Geography of Earth
Soon after the formation of the earth, it was divided into metallic core,% silicate mantle and crust – which, along to with surface water, made it different from the other planets in our Solar System. The formation of the early mantle was important as it consisted primarily of ferromagnesium silicate minerals, some of which contained water as an essential component. The Earth’s atmosphere and hydrosphere developed from the degassing ( loss of gaseous elements such as carbon, hydrogen and oxygen ) of the early – formed core and mantle during this volcanic activity.
Geographic Structure of Earth
Geography of Earth’s Core
- It has found that at a depth of about 2,900 kms. ( 1,800 miles ), the S Earthquake waves which can pass through only solid objects, suddenly disappear. Further, at this depth the velocity of the P earthquake waves which can travel through solids, liquids as well as gases, abruptly decreases from 13.7 kms per second to 8 kms per second. I this has been identified as the outer limit of the core. It is possible to determine this limit very clearly because at this depth the P waves suddenly decrease in velocity and the S waves are unable to penetrate.
- On the basis of evidence of meteorites and our knowledge about density it has been concluded that the core is primarily made of iron which is in liquid condition. At a depth of 5,080 kms ( 3,160 miles ) there is again some increase \ in the velocity of P waves. From this it is concluded that the interior of the, core is probably solid where the high pressure ash solidified the iron.
- Thus it is possible to subdivide the core into two parts the inner core and the outer core. The radius of the inner core is approximately 1,250 kms ( 1,400 miles ). In other words, the combined radius of the inner and the outer core taken together is 3,500 kms or about 2,200 miles which is more than half of the radius of the earth. It is estimated that the maximum temperature of the core is 5,500° C ( 10,000° F ) or slightly less and the specific gravity is 13. The outer core has sometimes been also referred to as the Shell.
- The inner and the outer cores taken together are equivalent to the nife of Sucess.
Geography of Mantle
- Above the core and below the crust there is a thick intermediate layer called the mantle. Its thickness is about 2,900 kms ( 1800 miles ). This is solid, as both P and S waves pass through it quickly. The specific gravity varies from 3 to 3.5 in the upper part of the mantle to about 4.5 or more in the lower part of the mantle.
- The mantle is composed of dense and rigid rocks which have predominance of minerals like magnesium and iron. These rocks are probably very similar to dunites and peridotites.
- It is possible that the mantle is sub – divided into thinner layers each containing a distinctive group of minerals. But broadly speaking, we may subdivide it into two parts the lower mantle or the mesosphere and the upper mantle or the asthenosphere.
- The lower part of the asthenosphere like the mesosphere, is solid, but the upper part of the asthenosphere is plastic and is in a partially molten condition. The velocity of the earthquake waves decreases in the asthenosphere, and the asthenosphere is, therefore, referred to as the low Velocity Zone.
Geography of Earth’s Crust
- This is the uppermost and the thinnest layer of the earth. Its thickness varies from 16 to 40 kms ( 10 to 25 miles ). In the continental areas its thickness is about 40 Kms ( 25 miles ) and has been found to vary from 30 to 65 kms ( 18 to 40 miles ).
- It is composed of various kinds of rocks. In its uppermost part we find sedimentary rocks. This sedimentary layer is not continuous over the entire surface of the earth and is generally thin. Its thickness is usually less than 2 mifes ( 3.2 krns ), but in areas of folded mountains this may increase to 20 miles ( 32 kms ) or more.
- Below the sedimentary cove is a layer of crystalline rocks, consisting of granites and gneisses in its upper section and basaltic rocks in its lower section. Some times these crystalline rocks cover wide areas on the surface of the earth, such as in Western Australia, Peninsular India, Middle Africa, Brazilian plateau, Eastern Canada, Scandinavia and North – east Asia. Underneath the ocean basins, the thickness of crust is less, and varies from 5 to 10 kms ( 3 to 6 miles ). Here the sedimentary layer is either thin or absent. Even the granitic layer is absent, and we come across mostly basaltic rocks.
