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1. Deep Ocean drilling reveals information through analysis of materials collected at different depth.
2. Volcanic eruption delivers information by the means of molten magma that comes out of Earth’s interiors. But, it's difficult to determine the depth of such magma’s origin.
3. Surface rocks are readily available earth material.
4. Gold mines go to a depth of 5 km on an average, these serve as good opportunities for studying the depths of earth.
1. Temperature and pressure patterns through mining activity: An increase in temperature and pressure with depth means an increase in density as well. Hence it becomes possible to determine the rate of change of characteristics of material of earth. This has lead to the knowledge of the layers of earth.
2. Meteors : These are extra-terrestrial masses reaching the earth’s surface. They have material and structure similar to earth and give information about the materials of which earth is formed of.
3. Gravitation force(g) : The force exerted by the Earth on all things in its range is not same along all latitudes, it is variable over different places. Observations suggest that gravitational force is greater at poles, and lesser at equator. This is due to increased distance from the core. This difference in (g) is also attributed to the uneven material mass distribution.
4. Magnetic surveys: The distribution of magnetic materials gives idea of magnetic field of earth which indicates density and type of material present in the interior of earth.
5. Seismic activity: This gives most important evidences of interior of earth. Earthquakes give a fair idea of interior of earth. We shall look into the details of Earthquake later on.
More than 90 percent of the Earth’s mass is composed of iron, oxygen, silicon, and magnesium, elements that can form the crystalline minerals known as silicates. However, in terms of chemical and mineralogical composition, as in physical properties, the Earth is far from homogeneous. Apart from the superficial lateral heterogeneities near the surface (i.e., in the compositions of the continents and ocean basins), the Earth’s principal differences vary with distance toward the centre owing to increasing temperatures and pressures and due to the original segregation of materials into a metal-rich core, a silicate-rich mantle, and the more buoyant, highly refined crustal rocks. The Earth is geochemically differentiated to a great extent. Crustal rocks contain about twice as much of the rock-forming element aluminum as does the rest of the solid Earth and nearly 50 times as much as uranium. On the other hand, the crust, which accounts for a mere 0.4 percent of the Earth’s mass, contains less than 0.1 percent of its average abundance of iron. Virtually all the iron is concentrated in the Earth’s core.
The increasing pressure with depth causes phase changes in crustal rocks at depths of roughly 60 kilometres, marking the boundary of the upper mantle. This transition area between crust and mantle, called the Mohorvicic discontinuity, is prominently revealed by seismic wave analysis. It is believed that most basaltic magmas are generated near the base of the upper mantle at a depth of about 400 kilometres. The upper mantle, which is rich in the greenish mineral olivine, shows significant lateral in homogeneities. Nearly 50 percent of the body of the Earth, down to a depth of 2,890 kilometers, consists of the lower mantle, which is composed chiefly of magnesium- and iron-bearing silicates, including high-pressure phases of olivine and pyroxene. The mantle is not static but rather slowly convects. One important feature is the production of temporary super plumes (huge, rising jets of hot, partially molten rock), which may originate as deep as the heterogeneous layer near the core-mantle interface. Much larger than ordinary thermal plumes, such as that associated with the Hawaiian Island chain in the central Pacific, super plumes may have had profound effects on the Earth’s geologic history and even on its climate. One outburst of global volcanism that began about 125 million years ago and lasted through most of the Cretaceous Period may have been associated with melting at the tops of one or more giant plumes that rose in the mantle beneath the Pacific and Indian plates.
About one-third of the Earth’s mass is contained in its core, most of which is liquid iron alloyed with some lighter, cosmically abundant components (e.g., sulfur or oxygen). Its liquid nature is revealed by the failure of shear-type seismic waves to penetrate the core. However, a small central part of the core, below 5,150-kilometre depth, is solid. Temperatures in the core are extremely hot, ranging from 4,000 to 5,000 K at the outer part of the core to 5,500 to 7,500 K in the Earth’s centre, probably hotter than the surface of the Sun. (Uncertainties in temperature arise from questions as to which compounds form alloys with iron.) The core’s reservoir of heat may contribute as much as one-fifth of all the internal heat that ultimately flows to the surface of the Earth.
Therefore the earth is almost a spherical body, made up of concentric zones. Physical composition of the different zones are
The centre of the earth is occupied by a spherical zone called core, about 3475 km in radius. The innermost part of the core may be solid or crystalline, with a radius of about 1255 km, while the outer core has properties of a liquid. The liquid core is considered to comprise of iron and small proportion of nickel. The temperatures in the earth’s core are between 6000 0C and 7000 0C. Pressures are as high as three to four million times the pressure of atmosphere at sea level.
The mantle lies outside the core, a layer about 2895 km thick, composed of mineral matter in a solid state. It is probably composed largely of magnesium iron silicate, which comprises an ultrasonic rock called dunite. This rock exhibits great rigidity and high density in response to earthquakes that pass through it. It can also adjust to unequal forces acting over great periods of time. The rigid part of upper mantle adjoining the crust is known as Lithosphere. The soft part below that is called Aesthenosphere.
It is the outermost and thinnest layer of the earth’s surface, about 8 to 40 km thick. The base of the crust is sharply defined where it contacts the mantle. This surface of separation between the mantle and the crust is called Moho (Mohorovicic Discontinuity). The crust varies greatly in thickness and composition as small as 5 km thick in some places beneath the oceans, while under some mountain ranges it extends up to 70 km in depth. The rocks of this layer can be subdivided in it (i) basaltic rocks, underlying the ocean basins, containing much iron and magnesium (ii) the rocks that make up the continents which are rich in silicon and aluminum and are higher in color and density.
THE COMPOSITIONS OF THE EARTH’S CRUST (by WEIGHT)
(i) BY ELEMENTS :
Oxygen
47%
Silicon
28%
Aluminium
8%
Iron
5%
Calcium
3.5%
Sodium
2.5%
Potassium
Magnesium
2.2%
Titanium
0.5%
Hydrogen
0.2%
Carbon
Phosphorus
0.1%
Sulphur
(ii) BY OXIDES :
Silica
59.1%
Alumina
15.2%
6.8%
Lime
5.1%
Soda
3.7%
Magnesia
Potash
3.15
Water
1.3%
Titania
1.0%
The other constitutes in order are titanium, hydrogen, phosphorus, barium and strontium. Elements like copper, lead; zinc, nickel and tin are present in very low percentage and are scarce elements.
1. Prof. Edward Suess divides the Earth’s interior into three parts : -
(i) BARYSPHERE – This is the Core or the Central Region of the Earth. The rocks of this layer are composed of Iron and Nickel. The relative densities of these rocks are about 11 or 12. Suess calls this layer as NIFE, this is, NI = Nickel + Fe = Ferum.
(ii) PYROSPHERE – This is the Middle Layer, surrounds the core on all sides. Its relative densities range from 2.90 to 4.75. It is composed of Silicon and magnesium. Suess calls it as SIMA (SI – Silicon + MA – Magnesium). An example of rocks having such density and other properties is Basalt.
(iii) LITHOSPHERE – This is an Outer Layer, surrounds the SIMA. Its relative density ranges from 2.75 to 2.90. It is composed of Silicon + Aluminum. It is called SIAL by Suess (SI – Silicon +AL – Aluminium. This layer is mainly composed of GRANITE.
2. Prof. R.A. Daly divides the Earth into four layers –
(i) CENTROSPHERE – It is crystalline and rigid with density of 11.0.
(ii) MESOSPHERE - It is comparatively rigid and 400 km deep. The average density of this zone is calculated from 9 to 11.0- a mixed zone of iron and silicate material.
(iii) ASTHENOSPHERE – It is the exterior, rigid and about 80 km deep, mostly made up of silicate material, with a density of 3.0.
3. Sir Harold Jeffrey’s divides the Earth’s interior into four parts –
(i) OUTER LAYER – Mostly composed of sedimentary rocks.
(ii) SECOND LAYER - Made up of Granite rocks.
(iii) THIRD LAYER OR INTERMEDIATE – Mostly composed of Thachylyte or Diorite rocks.
(iv) LOWEST LAYER - A mixed layer of Dunite, Peridolite and Eclogite rocks.
4. Prof. Vonder Gracht divided the Earth’s interior into four parts –
(i) OUTER SIALIC CRUST – Depth varies : 60 km under continents and 20 km under Atlantic Ocean. Density 2.75 to 2.9.
(ii) INNER SILICATE AND MANTLE SIMA - The thickness is 60 to 1140 kilometres with a density of 3.1 to 4.75.
(iii) MIXED METALS AND SILICATE ZONE - The depth of this zone is 1140 km to 2900 km with a density of 4.75 to 5.0.
(iv) METALLIC NUCLEUS - 2900 km to 6378 km density 11.0.
5. Prof. Gutenberg divides the Earth’s interior into five parts –
(i) CORE - The depth of the core is between 2902 to 6370 km. The density of the core is calculated at 11.0.
(ii) ULTRABASIC LAYER - This layer extends below MOHOROVICIC discontinuity at the depth of 2900 km.
(iii) BASIC ROCKS - This zone extends between Mohorovicic discontinuity the velocity of the seismic waves increases rapidly. The existence of several such discontinuities of the second order is, for instance, at 1200, 1900 and 2150 km depth.
(iv) GRANITE LAYER – Average depth is 15 to 30 kilometer.
(v) SEDIMENTARY LAYER – This layer is unequally distributed below the continents and oceans. This is also known as upper SIALIC layer with a density of 3.5.
The discontinuity between the Earth’s Crust and MANTLE is known as the MOHOROVICIC DISCONTINUITY, so called after the scientist A. Mohorovicic, discovered it in 1909 while studying a Balkan earthquake. The discontinuity greatly affects the speed at which earthquake waves travel. American scientists made preliminary trials towards drilling a hole through the crust into the mantle (Operation Mohole’, 170 miles north-east of Hawaii, abandoned 1966). The Mohorovicic discontinuity lies at a depth of about 20 miles under the continents, but at only 4 to 6 miles under the oceans.
The discontinuity at a depth of about 2900 km form the surface of the Earth, between the MANTLE and the Core, is called the GUTENBERG DISCONTINUITY, after the scientist B Gutenberg, who discovered it in 1914. At this depth the S-waves of earthquake disappear, while the P-waves travel on at a reduced speed, that is, it is likely that Gutenberg Discontinuity marks a change from a solid to a liquid medium, though of much greater density and under enormous pressure.
Discontinuity
Boundary line
Between
1.
Moho or Mohorovicic Discontinuity
Crust and Mantle
2.
Cornard Discontinuity
Outer and Inner Crust
3.
Weichert-Gutenberg Discontinuity
Mantle and Core
4.
Repetti Discontinuity
Outer and Inner Mantle
5.
Transition Discontinuity
Outer and Inner Core
The Earth’s outermost rigid, rocky layer is called the lithosphere. It is broken, like a slightly cracked eggshell, into about a dozen separate rigid blocks, or plates. There are two types of plates, oceanic and continental. An example of an oceanic plate is the Pacific Plate, which extends from the East Pacific Rise to the deep-ocean trenches bordering the western part of the Pacific basin. A continental plate is exemplified by the North American Plate, which includes North America as well as the oceanic crust between it and a portion of the Mid-Atlantic Ridge, an enormous submarine mountain chain that extends down the axis of the Atlantic basin, passing midway between Africa and North and South America.
