which is responsible for magnetic fi eld of earth.

WORLD PHYSICAL GEOGRAPHY

17www.iasscore.in

Earth’s Interior According to Suess Suess identifi ed three layers of earth – the upper layer, the intermediate layer and the central core. He called these layers as Sial, Sima and Nife.

Sial: This is the outermost layer composed of Silicon(Si) and Aluminium(Al) chiefl y. Its rock composition is mainly granite and gneisses. This layer forms the landmasses or continents. Sima: This is the intermediate layer basically composed of igneous rocks such as basalt and gabro. Its main elements are Silicon (Si) and Magnesium (Mg). Nife: It forms the core of the earth. It consists of heavy metals such as Nickel(Ni) and Iron(Fe).

Earth’s Division on the basis of Mechanical Rigidity

Lithosphere: Lithosphere consists of crust and upper part of upper mantle. It extends up to about 100 km depth. It is brittle in nature and broken into different lithospheric plates. The lithosphere is always moving, but very slowly. It is broken into huge sections called tectonic plates. The extreme heat from the mantle part of the lithosphere makes it easier for the plates to move. Asthenosphere: The asthenosphere is a portion of the upper mantle just below the lithosphere that is involved in plate tectonic movements and isostatic adjustments. In spite of its heat, pressures keep it plastic, and it has a relatively low density. Seismic waves pass relatively slowly through the asthenosphere, compared to the overlying lithosphere mantle, thus it has been called the low-velocity zone (LVZ), although the two are not exactly the same. Barysphere: Barysphere represents the innermost zone of the interior of the earth and extends from 2800 km depth to nucleus of the core. The average density is between 8 and 11.

Discontinuities within the Earth’s Interior:Conrad Discontinuity: The Conrad discontinuity corresponds to the sub-horizontal boundary in continental crust at which the seismic wave velocity increases in a discontinuous way. This boundary is observed in various continental regions at a depth of 15 to 20 km, between outer and inner crust however it is not found in oceanic regions.Mohorovicic Discontinuity: The Mohorovicic Discontinuity, or “Moho,” is the boundary between the crust and the mantle.Repetti Discontinuity: This discontinuity is found between upper and lower Mantle. This is marked by general decrease in velocity of seismic waves between upper and lower mantle.Gutenberg Discontinuity: The Gutenberg discontinuity occurs within Earth’s interior at a depth of about 1,800 mi (2,900 km) below the surface, generally between mantle and core ,where there is an abrupt change in the seismic waves (generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material, and cannot transmit through liquids, so it is believed that the unit above the discontinuity is solid, while the unit below is in a liquid, or molten, form. This distinct change marks the boundary between two sections of the earth’s interior, known as the lower mantle (which is considered solid) and the underlying outer core (believed to be molten).

Geomagnetism

Geomagnetism is the study of Earth’s magnetic fi eld and it has revealed much about the way the Earth works. The existence and characteristics of the fi eld essentially demand that the fl uid outer core be made of electrically conducting material that is convecting in such a way as to maintain a self-sustaining dynamo. Paleomagnetism - The study of the magnetic fi eld as it is recorded in rocks is known as paleomagnetism. The Earth’s magnetic fi eld is generated within its molten iron core through a combination of thermal movement, the Earth’s daily rotation, and electrical forces within the core. These elements form a dynamo that sustains a magnetic fi eld that is similar to that of a bar magnet slightly inclined to a line that joins the North and South Geographic Poles. A compass placed in this magnetic fi eld thus does

WORLD PHYSICAL GEOGRAPHY

18 www.iasscore.in

not point due north, declination measures the angle between the compass reading at any point on the Earth’s surface and true north (measured in degrees). The intensity and structure of the Earth’s magnetic fi eld are always changing, slowly but erratically, refl ecting the infl uence of the fl ow of thermal currents within the iron core. This variation is refl ected in part by the wandering of the North and South Geomagnetic Poles. There have been cases of reversal of magnetic poles of earth in history. This reversal does not take place in physical sense. The strength of magnetic fi eld weakens in due course of time and at one time it almost vanishes. After that the pole starts developing opposite polarity. The Earth’s magnetosphere plays an important role in protecting us from cosmic ray particle radiation, because the incoming ionized particles can get trapped along magnetic fi eld lines, preventing them from reaching Earth.

**********