- Thus we may subdivide the earth’s crust into two parts – ( i ) an upper discontinuous layer composed mostly of the continuous layer which is exposed on the floor of the ocean basins but is found below the surface on the continents. The average specific gravity of this lower layer is 3.0. These two sub – divisions are roughly equivalent to the sea and sima, though sima was considered to be equivalent to the intermediate layer ( mantle ) by Suess.
- The contact zone of the crust and the mantle is called Mohorovicic or Moho discontinuity. Here the rocks are different in chemical composition from those below and above. Here also the earthquake P waves suddenly increase in velocity from 6 to 7 kms per second in the crust to 8.2 kms per second.
- Moho is found at the depth of 45 to 60 kms and sometimes 70 kms from the surface in the principal fold mountain areas, as the crust is thicker in these areas. On the other hand, Moho is found only at the depth of 5 to 7 kms below the floor of the oceanic plains, as the crust is thinner here.
- Outside the mountainous zones. Moho is usually met with at the depth of 30 to 35 kms below the continental surfaces.
- Lithosphere actually includes the crust and the uppermost part of the plastic asthenosphere. The land masses are composed of mainly brittle rocks and are 70 to 100 kms ( 80 to 100 miles ).
- As the thin layer of lithosphere is located on the partially melted layer of the asthenosphere, the lithosphere is unstable and is liable to be easily deformed – This has important implications for, the recent theories of plate tectonics, sea – floor spreading as well as mountain building and volcanic action.
- The forces or movements responsible for the formation of relief features and changes occurring in them are known as Earth Movements. These forces are divided into two broad categories :
- Endogenic Forces which cause land upliftment, subsidence, folding faulting, earthquakes, volcnism etc.
- Exogenetic Force which cause destruction of relief features through their weathering, erosinal and depositional activities.
Geography of Endogenetic Forces
- The forces coming from within the earth and causing horizontal and vertical movements are known as Endogenetic forces. It is these movements which lead to land upliftment and subsidence, folding and faulting, earthquakes and valcanism, etc. Endogenetic movements are responsible for giving birth to major relief features such as mountains, plateaus, plains, valleys, etc. These endogenetic movements fall into two major categories, on the basis of intensity.
- Sudden Movements : Sudden movements result into sudden and rapid events such as earthquakes and volcanic eruptions. It must be noted that these events such as earthquakes and volcanic eruptions. It must be noted that these events are the result of long period of preparation deep within the earth. Only their effects in the form of earthquakes and volcanic eruptions are experienced as sudden events. These are also termed as constructive movements as they produce certain relief features such as volcanic mountains and lave plateaus ( e.g. Deccan ).
- Diastrophic Movements : These movements comprising both vertical and horizontal movements operate very slowly and their effects become perceptible after thousands and millions of years. These movements are further sub – divided into :
- Epeirogenetic Movements : These movements affect the continental masses, causing their uplift and subsidence or emergence and submergence through upward and downward movements respectively. In fact Epeirogenetic movements are vertical movements.
- Orogenetic Movements : These movements are caused by the endogenetic forces working in horizontal manner and they involve fording, bending, faulting and thrusting. These endogenetic forces, also known as tangential forces, are of two types :
- Compressional movement ( convergent movement ) : When ore genetic or horizontal forces operate face to face. They cause folding of the rock strata on the earth.
- Tensional movement ( Divergent Movement ) : When horizontal forces operate in opposite directions. They produce cracks, fracture and faults in the crustal parts of the earth.
Geography of Folds
- Wave – like bends are formed in the crustal rocks due to tangential compressive force resulting from horizontal movement caused by the endogenetic force originating deep within the earth. Such bends are called ‘folds’ wherein some parts are bent up and some parts are bent down. The upfolded rock strata in arch – like form are – called ‘anticlines’ while the down folded structure forming through – like feature is called ‘syncline’. The two sides of a fold are called limbs of the fold.