The upper layer of the lithosphere is termed the crust. It is composed of low-density, easily melted rocks; the continental crust is predominantly granitic, while the oceanic crust is basaltic. Analysis of seismic waves, generated by earthquakes within the Earth’s interior, show that crustal compositions extend beneath the continents to depths of about 50 kilometres, but only 5 or 10 kilometres beneath the ocean floors. The denser lithospheric plates (60 kilometres thick beneath the oceans and ranging from about 100-200 kilometres beneath the continents) ride on a weak, perhaps partially molten, layer of the upper mantle called the Aesthenosphere.
Slow convection currents deep within the mantle generated by radioactive heating of the interior are believed to drive the lateral movements of the plates (and the continents that rest on top of them) at a rate of several centimeters per year. The plates interact along their marginal zones, and these boundaries are classified into three general types on the basis of the relative motions of the adjacent plates: divergent, convergent, and transform (or strike slip).
In areas of divergence, two plates move in opposite directions. Buoyant upwelling motions force the plates apart at rift zones (such as along the middle of the Atlantic Ocean floor) where magmas from the underlying mantle rise to form new oceanic crustal rocks.
Lithospheric plates move toward each other along convergent plate boundaries. When a continental plate and an oceanic plate come together, the leading edge of the oceanic crust is forced beneath the continental plate (i.e., is subducted). Only the thinner, denser slabs of oceanic crust will subduct, however. When the thicker, more buoyant continents come together at convergent zones, they resist subduction and tend to buckle, producing great mountain ranges. The Himalayas, along with the adjacent Plateau of Tibet, were formed during such a continent-continent collision when India was carried into the Eurasian Plate by relative motion of the Indian-Australian Plate.
At the third type of plate boundary, the transform variety, two plates slide parallel to one another in opposite directions. These areas are often associated with high seismicity, as stresses that build up in the sliding crustal slabs are released to generate earthquakes. The San Andreas Fault in California is an example of this type of boundary, which is also known as a fault or fracture zone.
Most of the Earth’s active tectonic processes, including nearly all earthquakes, occur near plate margins. Volcanoes form along zones of subduction, because the oceanic crust tends to be remelted as it moves into the hot mantle and then rises to the surface as molten lava. Chains of active, often explosive, volcanoes are thus formed in such places as the western Pacific and the west coasts of the Americas. Older mountain ranges, eroded by weathering and runoff, mark zones of earlier plate-margin activity. The oldest, most geologically stable parts of the Earth are the central cores of some continents (such as Australia, southern parts of Africa, and northern North America) where little mountain-building, faulting, or other tectonic processes have occurred for hundreds of millions to billions of years. Because of the stability, erosion has flattened the topography, and geologic evidence of crater scars from the rare, often ancient impacts of asteroids and comets is preferentially preserved. In contrast, much of the oceanic crust is substantially younger (tens of millions of years old), and none dates back more than 200 million years.
It is not known when the original continental cores formed or how long ago modern plate-tectonic processes began to operate. Certainly the processes of internal convection, thermal segregation of minerals by partial melting and fractional crystallization, and basaltic volcanism were operating even more extensively and thoroughly in early epochs. But the assembling of continental landmasses had to compete with giant impacts, which tended to disaggregate them until the impact rate decreased nearly four billion years ago. It is thought that a single super continent that had been created by the amalgamation of many smaller continental cores and island arcs was broken up approximately 500 million years ago into at least three major continents: Gondwana (or Gondwanaland), Laurentia, and Baltica. These three landmasses were widely separated by the so-called Iapetus Ocean (a precursor to the Atlantic). By about 250 million years ago, the continued drifting of these continents resulted in their fusion into a single super continental landmass called Pangea. Some 70 million years later, Pangea began to fragment, gradually giving rise to today’s continental configuration. The distribution is still asymmetric, with continents predominantly located in the Northern Hemisphere opposite the Pacific basin.
The entire conceptual framework in which geologists and geophysicists now understand the evolution of the Earth’s lithosphere is termed plate tectonics (see the article plate tectonics). Analogies from plate tectonics have been applied to understanding surface features on Venus and Mars, as well as to some of the icy satellites of the outer solar system, but with only moderate success.
The Magnetic Field of the Earth is generated by the motion of molten iron alloys in the Earth’s outer core.
The solid inner core is too hot to hold a permanent magnetic field, but the outer core gives rise to Earth's magnetic field. The geomagnetic field extends from outer core to where it meets the solar wind. At the surface of Earth, the magnitude of Earth’s magnetic field ranges from 25 to 65 microteslas (0.25 to 0.65 gauss).
The magnetic field deflects most of the charged particles emanating from the Sun in the form of solar winds. If there were no magnetic field, the particles of the solar wind would strip away the ozone layer, which protects the Earth from harmful ultraviolet rays. One of the reasons that there is no atmosphere at Mars is that its magnetic field is turned off which led to the loss of carbon dioxide due to scavenging of ions by the solar wind.
The Earth’s magnetic field is believed to be caused by electric currents in the liquid outer core, which is composed of highly conductive molten iron. The motion of the fluid is sustained by convection, motion driven by buoyancy. At the core, the pressure is so great that the super hot iron crystallizes into a solid. The higher temperature of the fluid lower down makes it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is played to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This Is called compositional convection.
Based on data from ancient and new rocks, it has been observed that Earth’s north and south
magnetic fields have reversed polarity many times. This is because the polarity of the Earth’s magnetic field is recorded in sedimentary rocks. The switching from north to south (an individual reversal event)seems to take around a couple thousand years to complete; once the reversal takes place, periods of stability seem to average about 200,000 years. Nobody has been able to explain why the poles reverse, but theories range from the changes in lower mantle temperatures to the imbalance of land masses on our world (most of the continental landmass is in the Northern Hemisphere).
The last magnetic reversal was 780,000 years ago, which gives us current northern and southern magnetic poles.It is believed that geomagnetic field is slowing weakening, so Earth might be heading for a long-overdue magnetic reversal. Reversals tend to occur when there is a wide divergence between the magnetic poles and their geographic equivalent
Magnetic dip or magnetic inclination is the angle made with the horizontal by the compass needle of a vertically held compass. This angle varies at different points on the Earth’s surface. In the northern hemisphere, the field points downwards. It is straight down at the North Magnetic Pole and rotates upwards as the latitude decreases until it is horizontal (0°) at the magnetic equator. It continues to rotate upwards until it is straight up at the South Magnetic Pole. North Magnetic Pole on the surface of Earth’s Northern Hemisphere at which the planet’s magnetic field points vertically downwards. In2001, it was in Canada, but now, it has moved out of Canada’s territory towards Russia. The south magnetic pole was off the coast of Wilkes Land — a part of Antarctica — about 2750 km from South Pole.
Geomagnetic Equator & Equatorial Electrojet
Contour lines along which the dip measured at the Earth’s surface is equal are referred to as isoclinic lines.
The locus of the points having zero dip is called the magnetic equator or aclinic line. In the following graphics, the green line shows the magnetic equator, which runs very close the southern tip of our country. This is the important reason for the establishment of the Vikram Sarabhai Space Centre Thumba, which is close to Geomagnetic Equator. The reason is that the magnetic equator differs significantly from the geographic equator. Directly above the magnetic equator, at altitudes of around 110 km in the atmosphere, a system of electric currents exists that flows from west to east along the magnetic equator. It is known as Equatorial Electrojet.The closer we are to the magnetic equator, the better we are placed to study the Equatorial electrojet.In the early 1960s, there were very few places in the world close to the magnetic equator with adequate infrastructure to support research in this field. That is the reason that Thumba was chosen.
Thumba is located in the outskirts of Thiruvananthapuram. Here, Thumba Equatorial Rocket
Launching Station (TERLS) was launched in 1963. Eventually, TERLS have given birth to the
Vikram Sarabhai Space Centre (VSSC) and to the Indian Space Research Organisation (ISRO).
Connected Concept
Structure of the Magnetosphere
The complex structure of Earth’s magnetosphere is the result of the interplay between the charged particles originating in the upper layers of the terrestrial atmosphere, whose motion is guided by the Earth's magnetic field, and the solar wind particles carrying the interplanetary magnetic field.
The Magnetosphere is basically a space filled primarily with particles from terrestrial origin.
The shape of magnetosphere keeps changing throughout the day and night, with Earth’s rotation, revolution and during solar storms and other such events which can affect it.
The solar wind consists of particles that are mainly of solar origin. It is pervaded by the interplanetary magnetic field. A bow shock is formed in front of the Earth’s magnetosphere, which demarcates the region where the solar wind flow is impeded by the presence of the Earth. The solar wind in the magneto sheath, the region between the bow shock and the Earth’s magnetosphere, is forced to flow around the Earth’s magnetosphere and is compressed.
The impermeable outer surface of the magnetosphere, where the total pressure of the compressed solar wind precisely balances the total pressure inside the magnetosphere, is called the magnetopause.
The substance formed of one or more minerals and a part of the Earth’s crust is called rock. It is usual to call the hard and rigid part of the Earth’s crust as rock. Geographically, this definition of rock is incomplete; Rock may be hard, soft, of large size or fine as a small grain and colored or uncolored. For example, granite is hard; graphite is soft. Basalt has a large size; while sand is fine. The rocks have been formed of about 2000 minerals but most of the rocks are composed of six minerals.
Feldspar
Quartz
Orthoclase
Pyroxenes
Plagioclase
Amphiboles
Microcline
Mica
Olivine
(1) Apatite: It is a complex compound of Calcium phosphate. It compound of Calcium phosphate. It is red, brown yellow or green. Phosphorous and Fluorine is obtained from it.
(2) Barite: It is Barium Sulphate. It has a pearly lustre. It occurs in granular or crystalline masses. It is used in glass, rubber, chemical and other industries.
(3) Bauxite: It is noncrystalline and soft and occurs in small pellets. It is a Hydrous Oxide of Aluminium. In pure from it is white and brown. It is also red, yellow and brown if there is an impurity of iron in it. Aluminium is obtained from it.
(4) Calcite: It is calcium carbonate. It is white or colorless. It is an important ingredient of limestone, chalk and marble.
(5) Chlorite: It is hydrous Magnesium Iron. Just like Mica crystals, it has cleavages but unlike Mica it is non-flexible and inelastic. It has a green color.
(6) Cinnabar: It is Mercury Sulphide. It is a soft, heavy, deep red or brown, Mercury is obtained from it.
(7) Corundum: It is Aluminium Oxide. It is hard, heavy, brown or light blue and has six sided crystals. Examples: Ruby and Sapphire.
(8) Dolomite: It is a double carbonate of calcium and Magnesium. It is white. It is used in making cement and is also used in iron and steel industries.
(9) Galena: It is Lead Sulphate. It is black, brittle, heavy and occurs in cubes and lead is obtained from it.
(10) Gypsum: It is hydrous Calcium Sulphate: It is soft, white or colorless and pearly in crystals. Many idols and objects of arts are made of it.
(11) Hematite: It is red oxide of Iron. It is found in sedimentary rocks iron is obtained from it.
(12) Kaolinite: It is known as China clay. It is hydrous Aluminium silicate. It is found in light colours in the wet condition, it is plastic and can be converted into any shape. It is used for making paper, china clay, tiles and crockery.
(13) Manganese: It is white and noncrystalline. It is known as magnesium Carbonate. It is used as refractory material in furnaces.
(14) Magnetite: It is black Iron Oxide. It is found in igneous rocks. Iron is obtained from it.
(15) Pyrite: It is Iron Sulphide. It is hard, yellow and brittle, Iron and Sulphuric acid are obtained from it.
(16) Talc: It is hydrous Magnesium Silicate. It is not, soapy, white or brown or light yellow. It is used for making paints, rubber, crockery, roofs, paper, powder, plastic and insecticide.
On the Basis of Formation, the rocks are composed of minerals. Beside minerals the structure, form etc., also depend upon their mode of formation. On the bases of formation the rocks are divided into three classes:
(i) Igneous Rocks
(ii) Sedimentary Rocks
(iii) Metamorphic Rocks
The igneous rocks were formed of the solidification of the molten material of the earth’s crust at a depth ranging from 16 to 20 km. from the surface of the Earth, the rocks are in a molten form and are known as Magma. When this magma rises and comes into contact with cooler parts, it is converted into solid form. This solidified magma is called igneous rock.
Properties of Igneous Rocks: The following properties are found in the igneous rocks:
1. Magmatic consolidation
2. Both Crystalline and Non-Crystalline
3. Nonporous.
4. Poorly eroded.
5. Content of Silica.
6. Non Fossils.
On the basis, the igneous rocks are divided into subgroups:
i. Acid Igneous Rocks: It has an excess of acid forming radical rocks and silica, the rest has Magnesium, sodium, Potassium, etc. It contains orthoclase, Feldspar, quartz and small amount of mica, Iron and Magnesium.
ii. Basic Igneous Rock: The silicon content in this rock is below 40%. It has 40% Magnesium, and the rest 20% contains Iron, Aluminium and Potassium.
Some of the igneous rocks cool down on the surface of the Earth but most of them remain confined below the surface. On this basis, the rocks are divided into two subgroup.
i. Extrusive Igneous Rocks
ii. Intrusive Igneous Rocks
The rocks can be divided into three types according to the depth of their situation:
i. Minor Igneous Rocks.
ii. Intermediate Igneous Rocks.
iii. Major Igneous Rocks.
The hot magma, which rushes out of the interior, gets accumulated wherever it gets space and cools into various forms. Some of the main forms are described here.
i. Laccoliths
ii. Batholiths and Stock
iii. Lapolith
iv. Phacolith
v. Sills and Dykes
In the 16 km., thick crust of the earth, 95% rocks are igneous and 5% sedimentary. However 80% of the surface rocks are sedimentary and the rest are igneous. Weathering and erosion take place on the earth’s surface. This loosens the particles of the rocks and is then transported from one place to another and deposited as layers overlying each other. These layers hardened, stratified and consolidated are known as sedimentary rocks.
Calcareous Rocks
Sand Stone
Clayey Rock
Carbonaceous Rock
Conglomerate Rocks
Breccia
Aqueous Rocks
Lacustrine Rocks
Riverine Rocks
Chemically Formed Rocks
Glacial Rocks
Aeoline Rocks
The sediment, which forms sedimentary rocks, is divided into the following groups:
i. Land derived
ii. Organic
iii. Volcanic
iv. Magmatic
v. Meteoritic.
Sedimentary rocks have different characteristics from igneous rocks. The following are the main characteristics of Sedimentary rocks:
i. Stratification
ii. Fossilization
iii. Porosity
iv. Marks and Imprints
v. Rapid Erosion
These rocks are formed of igneous as well as of sedimentary rocks but are different from both of them. The formation of rocks takes place in two ways:
i. By high pressure and high temperature.
ii. By basic change in the structure of rocks.
1. Orogenic (Mountain building)
2. Iava Inflow
3. Geodynamic Forces
4. Action of Underground Water
IGNEOUS
METAMORPHIC
(i)
Granite
Gneiss
(ii)
Basalt
Amphibolites
(iii)
Gabbros
Serpentine
SEDIMENTARY
Sandstone
Quartzite
Limestone
Marble
Shale
Slate
(iv)
Coal
Graphite, Diamond
REMETAMORPHOSED
Schist
Phylite
METAMORPHISM is the processes by which an already consolidated rock undergoes changes in or modifications of texture, composition or structure, either physical or chemical.
These changes may be brought about by-
(i) The PRESSURE involved in earth-movements (DYNAMIC or REGIONAL Movement)
(ii) The HEAT, caused by the intrusion of mass of molten rock (THERMAL or CONTACT MOVEMENT)
Hence METAMORPHIC ROCKS, tend to be compact and resistant to denudation, and form masses involved within areas of MOUNTAIN-BUILDING.
Metamorphism is classified on the basis of agency that brings about metamorphism and on the Zones of influence where it is caused.
I. On the basis of Agency: Metamorphism is caused by thermal and dynamic agencies.
a. Thermal metamorphism
b. Dynamic Metamorphism
II. On the bases of zones of Influence: On this basis the metamorphism is of two types:
a. Contact Metamorphism
b. Regional Metamorphism
i. Slate
ii. Phyliite
iii. Schist
Examples of metamorphic rocks metamorphosed from basic rocks.
Like land forms themselves, rocks do not remain in their original form indefinitely but instead are always in the process of transformation.
Igneous rocks are basic rocks and from the first stage of rock cycle. When magma is cooled, IGNEOUS ROCKS are formed. Igneous rocks can be returned to a molten condition (MAGMA) through the addition of heat, or they can be changed into METAMORPHIC ROCKS, through the application of heat, pressure and/or chemical action, or their weathered particles may form the basis of SEDIMENTARY ROCKS.
SEDIMENTARY ROCKS can be formed from the weathered particles of either Igneous or Metamorphic rocks.
Finally, METAMORPHIC ROCKS can be created out of either igneous or sedimentary rocks. In addition Metamorphic rocks can be heated sufficiently be become MAGMA.
A supercontinent is a single continental landmass made of all or most of the continental lithosphere at the time. There seems to be a cycle of supercontinents that form and split up every 400 or 500 million years, driven by plate tectonics. Scientists estimate that there was a cycle of at least seven supercontinents on Earth. The last supercontinent, the famous Pangea, formed around 300 million years ago and broke up about 100 million years later. Within several hundreds of million years all the continents should join together to form once again a new supercontinent.
The concept of continental drift was first put forward in 1915 in a book entitled The Origin of Continents and Oceans by the German meteorologist Alfred Wegener, who recognized that continental plates rupture, drift apart, and eventually collide with one another.
According to Wegner, all the continents were compact and one in the Paleozoic era. This Super and compact continent was called Pangaea .This land was surrounded by a giant ocean known as Panthalassa. The Pangea was composed of light Sial material while the oceans consisted of heavy Sima. Wegner does not subscribe to the view that the whole earth had a covering of sial. The continents had a sea in its middle part. This sea was called Tethys by Wegner. Laurentia land mass was situated to the north of Tethys and consisted of North America, Europe and Asia. Gondwana land was another landmass situated to the south of the Tethys and comprised Australia. Africa, South America, Antarctica and Peninsular India.
In those days, the landmasses were covered with shallow peninsular seas. The South Pole existed in the middle of Pangea near the South African coast (Natal). About 200 million years ago, Pangaea began to break up into two large masses called Gondwanaland and Laurasia, which in turn separated into the continents as they are today, and which have drifted to their present locations. According to Wegner, the Pangea broke up in carboniferous period (Mesozoic Era) and began to drift.
The continents had two directions of drift:
(i) Towards the Equator.
(ii) Towards the West.
The movement towards the Equator is considered to be caused by the relation of the forces of gravitation and Buoyancy. The centre of gravity and the centre of buoyancy existed in such a position that the continents began to drift towards the Equator. The movement towards the west is ascribed to the tidal forces affecting the Earth.
The drift in this way caused the redistribution of continents and ocean basins resulting in the present form and shape. According to Wegner, the drift has not come to a stop but is still continuing.
In the early 1960s scientists discovered that most earthquakes occur along lines parallel to ocean trenches and ridges, and in 1965 the theory of plate tectonics was formulated by Canadian geophysicist John Tuzo Wilson; it has now gained widespread acceptance among earth scientists who have traced the movements of tectonic plates millions of years into the past. In 1995 US and French geophysicists produced the first direct evidence that the Indo-Australian plate has split in two in the middle of the Indian Ocean, just south of the Equator. They believe the split began about 8 million years ago.
Theory formulated in the 1960s to explain the phenomena of continental drift and seafloor spreading, and the formation of the major physical features of the Earth's surface. The Earth's outermost layer, the lithosphere, is regarded as a jigsaw puzzle of rigid major and minor plates that move relative to each other, probably under the influence of convection currents in the mantle beneath. At the margins of the plates, where they collide or move apart, major landforms such as mountains, volcanoes, ocean trenches, and ocean ridges are created. The rate of plate movement is at most 15 cm per year.
The concept of plate tectonics brings together under one unifying theory many previously unrelated phenomena observed in the Earth's crust. The size of the crust plates is variable, as they are constantly changing, but six or seven large plates now cover much of the Earth's surface, the remainder being occupied by a number of smaller plates. Each large plate may include both continental and ocean crust. As a result of seismic studies it is known that the lithosphere is a rigid layer extending to depths of 50-100 km/30-60 mi, overlying the upper part of the mantle (the asthenosphere), which is composed of rocks very close to melting point, with a low shear strength. This zone of mechanical weakness allows the movement of the overlying plates. The margins of the plates are defined by major earthquake zones and belts of volcanic and tectonic activity, which have been well known for many years. Almost all earthquake, volcanic, and tectonic activity is confined to the margins of plates, and shows that the plates are in constant motion.
Plates ranges in size from 10,000 square Km and 100,000,000 square km. There are seven major plates (above 10,000,0000), Eight intermediate sized plates (1,000,000 km to 10,000,000) and 20 minor plates (10,000 km top 1,000,000 km).
Major Plates
African Plate.
Antarctic Plate.
Eurasian Plate.
Indo-Australian Plate.
North American Plate.
Pacific Plate.
South American Plate.
Minor Plates
Arabian Plate.
Caribbean Plate.
Cocos Plate.
Juan de Fuca Plate.
Indian Plate.
Nazca Plate.
Philippine Sea Plate.
Scotia Plate
Plate tectonic theory deals with the movement of rocks structures. These plates are basically moved in three different directions, such as:
Divergent Plates: it is also known as Constructive Margins
Convergent Plates: also known as Destructive Margins
Parallel Plates: also known as Conservative Margins.
Where two plates are moving apart from each other, molten rock from the mantle wells up in the space between the plates and hardens to form new crust, usually in the form of an ocean ridge (such as the Mid-Atlantic Ridge). The newly formed crust accumulates on either side of the ocean ridge, causing the seafloor to spread; the floor of the Atlantic Ocean is growing by 5 cm2 in each year because of the welling-up of new material at the Mid-Atlantic Ridge.
Where two plates are moving towards each other, the denser of the two plates may be forced under the other into a region called the subduction zone. The descending plate melts to form a body of magma, which may then rise to the surface through cracks and faults to form volcanoes. If the two plates consist of more buoyant continental crust, subduction does not occur. Instead, the crust crumples gradually to form ranges of young mountains, such as the Himalayas in Asia, the Andes in South America, and the Rockies in North America. This process of mountain building is termed orogenesis.
Sometimes two plates will slide past each other - an example is the San Andreas Fault, California, where the movement of the plates sometimes takes the form of sudden jerks, causing the earthquakes common in the San Francisco-Los Angeles area. Most of the earthquake and zones of the world are found in regions where two plates meet or are moving apart.
Before the plate tectonics theory was proposed, it was believed that the earth is a solid motionless body. But the concept of seafloor spreading and plate tectonics have highlighted that the earth’s crust is not static but dynamic and constantly evolving due to relative motion of tectonic plates.