Geography Types of Folds
- Symmetrical Folds are simple folds, the limbs ( both ) of which incline uniformly. These folds are an example of open fold.
- A symmetrical Folds are characterized by unequal and irregular limbs. Both the limbs incline at different angles.
- Monoclinal Folds are those in which one limb inclines moderately with regular slop while the other climb inclines steeply at right angle and the slope is almost vertical. It may be pointed out that vertical force and movements are held responsible for the formation of monoclinal folds. It is also opined that monoclinal folds are also formed due to unequal horizontal compressive forces coming from both the sides.
- Isoclinal Folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel but not horizontal..
- Recumbent Folds are formed when the compressive forces are so strong ( hat both the limbs of the fold become parallel as well as horizontal ).
- Overturned Folds are those folds in which one limb the fold is thrust upon’ another fold due to intense compressive force.
- Plunge Folds are formed when the axis of the fold instead of being parallel to the horizontal plane becomes tilted and forms plunge angle which is the angle between the axis and the horizontal plane.
- Fan Folds represent an extensive and broad fold consisting of several minor anticlines and synclines. Such fold resembles as fan. Such feature is also called as anticlinorium or synclinorium.
- Open Folds are those in which the angle between the two limbs of the fold is more that 90° but less than 180° ( i.e. obtuse angle ). Such open folds are formed due to moderate nature of compressive force.
- Closed Folds are those folds in which the angle between the two limbs of a fold is acute angle. Such folds are formed because of intense compressive force.
- When the tensional force is moderate, the crustal rocks develop only cracks ( fractures ) but when intense tensional force work the rock beds are dislocated and displaced also, resulting into formation of faults. Thu§, faults are those fractures in the rock body along which there has been an observable amount of displacement.
- Normal Faults : The faults having displacement of both the rock blocks in opposite direction are called normal faults. Movement of rocks take place vertically, so that one side is raised or upthrown. In the case of normal faults, there occurs extension of the faulted area.
- Reverse Faults ( Thrust Fault ) : On account of extreme compression, along with the tensional force, rocks snap and one block of fractured rock is pushed over the underlying block. Fractured rock blocks move towards each other in the Reverse faults. There is thus shortening of the crust in these faults.
- Lateral or Strike – Slip Fault : This fault is formed when the rock blocks are displaced horizontally along the fault plane due to horizontal movement. They are commonly produced where one tectonic plate slides past another at a transform fault boundary. Landforms related to faulting.
- Rift Valley : A linear depression or trough created by the sinking of the intermediate crustal rocks between two or more parallel faults is known as a Rift Valley. The East African Rift Valley System and Rhine rift valley are famous examples of these morphological features.
- Dead Sea, the second most saline lake in the world after lake Van is situated in a rift valley. Narmada and Tapti, rivers are believed to be flowing in a rift valley.
- Ramp Valley : When both the side block of rocks are raised and the middle portion remains standstill, the resultant trough is known as a Ramp Valley. Brahmaputra Valley is regarded as a Ramp Valley.
- Block Mountain : Also known as fault block mountains, these mountains are the result of faulting caused by tensile and compressive forces. They represent the upstanding parts of the ground between two faults or on either side of a rift valley. Noted examples are.Vosges and Black Forest mountains bordering the faulted Rihne rift valley, Wasatch range in USA and Sierra Navada mountains of California ( considered to be the most extensive Block mountain of the world ).
Geography of Rocks
- Rocks are solid material which made up of Earth’s crust. They include hard and resistant material like granite and marble, and the loose material like silt and sand. The commonly found minerals in the rock are feldspar and quartz. The metal compound of rocks is known as ‘ores’.
- The igneous rocks is made up of ( i ) solidification of Magma and ( ii ) granitisation. It is the ancestor of all other rocks and make up 85 percent of or more of the earth’s crust, ft is also called primary rocks from which all the rocks is made.
- The term ‘Magma’ refers to molten underground material.
- When the molten material reaches the surface, it is known as ‘Lava’.