The movement of plates has been assigned various reasons by various scientists. Wegener suggested that the drifting in plates was due to ‘pole fleeing force’ generated due to rotation of earth or due to tidal forces generated by gravitational pull of sun and moon.
However, most of the scientists believe that these forces are too weak to generate any considerable motion in plates. Instead it is the convection currents in the mantle that are responsible for movement of plates. The temperature differences[1] in mantle are responsible for convection currents of hot magma beneath the earth’s crust. The mobile rocks in the mantle beneath the plates are believed to be moving in a circular manner. The heated material rises to the surface, spreads and cools, and then sinks back to the depths. This cycle is repeated and is called convection cell or convection flow. This slow movement of hot soft mantle beneath the plates is the force behind plate movement.
New plate material is generated along the mid- ocean ridges, where basaltic lava is poured out by submarine volcanoes. The theory of sea-floor spreading has demonstrated the way in which the basaltic lava spreads outwards away from the ridge crest at 1-6 cm/0.5-2.5 in per year. Plate material is consumed at a rate of 5-15 cm/2-6 in per year at the site of the deep ocean trenches.
Hess was first to propose this concept of sea floor spreading. He argued that constant eruptions along the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side thereby spreading the oceanic floor.
The younger age of oceanic crust[2] and the fact that spreading of one oceanic floor does not led to shrinkage of other made Hess argue that the oceanic crust gets consumed into the mantle i.e. as the oceanic floor spreads due to volcano at crest, the oceanic crest sinks down to get consumed.
For example, along the Pacific coast of South America. The trenches are sites where two plates of lithosphere meet; the one bearing ocean-floor basalts plunges beneath the adjacent continental mass at an angle of 45º, giving rise to shallow earthquakes near the coast and progressively deeper earthquakes inland. In places the sinking plate may descend beneath an island arc of offshore islands, as in the Aleutian Islands and Japan, and in this case the shallow earthquakes will occur beneath the island arc.
The term volcano has been derived from Greek word “Vulcan” which means deity of fire. It is defined as a vent on the surface of the earth through which magma, gases and ashes erupt. The volcanoes are generally dome shaped or conical shaped structures with truncated tops representing crater through which lava & other volcanic materials are poured out.
Classification of Volcanoes: Volcanoes can be classified into various types depending
a. On the basis of their record of activities
b. On the basis of mode of eruption and
c. On the basis of nature of eruption
It Is Classified into Three Types
i. Active Volcanoes
ii. Dormant Volcanoes
iii. Extinct Volcanoes.
A volcano is active when it is erupting continuously or intermittently. Volcanoes, which have not erupted for a long time but have the chance to erupt in future are, said to be dormant or slumbering type. Extinct volcanoes are those, which have totally stopped their activities i.e They were active in the geological past but which are not active today with a prospect of any future activity.
Barren Island- The only active Volcano of India.
Barren Island is situated in the Andaman Sea, and lies about 138 km (86 mi) northeast of the territory's capital, Port Blair. It is the only active Volcano along the chain from Sumatra to Myanmar and also the only active volcano in India. Barren Island is a part of the Indian Union Territory of Andaman and Nicobar Islands and is well known as a Submarine emergent Volcano, which lies above the subduction zone of India and Burmese plate. Well, Barren Island in Andaman has again started erupting in 2018, and many people across the globe are coming to the islands just to experience this phenomenon.
Volcano
Activity
Arenal, Costa Rica
ongoing
Bagana, Bougainville, Papua New Guinea
Colima, Mexico
Dukono, Indonesia
Fuego, Guatemala
Karymsky, Kamchatka, Russia
Kilauea, Hawaii
Manam, Papua New Guinea
Masaya, Nicaragua
Mount St. Helens, Washington
Sakura-Jima, Japan
Sangay, Ecuador
Santa Maria, Guatemala
Semeru, Java, Indonesia
Shiveluch, Kamchatka, Russia
Soufriere Hills, Montserrat, West Indies
Stromboli, Italy
Suwanose-Jima Ryukyu Islands, Japan
Tungurahua, Ecuador
b. On the basis of mode of eruption: Volcanoes are classified into two types depending on the basis of mode of eruption viz.
i. Central type: The products escape through a single pipe.
ii. Fissure type: Products escape through a long or a group of parallel fissures.
Volcanoes Are Classified Into
a. Explosive type
b. Quiet type
In the Explosive type the lava that is poured out is acidic, viscous & light colored while in the quiet type the lava that is poured out is basic, mobile and dark colored.
Examples of the above two lavas are block and ropy lavas.
We can also differentiate volcanoes based on the above two i.e. their degree of activity & nature of eruption as
a. Hawaiian type
b. Strombolian type
c. Vulcanian type
d. Vesuvian type
e. Plinian type
f. Pelean type
Hawaiian is the most silent effusion type of volcanoes while Pelean is the most violent type.
The products that generally erupt are in the form of solids, liquids and gases. Fragments of rocks ejected during an eruption are called pyroclastic materials. The rocks or solids that are ejected are classified into
Agglomerates are solids with diameter of 20mm. Lavas alone are poured out in liquid state. Among gases the most common is the steam constituent. Other gas constituent includes CO2, Sulphur dioxide, Hydrochloric acid, hydrogen etc. Volcanoes emitting sulphurous vapour are termed as Solfatras. Those emitting CO2 are Mofettas & those emitting boric acid vapours are termed as Saffioni.
India Meteorological Department (IMD), under Department of Science and Technology is the nodal agency for seismic observational network in India. At present 51 Seismological Observatories are being maintained by IMD under the National Network.
The Indian Ocean Tsunami Warning System is a tsunami warning system set up to provide warning to inhabitants of nations bordering the Indian Ocean of approaching tsunamis. It consists of 25 seismographic stations relaying information to 26 national tsunami information centers, as well as three deep-ocean sensors and is part of International Early Warning Programme.
Pacific Rim of fire: Approximately 90% of earth’s earthquakes occur in the basin of the Pacific Ocean in a 25,000 mile horseshoe shape, it’s associated with an almost continuous series of oceanic trenches, volcanic belts, volcanic arcs and/or plate movement. This region is a direct result of plate tectonics and the movement and collisions of lithospheric plates.
Landforms formed by highly fluid lava- The basic lava is very fluidic in nature and flows for long distances forming lava plains and Basalt plateaus like the Deccan plateau.
The fluidic lavas are also responsible for formation of shield volcanoes or lava domes with gentle slopes and flat broad tops.
Landforms formed by viscous lava- the viscous acidic lavas do not flow for great distances and solidify readily. When trapped in valleys, such viscous lava solidifies to form lava tongues and lava dammed lakes if they block the flow of a river.
The most common feature of viscous lava is a composite cone formed by successive accumulation of layers of lava around a conduit. The composite cone also consists of subsidiary dykes and parasitic cones.
Highly viscous lava often solidifies very quickly and blocks the opening or mouth of a crater. This leads to building up of pressure in the conduit leading to violent eruptions. These violent eruptions often cause the mouth of the crater to collapse thereby forming depressions called calderas on the mouth of the crater.
Accumulation of heat (radioactive, frictional & increase of heat with depth) produces magma which are present below the ground surface & they build up pressure if they do not escape. As soon as they find their way, they release the high pressures through thin and rather unstable crust. This causes volcanism.
Distribution: They are generally concentrated in a narrow belt called Circum Pacific Ring of Fire. The major areas includes
a. Hawaiian islands which covers Arabia, Madagascar, rift valleys of Africa, Mediterranean.
b. West Indies
c. Iceland
d. Atlantic
Important volcanoes include Cotopaxi, Lassen Peak, Katmai, and Fujiyama & Mayon.
Sill is a sheet of igneous rock created by the intrusion of magma (molten rock) between layers of pre-existing rock. A dyke, by contrast, is formed when magma cuts across layers of rock. An example of a sill in the UK is the Great Whin Sill, which forms the ridge along which Hadrian’s Wall was built. A sill is usually formed of dolerite, a rock that is extremely resistant to erosion and weathering, and often forms ridges in the landscape or cuts across rivers to create waterfalls.
PRINCIPAL ACTIVE VOLCANOES
Name
Location
Country
Last
eruption
Ojos del Salado
Andes
Argentina-Chile
1981-Streams
Guallatiri
Chile
1960
Cotopaxi
Equador
1975
Tupungatito
1964
Lascar
1968
Popocatepeti
Altiplano de Maxico
Maxico
1920-
Streams
Nevado Del Ruiz
Colombia
1985
Sangay
1976
Klyuchevskaya
Sredinny Khrebet
USSR
1974
Soplea
(Kanchatka Peninsula)
(now CIS)
Purace
1977
Wrangell
Wrangell Mts.
Alaska
Tajumulco
Guatemala
Rumbles
Mauna Loa
Hawaii
USA
1978
Tacana
Sierra Madre
Cameroon Mt.
(Monarch)
Cameroon
1959
Fuego
1962
Erebus
Ross Island
Antarctica
Rindjani
Lombok
Indonesia
1966
Pico de Teide
Tenerife Canary Island
Spain
1909
Semeru
Java
Nyiragongo
Virunga
Zaire
Koryakskaya
Peninsula Kamchatka
USSR (now CIS)
1957
Irazu
Cordillera
Costa Central
1967
Slamat
Mount Spurr
Alaska Range
1953
Mount Etna
Sicily
Italy
1979
Agung
Bali
Kilauea
1961
Stromboli
Islands (Lipari)
Mediterranean
Sea
1956
Surtsey
Off SE Iceland
Iceland
1963
Ana
Krakatoa
1929
In geology, hot spots are isolated rising plume of molten mantle material that may rise to the surface of the Earth’s crust creating features such as volcanoes, chains of ocean islands, seamounts, and rifts in continents. Hot spots occur beneath the interiors of tectonic plates and so differ from areas of volcanic activity at plate margins (see plate tectonics). Examples of features made by hot spots are Iceland in the Atlantic Ocean, and in the Pacific Ocean the Hawaiian Islands and Emperor Seamount chain, and the Galápagos Islands. Hot spots are responsible for large amounts of volcanic activity within tectonic plates rather than at plate margins. Volcanism from a hot spot formed the unique features of Yellowstone National Park, Wyoming, USA. The same hot spot that built Iceland atop the mid- Atlantic ridge in the North Atlantic Ocean also produced the voluminous volcanic rocks of the Isle of Skye, Scotland, at a time before these regions were rifted apart by the opening of the Atlantic Ocean.
Chains of volcanic seamounts trace the movements of tectonic plates as they pass over hot spots.
Immediately above a hot spot on oceanic crust a volcano will form. This volcano is then carried away by plate tectonic movement, and becomes extinct. A new volcano forms beside it, again above the hot spot. The result is an active volcano and a chain of increasingly old and eroded extinct volcanoes stretching away along the line traced by the plate movement. The chain of volcanoes comprising the Hawaiian Islands and Emperor Seamounts formed in this way.
Natural spring that intermittently discharges an explosive column of steam and hot water into the air due to the build- up of steam in underground chambers. One of the most remarkable geysers is Old Faithful, in Yellowstone National Park, Wyoming, USA. Geysers also occur in New Zealand and Iceland.
Ground water, coming in contact with the magma gets heated, which when comes to the surface is known as Hot spring. These hot springs are found in many places in India-Laddakh, Manali, Sohana, Rajgir, Rajmahal, etc., and also in the Volcanic regions of Iceland, Yellow Stone National Park and North Iceland of New Zealand.