- On the basis of origin the igneous rocks can be classified into intrusive and extrusive varieties.
- ‘Batholiths’ are the intrusive rock below the surface of the Earth.
- The igneous rocks can be classified broadly on the four bases : ( i ) Process of origin, ( ii ) Place of origin, (iii) Mineral composition, (iv) Texture.
- The underground igneous rocks may be classified into two categories ( i ) Hypabyssal Rocks – It is formed just below the surface of the Earth usually in dykes and sills ( ii ) Plutonic Rocks – It is formed deep beneath the ground in the plutons and batholiths.
- The igneous rocks are made mainly of silicate ( SiO2 ) and often combine with other oxides of aluminum, potassium, sodium, calcium, iron, magnesium etc. some of the important igneous rocks are granites, rhyolite, pegmatite, syenite, diorite, andesite gabbro, basalt, dolerite and periodotite.
- Igneous rocks are formed by the cooling, solidification and crystallization of molten earth materials, known as magma and lava.
- Igneous rocks are also called as Primary rocks or Parent rocks because these were originated first during the formation of crust through the process of cooling of the earth surface.
- They do not have distinct beds or strata like the sedimentary rocks.
- These are granular and crystalline rocks. The size of the crystals vary from one rock to another.
- Igneous rocks are generally hard and water percolates through them with great difficulty along the joints.
- Since water does not percolate easily, these rocks are less affected by chemical weathering.
- These rocks are more prone to mechanical weathering due to their granular structure.
- These rocks are non – fossiliferous.
- Most of the igneous rocks consist of silicate minerals.
- On the basis of chemical composition, Igneous Rocks are divided into : ( i ) Acidic Igneous Rocks having more silica. They are light rocks relatively, e.g. Granites ( ii ) Basic Igneous Rocks have lower amount of silica. They are dark – coloured due to pre – dominance of ferro – magnesium, e.g. Gabbro, Basalt, etc.
- On the basis of mode of occurrence, Igneous Rocks are classified into two major groups : Intrusive Igneous Rocks : When the rising magma is cooled and solidified below the surface of the earth, they are known as Intrusive Igneous Rocks. These are further sub – divided into :
a ) Plutonic Igneous Rocks : They result from the cooling of magma very deep inside the earth. Due to very slow cooling at that great depth, large grains are developed, e.g. Granite.
b ) Hypabyssal Igneous Rocks : They are formed when magma cools & solidify just beneath the earth surface. They take different shapes and forms depending upon the hollow places in which they solidify.
- Batholith : These are large intrusive mass of igneous rocks, usually granite, formed by the deep – seated intrusion of magma on a large scale. They are known to be the largest kind of intrusive bodies and are usually dome shaped with very steep walls. They are known to be present in the core of most of the mountains.
- Lacolith : These are of mushroom shape having convex upper suface and a relatively flat lower one. The ascending magma forces the upper layer of the sedimentary rocks to take the form of a convex arch or a dome.
- Lopoliths : They represent inter – stratal bowl – like bodies formed by the solidification of magma in a concave shallow basin and the sagging of rocks under the weight of the intruded magma.
- Phacoliths : These lens – shaped bodies are formed due to injection of magma along the anticlines and synclines in the folded strata.
- Sills : They are bed – like intrusive bodies formed by the solidification of magma parallel to the bedding planes of the sedimentary rocks.
- Dykes : These wall – like formation of solidified magma are found mostly perpendicular to the beds of sedimentary rocks.
- Sills, Lacoliths, Lopoliths and Phacoliths are concordant intrusive bodies while Batholiths and Dykes are discordant intrusive bodies.
- Extrusive Igneous Rocks : These igneous rocks are formed by the cooling and solidification of molten lava on the earth’s surface. Basalt is the most important example of extrusive igneous rocks, others being Gabbro and Obridian. These are generally fine grained or glassy because of quick rate of cooling of lava. The extrusive igneous rocks are divided into two sub – groups :
( i ) Explosive Type : Volcanic materials of violent volcanic eruptions include ‘bombs’ ( big fragments of rocks ), lapilli ( pea – sized fragments ) and volcanic dusts and ashes.