Unlike a geyser, the hot water gently seeps out of the surface.
Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air. The Tephra is all the volcanic material such as Ash, Plumes, Volcanic Bombs, Volcanic Blocks,lapilli etc.
Pieces of Viscous lava often 2.5 inch size are ejected from the volcanoes. They are viscous rounded shaped half semi solid pieces called Volcanic Bombs. They are either round or spindle shaped orribbon shaped. Sometimes referred to as Volcanic Blocks, however, Volcanic blocks are thought almost same size, are solid. The smaller particles less than 2.5 inch are called Lapilli. The pieces of rocks that erupt violently are also called ballistic fragments.
Lapilli mean “little stones.” These are round to angular rock fragments, measuring 1/10 inch to 2 1/2 inches in diameter, which may be ejected in either a solid or molten state.
The Ash from the Volcanoes is hard and abrasive type which is made up of rock particles, minerals and Volcanic glass fragments. The cloud made by the Volcanic Ash is called Ash Cloud. When thisash falls on the ground, it is called Volcanic Ash Fall. The clouds are called Avalanches sometimes.
Interconnected, sack-like bodies of lava formed underwater.
It is the fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.
A cone shape hill of volcanic fragments that accumulate around and downwind from a volcanic ventis a cinder cone. There is usually a bowl-shaped crater at the top. As the gas-filled lava erupts into the air, the lava fragments and forms cinders.
The time lag between the volcanic eruptions is called repose.
Volcanic Explosivity Index is a scale that measures the Volume of Volcanic Products, Height of
Plume and other observations to decide which volcano is more explosive. Highest Magnitude is 8.
Earthquakes are tremors which are produced by the passage of vibratory wave through the rocks of the earth. These vibratory waves are generated due to the relative motion of tectonic plates.
The term focus is used to designate the point where earth vibrations originate and from which they radiate. This is usually at some depth below the surface of the earth. An area on the surface vertically above the focus is called the epicentre; this is where the greatest damage occurs. An isoseismic line is a line connecting all points on the surface of the earth where the intensity of shaking produced by the earthquake waves is the same.
Instruments of great precisions and sensitivity called seismographs have been constructed for the purpose of recording earthquake shocks. A seismographic record is called a seismogram. A good seismograph is practically automatic in its operation. Earthquake waves are of three types. These are:
P waves, or Primary waves, are the first waves to arrive at a seismograph. P waves are the fastest seismic waves and can move through solid, liquid or gas. They act like sound waves. They leave behind a trail of compressions and rarefactions on the medium they move through. P waves shake the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.
Certain animals, such as dogs, can feel the P waves much before an earthquake hits the crust (surface waves arrive). Humans can only feel the ramifications it has on the crust.
S or shear waves also called transverse waves,because the particles vibrate at right angles to the direction of propagation. They closely follow the P waves. The velocity of the S wave is about one-half that of the P wave, but the S wave is more destructive. On reaching the surface of the earth, these waves cause the rocking motion of the earthquake.
After the P or S waves reach the surface of the earth, a surface wave L also known as long waves may be formed which travels around along or near the surface portion of the earth with a lower velocity than the other two. L waves are also very destructive. In a great earthquake, the L waves may throw the ground into a series of actual undulations which may rise in long, low, rapidly moving waves that cause trees and tall structures to sway violently.
A seismic shadow zone is an area of the Earth's surface where seismographs can only barely detect an earthquake after its seismic waves have passed through the Earth. When an earthquake occurs, seismic waves radiate out spherically from the earthquake's focus.
The shadow zone for ‘P’ waves is an area that corresponds to an angle between 103 and 142.P waves are refracted at the mantle-outer core boundary due to liquid nature of outer core.
S waves radiate spherically away from an earthquake in all directions however they do not appear beyond an angular distance of ~103°. They are stopped by the liquid outer core. It must be noted that shadow zone of S waves is bigger than shadow zone of P waves because S waves do not appear after 103 degrees.
Moreover, shadow zone of each earthquake is different, and it depends on focus of the earthquake.
Surface waves are of two types viz. Rayleigh Waves and Love waves
L Waves or Surface Waves travel near the earth’s surface and within a depth of 30-32 kilometers
from the surface. These are also called Rayleigh waves after Lord Rayleigh who first described these waves. Behave like water waves with elliptical motion of material in wave. Generally slower than Love waves.
Love waves make the ground vibrate at right angles to the direction of waves . They are a variety of S-waves where the particles of an elastic medium vibrate transversely to the direction of wave propagation, with no vertical components. Involve shear motion in a horizontal plane. Most Destructive kind of seismic wave.
Largest Earthquakes Since 1900
Date
Magnitude
May 22, 1960
9.5
Prince William Sound, Alaska
March 28, 1964
9.2
Andreanof Islands, Aleutian Islands
March 9, 1957
9.1
Kamchatka
Nov. 4, 1952
9.0
Off western coast of Sumatra, Indonesia
Dec. 26, 2004
6.
Off the coast of Ecuador
Jan. 31, 1906
8.8
7.
Rat Islands, Aleutian Islands
Feb. 4, 1965
8.7
8.
Northern Sumatra, Indonesia
March 28, 2005
9.
India-China border
Aug. 15, 1950
8.6
10.
Feb. 3, 1923
8.5
(i) Circum-Pacific areas (70 per cent of Earthquakes) with most frequent occurrence along the ‘pacific Ring of Fire’.
(ii) Mid-continental belt (20% of Earthquake) – includes the Mediterranean-Himalayan belt.
(iii) Mid-Atlantic Ridge (10% of Earthquake) – includes the Earthquakes of New Madrid, Charleston, Boston and Koyna, etc
Richter scale: A logarithmic scale, devised in 1935 by an American, C.F. Richter, to identify the Magnitude of An Earthquake. Designed to compare the magnitudes of Californian earthquakes, it is now universally adopted and extended. It is more precise than the Mercalli Scale, which refers to the Intensity of an Earthquake. The Richter scale reflects the total amount of elastic energy released when overstrained rocks suddenly rebound to give a seismic shock. The relationship is given by: log E = a + bM, where: E is energy expressed in ergs, M is magnitude, a is 5.8, and b is 2.4. The scale is open-ended, ranging from zero up to 8.9, which is the largest recorded magnitude (Chile, 1960), and which produced energy of 1027.2 ergs. It has been calculated that the average annual energy release from all earthquakes ranges from 1025 and 1027 ergs. Moderate-to-strong intensities on the Mercalli scale (IV and V) are thought to be equivalent to a Richter magnitude of 4.3 to 4.8. Destructive-to-disastrous intensities (VIII, IX and X) are possibly equivalent to magnitudes of 6.2 to 7.3.
Modified Mercalli Scale: The Modified Mercalli Intensity (MMI) scale depicts shaking severity. An earthquake has a single magnitude that indicates the overall size and energy released by the earthquake. However, the amount of shaking experienced at different locations varies based on not only that overall magnitude, how far you are from the fault that ruptured in the earthquake, and whether you are on rock or thick valley deposits that shake longer and harder than rock.
The Mercalli intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.
Intensity
Shaking
Description/Damage
I
Not felt
Not felt except by a very few under especially favourable conditions.
II
Weak
Felt only by a few persons at rest, especially on upper floors of buildings.
III
Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
IV
Light
Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V
Moderate
Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI
Strong
Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII
Very strong
Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
VIII
Severe
Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX
Violent
Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X
Extreme
Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
XI
Total damage
Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground.
XII
Total damage.
Waves seen on ground. Objects thrown up into air.
Earthquakes can cause soil liquefaction where loosely packed, waterlogged sediments come loose from the intense shaking of the earthquake. The liquefaction is more prominent in areas such as river valleys, river plains and deltas. The randomly bunched together soil particles have spaces have formed between them. These spaces, called pores, can be filled with water or air. The pressure of the material in the spaces holds the particles apart and stabilizing the soil in its present configuration.
The effect of a seismic wave on granular soil and pore pressure is that it increases the water pressure and forces the particles apart as well as disrupts the contact point of the particles themselves.
At this point in time the soil will flow like a liquid. The end product is the collapse of the particles so that there is less space between them. The water that was in that space is then forced upward.
Liquefaction should have the following conditions for it to take place:
The impacts of the Liquefaction are as follows:
The underlying layer of water rich sand compacts and sends a column of water and fine sand up and out onto the surface. This phenomenon is called Differential Compaction. At the same time, depth of lakes, ponds, borrow areas, and other depressions becomes lower, because the sand is pushed through the ground. The buildings sink into the ground after the earthquake.
India has a very high frequency of great earthquakes (magnitude greater than 8.0) in comparison to the moderate earthquakes (magnitude 6.0 to 7.0). For example, during 1897 to 1950, India was hit by four great earthquakes.
However, since 1950, only moderate size earthquakes have occurred in India which should be no reason to assume that the truly great earthquakes are a thing of the past.
The reasons of high magnitude earthquakes in India are hidden in the tectonic setting of India. India Is currently penetrating into Asia at a rate of approximately 45 mm/year and rating slowly anticlockwise.
This rotation and translation results in left-lateral transform slip in Baluchistan at approximately 42 mm/year and right-lateral slip relative to Asia in the Indo-Burman ranges at 55mm/year. At the same time, deformation within Asia reduces India’s convergence with Tibet to approximately 18 mm/year. Since Tibet is extending east-west, there Is a convergence across the Himalaya that results in the development of potential slip available to drive large thrust earthquakes beneath the Himalaya at roughly 1.8 m/century.
Indian subcontinent has a long history of devastating earthquakes, partially due to the fact that India Is driving into Asia at a rate of approximately 47 mm/year. More than 50% area of Indian Subcontinent is vulnerable to earthquakes. According to the IS 1893:2002 India has been divided into four seismic zones viz. Zone-II, -III, -IV and -V unlike its previous version which consisted of five zones for the country.
India has suffered some of the greatest earthquakes in the world with magnitude exceeding 8.0. For Instance, in a short span of about 50 years, four such earthquakes occurred: Assam earthquake of 1897 (magnitude 8.7), Kangra earthquake of 1905 (magnitude 8.6), Bihar-Nepal earthquake of 1934 magnitude 8.4) and the Assam-Tibet earthquake of 1950 (magnitude 8.7).
This 8.3 magnitude earthquake took place on the west coast of India and caused ground motion
which was perceptible as far as Calcutta. It created a fault scarp of about 16 mile long and about 10 feet high which was later named as “Allah Bund”.
This 8.7 magnitude earthquake caused severe damage in an area of about 500 km radius and caused extensive surface distortions in the area. The earthquake caused extensive liquefaction in the alluvial plains of Brahmaputra.
This 8.4 magnitude earthquake caused widespread damage in the northern Bihar and in Nepal. Dueto extensive liquefaction, most buildings tilted and slumped bodily into the ground in an area of about 300 km long and of irregular width. This area was termed as the “slump belt”.
This 6.5 magnitude earthquake occurred close to 103 meter concrete gravity dam at Koyna. Prior to this earthquake, the area used to be considered a seismic. However, after the construction of dam and filling up of reservoir in 1962, the seismic activity increased significantly. The main shock of December 10, 1967 caused widespread damage, killing about 200 persons and injuring more than 1500 persons. This was an example of the reservoir-induced seismicity in India.
The dam, designed keeping in mind the possible seismic activity, performed quite well with only
nominal damage to the dam. This earthquake lead to the revision of Indian seismic zone map wherein the area around Koyna was brought in zone IV from zone I, and seismic zone for Bombay Was upgraded from zone I to zone III.