( ii ) Quiet Type : In this, lava appear on the surface through cracks and fissures and their continous flow form extensive lava plateaus, e.g., Deccan Plateau, Columbia Plateau ( USA ).
- Sedimentary rocks are constituted of sediments, a material from air and water that settle down. About 70 per cent of the rock exposed to the surface of the earth is sedimentary rocks. It is also called stratified rocks because it is found in the layers.
- The fossils are found in the layers of sedimentary rocks. A fossil refers to any part of the once living things preserved in the rock. It may be entire body, a single bone or a set of footprints. It tells up about the life in past and they help us to date environment. Fossils also show what kind of animals lived in the past
- The layers of sedimentary rocks hold all reserve of coal, oil and natural gas.
- The ‘Lithification’ is a process that turns loose sediments into rock. It takes millions of years.
- The ‘compaction’ refers to squeezing of sediments to form hard rock.
- The ‘cementation’ refers to binding together of the compact sediments.
- ‘Limestone’ is a chemically precipitated sedimentary work which is formed by the compaction and lithification of the shells from marine organism. Sandstone is formed by the compaction of quartz grains.
- The five factors which controls the properties of sedimentary rocks are – ( i ) Kind of rock in the source area ( ii ) Environment of the source area ( iii ) Earth movement ( iv ) Environment of deposit in areas ( tectonism ) and ( v ) Post depositional change of the sediments.
- The mechanically formed sedimentary rock contain pieces of other rocks. Agents like running water, wind and moving ice break them into smaller pieces and deposits them at new sites where they form new sedimentary rocks.
- Organically formed sedimentary rocks consists of the remains of animals and plants. Limestone, chalk and corals are the most common of this type of sedimentary rocks.
- Chemically formed rocks are formed by the direct precipitation of mineral matter from solution. Rock – salt is an example of such rocks. Gypsum is also formed in a similar manner.
- ‘Sandstone’ is a common sedimentary rock, is formed mainly of quartz particles cemented together by silica, lime or iron oxide.
- ‘Shale’ is most abundant of all sedimentary rock. It is compacted silt and clay. Kaolin and clay minerals are abundant in it.
- Rock gypsum is a white to reddish in colour. Gypsum and rock salt are formed by the evaporation of sea water and salt lakes.
- ‘Chalk’ is a calcareous rock made up of microscopic skeletal elements from a varieties of limes secreting organism. It is composed of almost pure calcium carbonate.
- Rocks formed from material derived from pre – existing rocks and from organic sources by the process of denudation are known as sedimentary rocks : In other words, rocks formed due to aggregation and compaction of sediments are called sedimentary rocks.
- Sedimentary rocks contain different layers of sediments.
- About 75% of the surface area of the globe is covered by the sedimentary rocks while rest 25% area is occupied by the igneous and metamorphic rocks.
- Though sedimentary rocks cover largest area of the earth’s surface, they constitute only 5% of the composition of the crust while 95% of the crust is composed of igneous and metamorphic rocks.
- Layers of sedimentary rocks are seldom found in original and horizontal manner. They are prone to folding and faulting due to compressional and tensional forces.
- Most of the sedimentary rocks are permeable and porous but a few of them are also non – porous such as clay.
- Shale is the most abundant sedimentary rock.
Geography of Metamorphic Rocks
- The Metamorphic rocks are formed when igneous or sedimentary rocks are transformed underground by the altering of sedimentary igneous rocks. It is generally caused by heat, pressure, chemical action, volcanic activity or movement at earth’s crust.
- The ‘Metamorphism’ is a process by which an already consolidated rocks undergoes changes in or modification of texture, composition or structure either physical or chemical.
- The metamorphic rocks may be classified into two categories :
( i ) The Foliated and ( ii ) Non – Foliated. The foliated rock is characterised by parallel arrangement of slaty minerals such as mica. In the non – foliated metamorphic rock, the minerals grains are equdiamensional e.g. quartzite and marble.