This 6.6 magnitude earthquake shook the districts of Uttarkashi, Tehri, and Chamoli of current Uttarakhand.
This was a magnitude 6.4 earthquake that shook the area near village Killari in Latur district killingabout 8,000 persons. Until this earthquake the area was considered non-seismic and placed in the lowest seismic zone (zone I) by the Indian code (IS:1893-1984).
The affected area did not have any modern towns, modern buildings or major industries. In some of the villages more than 30% of the population was killed. This earthquake will be known for outstanding rescue, relief and rehabilitation.
This magnitude 6.0 earthquake is only example of such earthquakes which occurred close to a major Indian city in recent times.
The 2004 Indian Ocean earthquake was an undersea mega thrust earthquake with an epicenter of the west coast of Sumatra, Indonesia, and it is known as Sumatra–Andaman earthquake of 2004 Indian Ocean tsunami or South Asian tsunami, Indonesian tsunami, and the Boxing Day tsunami. It killed 230,000 people in fourteen countries, and inundating coastal communities with waves up to 30 meters.
These forces are caused by contraction of the Earth on cooling, or change in the rotation of the Earth, or due to action of the radioactive elements.
On the basis of INTENSITY, these forces can be divided into two sub-groups.
The Diastrophic Forces can be divided into two further sub-groups – EPEIROGENETIC and OROGENETIC.
When TENSION is produced by these forces, the Earth’s surface displays fissures and when COMPRESSION is produced, folds result in the sedimentary rocks.
These are external forces that produce physical features-i.e. those forces which operate externally on earth’s surface and change it by- (i) Denudation or by (ii) Deposition. The forces of denudation are weathering and erosion. The forces of deposition are water, ice, wind and living organisms.
Weathering is the combined action of all processes whereby a rock is decomposed and disintegrated in situ, because of exposure at or near earth’s surface. This process changes hard, massive rock material into finely fragmented, soft residual material, the parent matter of soil. For this reason, weathering is often described as the preparation of rock materials for transport by the agents of land erosion-flowing water, glacial ice, and wind. Due to the gravitational force, both bedrock and the products of weathering tend to slide, roll, flow or creep down all slopes in a variety or type of earth and rock movements grouped under the term mass washing.
Weathering may result from (i) physical (mechanical) processes e.g. heating, cooling, freezing and mechanical action of plants and animals (ii) chemical processes, e.g. carbonation, hydration, hydrolysis, solution, Organic weathering, which consists of both physical and chemical weathering, is caused by plants, and animals, particularly when they bury or release acidic substances into the rocks. Lichens, for example, cause breakdown as they extract nutrients directly from the rocks.
Erosion refers to the disintegration of rocks which lie exposed to what are called the agents of erosion, i.e. gravity, running water, wind, and moving ice, and on the coasts, waves, tides and currents.
Exfoliation is aptly called onion weathering. It is a process of physical weathering in which the outer layers of a rock split off into thin sheets or scales. It has been suggested that it is caused by the alternate expansion and contraction resulting from a wide diurnal range of temperature in arid regions, which would lead to stresses in the rock.
Frost action shatters rocks through repeated freezing and thawing of water which seeps into rocks and expands on freezing. This is common in winter in the temperate regions and in some high mountains all the year.
In tropical areas or in coastal areas where rocks are repeatedly exposed to water weathering occurs. When the rocks are wetted, outer layer absorbs moisture and expand. When the moisture dries, shrinkage occurs. This repeated expansion and shrinkage produces stresses leading to peeling/splitting of outer layers.
Exposure of rocks to air and water often sets up chemical reactions thereby leading to either weakening or entire dissolution of exposed surfaces. Rocks with a pitted surface and pore spaces are more affected by chemical weathering as water and air gets trapped into them thereby speeding up chemical reactions. Also chemical weathering is most potent in hot and wet climate and least in dry climate.
Chemical weathering can be classified under following heads-
Solution- Rain water contains some carbon dioxide that makes it slightly acidic. In limestone regions, this weak acid dissolves the calcium carbonate of which limestone is chiefly formed. Though solution is most evident in limestone, other rocks are also subject to solution but at a slower rate.
Oxidation- Certain rocks contain iron or oxides of iron. When exposed to the oxygen in the air iron oxide is formed which easily crumbles or peels thereby loosening the overall structure of rock.
Decomposition by organic acids- Organic acids are released by bacteria thriving on plant and animal matter present in the soil which lead to chemical weathering of rocks.
The weathered material is transported to other places by agents like wind, running water, glaciers, waves etc. and deposited to give rise to depositional geographical features like alluvial plains, beaches etc.
A mountain is that land block whose height is at least 900 meters above sea level and its slope makes an angle of 250 to 350 with the horizontal plane. The lower mountains are called hills. In fact mountain and hill are relative terms.
1. Ridge: It is a high and narrow continuation of hills. It has many passes.
2. Mountain Range: It is a big and complex ridge. It is a series of ridges, which are mutually related, form a compact unit and stand over an extensive area.
3. Mountain System: It is a group of mountain ranges, which are similar in their form, structure and extension. Besides, they should owe their origin to the same cause.
4. Mountain Chain: It is an elongated unit of these mountain ranges or chains whose size and age need not necessarily be similar or the same.
5. Cordillera: It is a Spanish word and is the highest unit of mountain.
Mountains are classified on many bases. Some classifications are given here.
1. Classification based on Height: Mountains are usually divided into 4 classes on the basis of height.
i. Low Mountains: The heights of low mountains range from 1,000 to 1,500 meters.
ii. Mountains of Ordinary height: The height of such mountains varies from 1,500 to 2,000 meters.
iii. High Mountains: The height of these mountains is above 2,000 meters.
2. Classification based upon Structure: On the basis of structure, mountain can be divided into the following classes:
These mountains are formed by compression generated by plate tectonics. When the compression acts upon the accumulated sediment, the latter begins to rise. In the process of uplifting, the layers of sediment are folded. Mountains formed in this way are called Fold Mountains. There are many folds in these mountains, which break down under great pressure. Such folds are called Nappe.
These are also known as Fault-block landforms (mountains, hills, ridges, etc.) and are formed when large areas of bedrock are widely broken up by faults creating large vertical displacements of continental crust.
Vertical motion of the resulting blocks, sometimes accompanied by tilting, can then lead to high escarpments. These mountains are formed by the Earth's crust being stretched and extended by tensional forces. Fault block mountains commonly accompany rifting, another indicator of tensional tectonic forces.
The uplifted blocks are called block mountains or horsts. The intervening dropped blocks are termed graben. These can be small or form extensive rift valley systems. This form of landscape can be seen in East Africa, the Vosges, the Basin and Range province of Western North America, in south-central New England, and the Rhine valley. These areas often occur when the regional stress is extensional and the crust is thinned.
These are the mountains which come into the existence due to the accumulation of the material. They can be of various types according to their geological features and accumulated material, such as volcanic mountains, mountains of glaciers etc.
Islands are broadly divided into four types :-
(i) Continental Islands,
(ii) Oceanic Islands,
(Iii) Tectonic Island, And
(Iv) Coral Island.
CONTINENTAL ISLANDS are those islands that rise from the ‘continental shelf’, for example, the British Isles and Newfoundland. These islands have the same geological structure, as the continents to which they are related.
OCEANIC ISLAND are those that rise from the BOSOM of the oceans. Their geological structure will have no geological relation to that of the nearest shores. They are very often the tops of submarine mountains or submarine volcanoes.
WORLD’S LARGEST ISLANDS
Largest Islands
Area in square Km.)
Australia(Geographically regarded as a continent)
Indian Ocean
7618493
Greenland
Arctic Ocean
2175600
New Guinea
West Pacific
770000
Borneo
725545
Malagasy Republic
590000
Baffin Island
476065
Sumatra
473600
Honshu
North-west Pacific
228000
Great Britain
North Atlantic
218041
Victoria Island
212197
Ellesmere Island
196236
Celebes
189035
South Island (New Zealand)
South-west Pacific
150460
126295
North Island (New Zealand)
114687
Cuba
Caribbean Sea
114522
Newfoundland
112300
Luzon
104688
103000
Mindanao
94226
Ireland (Northern Ireland and Republic of Ireland)
82460
Hokkaido
77900
Hispaniola (Dom. Republic and Haiti)
76192
Sakhalin
74060
Tasmania
67900
Srilanka
65600
ASSENSION and TRISTAN DE CUNHA, for example, rise from the Central Atlantic ridge (mountain), while ST. HELENA and TENERIFFE are the islands formed by submarine volcanoes.
TECTONIC ISLAND are created by the movements in the Earth’s crust. The outer-most layer of the earth made of rigid plates are in very slow, but constant, motion. When one plate is pushed under another plate, the top plate may scrape off pieces of the bottom plate. Over millions of years, this material piles up to form an island. BARBADOS in the West Indies and KODIAK ISLANDS near Alaska were formed in the same manner.
CORAL ISLANDS are the work of minute sea organisms called CORAL POLYPS. They congregate in large colonies. When the organisms (constituting ecological community of coral reefs) die, their skeletons, which are made of a substance resembling limestone, form big clusters, some of which rise above the water.
When people think about deserts, they usually imagine vast sand dunes under a burning sun, with nothing to break the monotony of the landscape except for an occasional palm-filled oasis. Although places like this certainly exist, the word `desert´ encompasses a wide range of landscapes, many of which are rich in wild plants, animals, and people, adapted to difficult conditions. Characteristics of deserts include very low rainfall and humidity, high evaporation rates, and little cloud cover. Many deserts are hot but some can be bitterly cold; there are ice deserts in the polar regions.
Desert soils are generally poor because they contain little vegetable matter; if water is made available, most deserts are capable of sustaining agriculture.
Natural deserts occur when geography and climate combine to prevent significant amounts of rain falling on an area.
THE GREAT DESERTS
Desert
Continent
Area in
sq Km
14 million
Sahara
Africa
9.4 million
Arabian
Asia
2.3 million
Gobi Desert
1 million
Kalahari
900000
Great Victoria Desert
Australia
647000
Patagonian
South America
620000
Syrian desert
520000
Great Basin desert
North America
492000
Chihuahuan desert
450000
Tropical deserts are caused by the descent of air that is heated over warm land and has previously lost all its moisture.
Continental deserts, such as the Gobi in Mongolia, are too far from the sea and large inland waters to receive any significant moisture.
Rain-shadow deserts lie in the lee of mountains where the rain falls on the windward slope, as is the case in the Mojave Desert beyond the Sierra Nevada in California.
Coastal deserts, such as the Namib in southern Africa, are formed when cold ocean currents cause local dry air masses to descend thereby leading to high pressure.
Animals have also developed special adaptive strategies. In hot deserts, most lie dormant during the day; if the nights are cold, small mammals confine their activities to the hours of sunrise and sunset, and spend the rest of the time sheltered in their burrows. Birds migrating across desert regions follow routes that have remained the same for hundreds or thousands of years, stopping at occasional oasis of water and vegetation along the way.