- The formation of metamorphic rock refers that in course of time shale may get changed to slate and schist, limestones to marble, sandstone to quartzite and granite to gneiss.
- Shale after being squeezed sheared under mountain building force, is altered into slate. This grey or brick red rocks splits neatly into thin plates and is used as roofing shingles and as flagstones.
- Slate may change into schist with continued application of pressure and internal shearing. It is the most advanced grade of metamorphic rock.
- Limestone after going under metamorphism, becomes marble. Calcite and dolomite are the main rock forming minerals.
- Quartzite is a metamorphosed form of sedimentary rock. The slow movements of underground water carry silica into the sandstone and completely fill the space between the grains. Pressure and kneading of the rock is not essential in producing a quartzites.
- Gneiss is formed either from intrusive igneous rock or from elastic sedimentary rocks that have been in the close contact with the intrusive magmas. It is coarser than Schist.
- The Metamorphic rocks are generally hard and gems are found in the metamorphic rocks.
- Rock cycle is a general model that describes how various geological process create, modify and influence rocks. It is relationship between the three types of rocks. The first part of the rock cycle take place on the earth surface. It is a continuous process through which old rocks are transformed into new one.
- All of the rock type can be returned to the earth’s interior by tectonic forces at areas known as ‘subduction zones’.
Geography of Earthquakes
- When there is sudden disturbance of rocks in the earth’s interior, vibrations spread out in all directions from the source of the disturbance. An earthquake is the passage of these vibrations through earth’s crust.
- Earthquakes are caused by either volcanic explosions or by sudden movement of rocks generally along fault planes. Accordingly earthquakes are distinguished as volcanic & tectonic earthquakes.
- Tectonic earthquakes are felt over a much wider area & are more common. Volcanic earthquakes are generally of shallow origin & their area of disturbance is relatively small.
- Tectonic earthquakes may originate at depths, which may vary from only a few kilometers to over 700 Kms. They are classified as :
- Shallow or Normal ( Depth of origin is less than 60 Kms )
- Intermediate ( Depth of origin between 60 & 300 Kms )
- Deep. ( Depth of origin is between 300 to 700 Kms )
Intensity & Magnitude of Earthquack
- The place of origin of the earthquake below the ground is called the focus. The point or line on the surface vertically above the focus is called epicenter.
- The intensify of the earthquake is also maximum at the epicentre.
- In 1902, a scale of intensity based on the amount of damage to various types of structure was developed by Italian seismologist Mercalli.
- Magnitude of an earthquake is the total amount of energy released during an earthquake. The richter scale devised by CE Richter is used to describe the magnitude of an earthquake.
Effects of Earthquakes
- The geomorphological effects are not spectacular but create sudden topographie changes.
- Vertical displacements along faults are common. On one side of the fault, the surface rocks are raised while on the other side they may be depressed.
- Due to passage of land waves, fissure gape open at the crest.
- Glaciers are broken and icebergs suddenly become abundant.
- In alluvial plains, the sandy deposits gets compacted with the passage of earthquake vibrations and water filled sand escapes with great force.
- Groundwater may be disturbed by earthquakes in other ways. Lakes may be drained off by the opening of cracks and new lakes may be formed in depression.
- The appalling loss of human life is mainly related to the secondary events which are triggered by the earthquake such as the collapse of buildings, fires, landsides, floods & seismic Sea waves.
- The giant seawaves associated with earthquakes of high magnitude are called tsunami in Japan.
- In deep waters, the tsunami waves have wavelingth of hundreds of kilometers and travels at speeds of 700 to 1000 Km an hour. The energy transmitted in immense because the whole depth of water is involved. Therefore when these waves reach shallow coastal waters & narrow bays & intels, they have been known to grow into a wall of water 30m or greater.
Distribution of Earthquakes
- Earthquakes are not randomly distributed over the globe but they tend to occur in narrow continuous belt.
- These earthquake belts encircle large seismically quite regions, which
- constitute the plates of lithosphere. The plates are the continuous motion with respect to one another, and this relative movement of plates or plate motion is the fundamental cause of the earthquakes.
- Most earthquakes occur on the boundaries between lithospheric plates and arise directly from the motion between the plates, though there are some that cannot be so simply related to the movements of the plates.
- We may identify three well – defined belts or zones of seismic activity in the world there most earthquakes originate. These earthquake zones are – ( i ) a Circum – Pacific zone, ( ii ) a Mediterranean and Trans – Asiatic zone, and ( iii ) a zone following the mid – oceanic ridges with an extension along the East African rift valley system.
- The Circum – Pacific Zone : The Circum – Pacific zone follows the oceanic trenches and the associated island areas where plates converge and the oceanic lithosphere is thrust down into the asthenosphere and re-melted, and this melting supplies the magma for the volcanic arcs which occur behind the trenches.
- The epicenters of western side of the Pacific, this zone, starting from Alaska runs towards the south parallel to the Kurite, Japan, Marianas and Philippine trenches, beyond which it divides into two branches, one going west parallel to the Indonesian trench and the other towards the Keramac – Tonga trenches to the north – west to New Zealand.
- On the eastern side of the Pacific, the earthquake zone follows the west coast of North America, being particularly important in California, although there is no ocean trench associated with it there. It continues southwards parallel to the middle American trench and further south along the Peru and Chile trench on the west coast of South America.
- Shallow, intermediate as well as deep earthquakes are found to occur along the Circum – Pacific belt, but deep earthquakes are practically restricted to this zone.
- We have already referred to the existence of inclined earthquake zone known as Beniff zone along the Pacific coasts where the foci of earthquakes deepen from shallow through intermediate to deep in a landwatd direction from the trenches.
- This inclined earthquake or Beniff zone depicts the route along which the oceanic lithosphere descends into the mantle along eh trenches, and its downward progress is recorded by a series of earthquakes, which have their foci at various depths and many are as deep as 300 to 700 kms below the sea level.
- Further, the earthquakes are not restricted, to the plate boundary itself, but occur over a broad zone, several hundred kilometres wide, adjacent to the plate boundary. Such earthquakes may be called plate – boundary related earthquakes.
- They do not reflect the plate motions directly but are secondarily caused by “the stresses set up at the plate boundary. The best examples of such earthquakes are to be found in Japan where the plate boundaries are in the deep ocean trenches off the Japanese islands, and that is where the great plate boundary earthquakes occur.
- But many smaller earthquakes’ occur scattered throughout the Japanese island, caused by the overall compression of the whole region.
- The Mediterranean and Trans-Asiatic zone : This earthquake belt extends along the Alpine mountain system of Europe and North Africa, through Asia Minor and the Caucasus, Iran and Pakistan and China. This zone is characterized mostly by larger earthquakes of shallow origin and some of intermediate origin. Deep focus earthquakes are almost absent. This belt is not associated with oceanic trenches but with the Tertiary and Recent organic belts where continental plates collide and the lithosphere buckles under the force of the collision forming the great mountain ranges.
- The mid – oceanic ridges and the African rift system zone : This zone lies mostly along the mid – oceanic ridges and the transform faults and contains mostly earthquakes of the shallow variety. These constitute major fracture zones where the plates diverge and new oceanic crust is being formed by the upwelling of magma on the mid – ocean ridges. An extension of this belt is to the found along the Red Sea and the rift valleys of East Africa.
Geography of Volcanoes
- A volcano is a vent or opening usually circular in form through which heated materials consisting of gases, water, liquid lava and fragments of rocks are ejected from the highly heated interior to the surface of the earth.