Human cultures survive under harsh conditions. Human societies also exist in deserts, although, like any other animal, they have had to adapt. Most live on the edges of desert regions, and the only settled communities are around oasis. Truly desert cultures are nomadic, moving constantly to find food for their livestock or fresh supplies of water, and living in temporary accommodation. The Bedouin of Arabia and N Africa (the word Bedouin means `desert dweller ´ in Arabic) lived for centuries in tents made of animal skins, and traded in camels and horses, although they are now increasingly being forced to settle in one place. In the Gobi Desert in Mongolia, nomadic tribes lived in temporary structures known as yurts. The people of the Kalahari Desert in Botswana (formerly known as Bushmen) have no permanent dwellings at all and traditionally live a hunter-gatherer existence; their knowledge of desert survival is so acute that they spend only a small amount of time every day on gathering food and drink.
Deserts are expanding. Unfortunately, deserts can also be formed as a result of human mistakes. Degradation of a dry-land area - through deforestation, overgrazing, poorly designed irrigation systems, and soil erosion - can lead to a complex cycle of changes that result in desertification. Loss of trees and ground cover means that water is no longer trapped by vegetation, increasing the evaporation rates and often also the frequency and severity of drought. Mismanagement of irrigation can lead to a raised salt content in the soil, preventing crop growth and adding to risks of desert formation. Overgrazing, particularly by animals like goats that can strip vegetation bare, is often a contributory factor. Throughout the world, desertification is accelerating, and according to the United Nations, a hundred countries are affected.
The Earth has five main regions of natural desert.
By far the largest is the Afro-Asian desert, a vast belt that stretches from the Atlantic Ocean to China, and includes the African Sahara, the Arabian, Iranian, and deserts of the Middle East and central Asia, the Thar Desert in Pakistan and India, and the Takla Makan and Gobi deserts of China and Mongolia.
Also in Africa, the Namib and Kalahari deserts cover a large area in the southwest.
The North American desert covers much of southern USA and northern Mexico.
Further south, the Atacama Desert is a thin, arid strip on the coastal side of the Andes, and the Patagonian desert covers much of Argentina east of the Andes.
The Australian desert covers much of the interior of the world's largest island.
The main categories of DESERT are:
THE HOT (TROPICAL) DESERT-These are the areas of high atmospheric pressure around the subtropical high belt, with rainfall less than 25 cm, high summer temperatures; e.g., the Sahara, Arabian, Thar desert.
THE COASTAL DESERTS-These are on the western margins of continents in latitudes 15-30, with cold offshore currents, and low summer temperature; e.g., Atacama, Namib, Kalahari, Mojave deserts.
THE MID-LATITUDE DESERTS-These are the deserts of continental interiors far from rain bearing winds and moderating impact of oceans, with high summer and low winter temperature; e.g., Gobi, Taklamakan, Turkestan, Australian deserts.
THE ICE AND SNOW DESERTS-These are the cold deserts of polar lands : e.g., the Greenland, the Antarctica.
On The Basis Of Location
On The Basis Of Climate
On The Basis Of Surface Cover
Polar, e.g., Greenland, Antarctica.
Mid latitude Continental, e.g., Gobi, Nevada
Subtropical, e.g., Sahara, Atacama
Coastal, e.g., Namib, Atacama.
Physiological, e.g., Antarctica
Cold desert, e.g., Gobi, Laddakh, Patagonia.
Hot desert, e.g., Simpson, Gibson, Rub-al-Khali.
Sandy (erg), e.g., Rub-al-Khali
Rocky (hammada), e.g., Sahara
Stony (reg), e.g., Sinai, Negev (Israel)
Salty, e.g., Dasht-I-Kavir, Takla Makan
Salty marsh, e.g., Rann of Kachchh
Rivers are one of the greatest sculpturing agents at work in humid regions. In its youthful stage the river flows turbulently in a narrow, steep-sided valley whose floors are broken by pot holes and waterfalls. A youthful valley is ‘V’ Shaped, with steep gradient. The water of a fast-flowing river swirls if its bed is uneven. The pebbles carried by a swirling river cut circular depressions in the riverbed. These gradually deepen and are called potholes. Much larger but similar depressions form at the base of a waterfall under the consistent impact of falling water; these are called plunge pools.
Interlocking spurs are another feature of a youthful valley formed due to erosion of softer portions of bedrock in the valleys.
Some valleys have very steep sides and are both narrow and deep; these are called gorges. A gorge is often formed when a waterfall retreats upstream. One on the most famous gorges formed in this way lies below the Victoria Falls. A gorge will also form when a river maintains it course across a terrain which is being uplifted. The Indus, the Brahamaputra and the headwaters of the Ganga have cut deep gorges in the Himalayas. A huge gorge is called a canyon; they usually occur in dry regions where large rivers are actively eroding vertically and where weathering of the valley sides is at a minimum.
Valleys of the mature stage have the shape of a ‘V’ in cross-section. The gradient is gentler, river beds are more pronounced, spurs are removed by lateral erosion and there remains a line of bluffs on each side of the valley floor.
After the stage of maturity is reached the river begins to overflow its banks and it deposits fine silts and mud on the valley floor. This is the final stage in the growth of a flood plain. Meanders are pronounced and cutoffs develop and produce ox-bow lakes. The river builds up its banks with alluvium (the banks are called levees). The river thus flows between pronounced banks and above the level of the flood plain. In process of time, river erosion, transportation, and deposition turn the original surface into an almost level plain, which is called a peneplain.
River and their tributaries drain an area, which is called a ‘river basin’; its boundary formed by the crest line of the surrounding highland is the watershed of the basin.
A river at any stage of its development from youth to old age may be rejuvenated, and a young valley may occur in an old landscape. Where the river crosses from the original flood plain to the new flood plain, there may be a waterfall or rapids: this point is called knickpoint.
A delta is formed at the mouth of a river where it deposits more material than can be carried away, as the speed of the river is reduced by the time it enters a sea or lake. Also, fine clay particles carried in suspension in the river coagulate in the presence of salt water and get deposited.
In the first stage, deposition of sediment chokes the flow and divides the river into several distributaries. Spits and bars arise and lagoons are formed. (Lagoons are shallow stretches of water separated from the sea by a barrier such as a spit). In the next stage the lagoons begin to get filled in with sediments, and they become swampy. The delta begins to assume a more solid appearance. In the third stage the old part of the delta becomes colonized by plants. As a delta grows larger, the old parts merge imperceptibly with the flood plain, and they no longer have the appearance of a delta.
Arcuate: composed of coarse sediments such as gravel and sand and is triangular in shape. It always has a number of distributaries. River having this type of delta are “Nile”. “Ganga”, “Indus”, “Irrawady”, “Mekong”, “Hwnag Ho”, “Niger”.
Bird’s Foot/Digitate: It is composed, of very fine sediment called silt. The river channel divides into few distributaries only and maintains clearly defined channels across the delta. The “Mississippi Delta” is one of the best example.
Estuarine: Develops in the mouth of a submerged river i.e. river falling into deep sea. Rivers like “Amazon”, “Ebe”, “Ob”, “Vestula” from this type of Delta.
Cuspate: Only few rivers like Ebro of Spain form such type of delta. These have tooth-like projections and are formed when a river drops sediments on a straight shoreline with strong waves.
Delta can and do form on the shores of high tidal seas e.g. river Colorado (Gulf of California) and River Fraser (British Columbia)
Any rivers, irrespective of its development can build a delta. The “Kander” whose valley is in stage of youth has built delta lake in lake Thun (Switzerland)
Drainage refers to a body of flowing water, ranging in scale from a rill to river. On the basis of the origin and evolution of drainage system, Geological Structure, there are two broad categories of drainage systems.
Sequent Drainage System are system of stream which follow the regional slope and are well adjusted to geological structure. These are four types
1. Consequent Stream: Consequent streams are streams whose course is a direct consequence of the original slope of the surface upon which it developed, i.e., streams that follow slope of the land over which they originally formed. Most of the streams draining the coastal plains of India are of this type. The ideal landscape for the origin and development of consequent drainage systems are domes and volcanic cones.
2. Subsequent Stream: When the master consequent stream is joined by its tributary at right angels, it is called subsequent stream. For example, the river “Asan”, a tributary of Yamuna and river “Son”, a tributary of the Ganga are the subsequent streams..
3. Obsequent Stream: The stream which flows following the direction of slopes opposite to master consequent stream. For example, the Mahabharata Range of the Lesser Himalayas has originated several streams from its northern slope which flow as obsequent stream opposite to the direction of flow of master consequent streams like Ganga and Yamuna
4. Resequent Stream: Such streams whose course follows the original relief, but at a lower level than the original slope (e.g., flows down a course determined by the underlying strata in the same direction).
The streams which do not follow the regional slopes and drain across the geological structure randomly constitute insequent drainage system. These are of two types
1. Antecedent Drainage: The streams which originated before the upliftment of the surface on which they flow. For example, Indus; Sutlej and Brahmputra are antecedent rivers as they originated before the upliftment of Himalayas ranges and hence create deep gorges along the mountain ranges.
2. Superimposed Drainage: It is formed when the nature and characteristics of the valley and flow direction of a consequent stream develop on the upper geological formation and structure are superimposed on the lower geological formation of entirely different characteristic. For example, river Subarnarekha is superimposed on Dalma Hills to the west of Chandil in the Chota Nagpur plateau region of Bihar.
(On the basis of Nature of original surface and slope)
Dendritic Pattern: A drainage pattern consisting of a single main stream with tributaries resembling the branches of a tree. Such pattern is most fully developed where the underlying rocks are of uniform type and the structures are relatively simple.
Trellis Drainage: It is a rectangular pattern of river channels. It may develop where a slope is crossed at right angles by the strike of alternating hard and soft rock strata. Long streams develop along the soft rock strata, parallel to the strike and the short streams follow the slopes.
Radial Drainage: It is a drainage pattern in which streams radiate from a central peak or upland mass in all directions. It is also called centrifugal pattern. Dome structure commonly develops radial drainage as in the English Lake District. The entire drainage network of Sri Lanka, Hazaribagh Plateau, Parasnath Hills, Panchet Hills and Dalma Lava Plateaus are of such type.
Rectangular Drainage: A pattern of drainage consisting of two main directions of flow at right angles to one another. The pattern is most commonly caused by streams following fault lines that have a rectangular pattern. Here, both the mainstreams and its tributaries display right angled bends.
Annular Drainage: Here, streams follow roughly a circular pattern. Such patterns are usually produced on domed structures where the rivers follow the outcrops of weaker beds of rocks, alternating bands of hard and soft beds. It is also known as circular pattern, e.g., Sonapet Dome of Bihar.
Parallel Drainage: A pattern in which the mainstream and the tributary streams follow virtually parallel courses. This develops where there is a strong structural control in one direction, for example, where strata are gently dipping.
Barbed Drainage: In this pattern, the tributaries flow in opposite direction to their master streams. The tributaries join their master streams in a hook-shaped bend. Such pattern in generally developed due to river capture.
Centripetal Drainage: When the streams coverage at a point, which is generally a depression or a basin, they form centripetal or inland drainage pattern, for example, the Kathmandu Valley of Nepal.
Pinnate Drainage: It is developed in a narrow valley flanked by steep ranges. The tributaries originating from the steep side of parallel ridges join the longitudinal master consequence, occupying the valley at acute angles, e.g., the drainage network of angles, e.g., the drainage network of the upper Sone and Narmada river.
Action of the carbon dioxide in rain water on limestone converts the insoluble calcium carbonate into soluble calcium bicarbonate. Limestone is a well-jointed rock and its joints and bedding planes are opened up by rain and water, and in time the surface consists of broken and rugged rocks. One of the most noticeable features of the limestone landscape is the almost complete absence of surface drainage. Rivers rising in non-limestone areas, on entering the limestone region, disappear into vertical holes on the surface and continue to flow as underground rivers. The vertical holes called swallow holes or sink holes are formed by rivers and they are usually widened vertical joints. Swallow holes may join together to give a very large opening, called doline. Dolines may join together to give even larger opening called uvula. Rivers that flow inside limestone develop underground caves and caverns as they flow along joints and bedding planes. In the caves, deposits of minerals, especially calcium carbonate, are formed by evaporation of calcium carbonate charged water. The deposits hanging from the ceiling are known as stalactite, and those that grow from the floor of the caves are known as stalagmites. Sometimes the two join to form pillars or columns. Apart from these, there are flowstones in underground caves.
Limestone regions are generally barren with a very thin soil cover. The porous nature of rocks and absence of surface drainage means that vegetative cover is minimal. Only short grasses and turfs are able to grow and hence human settlements are also very few. But limestone regions yield important building material for the cement industry. Also some lead is found in limestone rocks.
Chalk is extremely pure form of limestone and being made of calcium carbonate is susceptible to solution by rainwater. But since chalk is of extremely Friable nature, it crumbles easily and is not able to form swallow-holes, underground caves and other features associated with limestone.
The level above which there is perpetual snow cover is called the snowline. The snowline varies with altitude and latitude. In the polar region it is at sea-level; in East Africa it is at 5000 m; in the northern hemisphere it is lower on the shady north-facing side of a mountain than the south-facing side. When the accumulation of snow in a region increases year by year it gradually turns into ice by its own weight. Masses of ice that cover large areas of a continent are called ice sheets, and those, which occupy mountain valleys, are called valley glaciers.
A glacier is defined as a mass of ice that moves under the influence of gravity along a confined course away from its source area, however, the movement is not of the glacier as a whole. Throughout the glacier bits of ice are melting, trickling downvalley and then turning back into ice the whole time. This means that within the glacier there is a gradual down valley movement.
Glacial erosion consists of two processes: (i) plucking or the tearing away of blocks of rock which have become frozen into the base and sides of a glacier, and (i) abrasion or the wearing away of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.
The erosional features produced by glaciers include the cirque or corrie. A glacier while moving downhill, abrades the floor thereby steepening it; and there is the process of Plucking on the back-wall. The resultant action leads to creation of horse shoe shaped basin called cirque. A cirque or corrie originates as a small hollow where snow accumulates. The snow becomes compacted to glacial ice, forming a cirque glacier, and eventually flows downslope under the influence of gravity. Many cirques form small circular lakes called tarns. Sometimes corries develop on adjacent slopes and only a knife-edge ridge, called an arete, separates them. Where three or more cirques cut back together, a pyramidal peak or angular horn is formed.
If a glacier extending down has to negotiate a bend or a precipitous slope, there develop cracks in it. These cracks are very deep and are called crevasses. As the amount of ice in valley increases, the power to erode by a valley glacier also increases. This results in the glacier deepening, straightening and widening a river valley. The over deepening of the valley gives it a characteristic U shape marked by wide flat floor and steep sides.
Hanging valleys are another common feature in areas that have been glaciated. These are tributary valleys that lie above the main valley and are separated from it by steep slopes down which streams may flow as a waterfall or a series of rapids. The hanging valleys are formed when the main valley is eroded much more rapidly than the valleys made by tributary glaciers. As a result these smaller valleys hang above the main valley.(Hanging valleys may also form during the retreat of a coastline under rapid erosion.).
A valley glacier carries a large amount of rock waste called moraines. The moraine forming along the sides of a glacier is called lateral moraine; that along the front of a glacier is called terminal moraine; that at the bottom of a glacier is the ground moraine. When two glaciers join together their inner moraines coalesce to give a medial moraine. Terminal moraine material is carried down-valley by the melting waters issuing from the glacier’s snout (front) and is deposited as a layer called an outwash plain. One of the most conspicuous features of lowlands, which have been glaciated by ice sheets, is the widespread morainic deposits. Because of the numerous boulders in the clay these are called boulder clay deposits. The deposits are sometimes several hundred meters thick and their surface is marked by long rounded hills, called drumlins. Large blocks of rock of a material quite different to that of the rocks of the region often occur in areas, which lay under ice sheets. These blocks are known as erratics. Rivers and streams occur inside most glaciers and these are heavily loaded with rock debris. As an ice front retreats the rivers build up ridge-like deposits called eskers. They develop on top of the boulder clay deposits.
Wind is a very effective agent of erosion, transportation and deposition and more so in arid areas where there is little vegetation or moisture to bind the loose surface materials. Erosion by wind is carried out by deflation, abrasion and attrition.
Deflation involves lifting and blowing away of loose materials by wind action thereby lowering the ground levels and producing depressions called deflation hollows. Abrasion takes place when the loose sand particles are carried by wind and hurled against the rocks thereby leading to abrasion or scratching of the exposed surfaces. Attrition takes place when wind born particles of sand wear each other away to form small rounded sand grains.
Due to abrasion or sand blasting by wind borne sand particles produces grooves and hollows in the rock surfaces thereby leading to formation of Rock Pedestals. Since the erosion is maximum near the base where most of the wind born sand particles strike, there results an undercutting in the rocks thereby giving it the form of a mushroom shaped rock.
Some regions have tabular masses of soft rocks lying beneath a layer of more resistant rocks. Wind action erodes the softer layers at a faster rate thereby giving rise to a ridge and furrow landscape called Zeugen. At times instead of lying over one another, the hard and soft rocks exist as alternate vertical bands. Wind abrasion excavates the bands of softer rocks into long narrow corridors separated by ridges of hard rocks called Yardangs.
Mesas or buttes are other features of wind erosion. These are flat table like land masses with a very resistant horizontal top layer that resists erosion and protects the softer layers underneath it. Generally a butte is smaller than a mesa but geographically both are similar.
Inselbergs are Island Mountains or isolated residual hills. They are the relics of an old plateau which has been entirely eroded away by the action of wind. A part of the original plateau being made up of hard granite or gneiss resists erosion and stands out as an isolated hill with rounded tops and steep sides and is called an Inselberg.
Ventifacts or dreikanters are pebbles polished and smoothened by abrasive action of wind.
At times the winds lower the ground surface by blowing away loose unconsolidated materials, thereby giving rise to depressions called the deflation hollows. At times the deflating action of wind lowers the deflation hollows to the extent that the water table is reached thereby leading to formation of oasis or swamps in arid regions.
The sand particles being carried away by wind may get deposited around an obstacle thereby leading to formation of hills of sands called sand dunes. Dunes rooted with vegetation are fixed dunes; while the ones which are on constant move are active or live dunes (The wind action keeps shifting the sand thereby causing the movement of the dunes).
Sand dunes are of two types namely Barchans and Seifs. Barchans are crescent or moon shaped dunes which occur transversely to the wind direction such that their horns thin out and become lower in the direction of the wind due to reduced frictional retardation of wind around the edges. The windward side is convex and gently sloping while the leeward side being sheltered is concave and steep. The sand is driven up on the windward side and after reaching the crest of the dune it falls down towards the concave side so that the dune advances or moves. Migrating sand dunes often threaten human habitations and trees and oasis.
Seifs are longitudinal dunes which are like narrow ridges of sand lying parallel to the direction of winds.
Loess plains are another depositional feature of the wind action. The fine dust of deserts is often blown away beyond the desert limits and is deposited on neighboring lands as loess. Loess is generally fine loam, rich in lime and highly porous. Loess plains are found in the basins of the Hwang-ho.
Waves are the most powerful agents of marine erosion as well as transportation. Tides and currents are weaker agents of erosion and mainly play the role of transporting the eroded material and depositing it as silt, sand and gravel along the coasts.
These agents of marine erosion transform the coastal landscape by the processes of –
The repeated action of waves against a coastal cliff gives rise to wave cut platforms. These are narrow flat areas often found at the base of a sea cliff or along the shoreline created by the erosive action of waves. Wave-cut platforms form when destructive waves hit against the cliff face, causing undercutting between the high and low water marks mainly by the actions of corrosion and hydraulic action thereby forming a notch on the cliff. This notch under repeated wave action develops to form a flat platform.
Similarly, on coasts containing rocks of varying resistance to erosion, the continuous wave action gives birth to capes and bays. Under erosive action of waves the softer rocks are worn back giving rise to bays and the resistant ones project out of the coastline as capes.
Continued wave action on cliffs excavates holes in the softer portion of cliffs. Such holes gradually enlarge to form caves. When two caves approach one another back to back they give rise to an arch. Eventually under continued wave action, the arch collapses to form a pillar of rock called stack. Further erosion of stacks give rise to short pillars called stumps.
Repeated erosive action of waves on the roof of a cave may give rise to blowholes or gloups. The continued action of waves on the cave roof will lead to enlargement of the gloup to such an extent that the roof will collapse giving rise to a geo.
Sand and gravel eroded from the coastlines is deposited along the shore by waves and tides as beaches. The Longshore Drift moving obliquely to the shore deposits the material along it. The coarser and heavier material like shingles, pebbles, boulders are deposited at the top of the beach while the finer materials like sand are carried by the backwash[4] and deposited closer to the sea.
At times the material carried by waves is deposited at the mouth of a river or a bay thereby giving rise to spits and bars. The water body enclosed by a bar is called a lagoon.
The strong onshore winds often carry fine sand from the coastal beaches inland thereby giving rise to marine dunes another landform associated with coastal landscapes.
AGENTS OF EROSION AND ASSOCIATED LANDFORMS
Agent
Processes
Landforms
Water (Fluvial action
Traction Abrasion Attrition Suspension Solution
Youth stage
Rapids, Waterfalls, Potholes, Plunge pools, Gorges, Canyons, Cataracts, River Capture
Mature stage
Meanders, River Cliffs and Slip off Slopes, Interlocking spurs
Old stage
Braided stream, Levees, ox-bow lakes, Delta, Flood Plain
Ice (Glacial action)
Abrasion Suspension Plucking
Highland Glaciation
Corrie, Aretes, Pyramidal Peaks, Bergschrund Creavasses, U-Shaped Valleys, Hanging Valleys, Rock Basins, Rock step, Moraines (Lateral, Medial, Ground, Recessional, Terminal)
Lowland Glaciation
Roche Moutonnee, Crag and Tail, Boulder clay or Glacial till, Erratics, Drumlins, Eskers, Outwash Plains, Terminal Moraincs, Kames, Kettle Lakes
Wind (Aeolian action)
Deflation Abrasion Attrition
Erosional
Rock Pedestals or Mushroom Rocks, Zeugen, Yardangs, Mesas, Buttes, Inselberg, Ventifacts or Drickanter, Deflation Hollows, Pediment
Depositional
Dunes (Barchans, Seifs), Loess plains, Bajada.
Waves (marine action)
Corrosion Attrition Hydraulic Action
Solution
Capes, Bays, Cliffs, Wave Cut platforms, cave Arch and stump, Geos and Gloups, Blow Hole
Beaches, spits and Bars, Marine dunes and Dune Belts.
Under ground water (Karst action)
Limestone Pavements Grikes, Clints, Swallow Holes, Doline, Uvala, Poljie, Stalactities, Stalagmites, Pillar, Coombes.
[1] The heat for this temperature difference comes from radioactive decay and residual heat within the earth.
[2] The age of rocks in oceanic crust is nowhere more than 200 million years old whereas the continental crust at places is as old as 3200 million years.
[3] See plate tectonics
[4] The retreating wave is called backwash.
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