- Volcanic eruptions are closely associated with several interconnected processes such as ( i ) the gradual increase in temperature with increasing depth at a rate of 1° C per 32m due to heat generated by degeneration of radioactive elements inside the earth, ( ii ) origin of magma because of lowering of melting point caused by reduction in pressure of overlying rocks due to fractures caused by splitting of plates ( iii ) origin of gases and water vapour due to heating of water ( iv ) Ascent of magma due to pressure from gases and vapour ( v ) occurence of volcanic eruption. These eruptions are closely associated with plate boundaries.
- Volcanoes are classified under different schemes.
- Classification on the basis of periodicity of eruptions.
a ) Active volcano e.g. Etna, stromboli, pinatubo etc.
b ) Dormant volcano e.g. visuvious, barren island volcano ( Andamans ).
c ) Extinct volcano e.g. where no indication of future eruption is estimated.
- Classification on the basis of the mode of eruption.
( i ) Central eruption type or explosive type e.g. Hawiian type, strombolian type, volcanian type, pelean type, visuvius type.
( ii ) Fissure eruption or quiet eruption type e.g. lava flow or flood, mud flow and fumaroles.
- Large quantities of lava quietly well up from fissure and spread out over the surrounding countryside. Successive lava flows results in the growth of a lava platform which may be extensive to be called a plateau like “Deccan” “Columbia Snake Plateau”, “Drakenberg Mountains”, “Victoria and Kimberley” districts of Australia, “Jawa Island”.
Topography Produced by Volcanoes
- Cinder or Ash cone : They are of low height and are formed of volcanic dust, ashes and pyroclastic matter. Its formation takes place due to accumulation of finer particles around the volcanoes vent.
- Composite cones : They are formed due to the accumulation of different layers of various volcanic materials.
- Parasite cones : When lava comes out of the minor pipes coming out of the main central pipe, parasite cones are formed.
- Basic lava cone : It has less quantity of silica in its lava.
- Acidic lava cone : It has more silica in its lava.
- Lava domes : These are formed due to accumulation of solidified lavas around the volcanic vents. .
- Lava plugs : They are formed due to plugging of volcanic pipes and vents when volcano becomes extinct.
- Craters : The depression formed at the mouth of a volcanic vent is called a crater. When it is filled with water it becomes a ‘crater lake’ e.g. lake lonar in Maharashtra.
- Calderas : Generally enlarged form of craters is called caldera. If is formed due to subsidence of a crater.
- Geysers : They are intermittent hot springs that from time to time spout steam and hot water from their craters.
- Fumaroles : It is a vent through which there is emission of gases and water vapour.
- About 15% of world’s active volcanoes are found along the “construction or divergent plate margins, whereas 80% volcanoes are associated with the” destructive or convergent plate boundaries.
- The ‘circum Pacific belt’ or Pacific ring of fire includes volcanoes of the eastern and western coastal areas of the Pacific Ocean, island areas and festoons off the east coast of Asia etc.
- The Mid – Continental Belt include volcanoes of Alpine mountain chain, the Mediterranean sea and the fault zone of eastern Africa e.g. Stramboli, Visuvious, Etna, Kilimanjaro, Mera, Elgon, Birunga etc.
- The Mid-Atlantic Belt : The volcanoes of these areas are mainly of fissures eruption type. The most active volcanic area is Iceland.
- Cerro Aconcagua ( 6960 metres ), the highest peak in Andes, South .America, is an extinct volcano while Kilimanjaro ( 5895 metre ).
- Tanzania, Africa and volcano Llullailiaco ( 6723 metres ) in Chile, South America are classified as dormant.
- The Hawaiian type volcanoe are characterised by the eruption of lava of the basic ( basaltic ) composition with the temperature about 1200 degree Celsius, which overflowing from the crater run down the slopes at the velocity of 8 – 10 miles / sec, thus forming lava stream the length of which is as great as 40 – 50 km and even 80 km the lava is relatively poor in gases, and the explosion are hardly ever noticed.
- The ‘Volcanic Pipes’ refers to a special type of the so called monogenic volcanoes, since there origin finds its expression in a single explosion without any emergence of lava. The diameter of a pipe usually comprises 80 to 100 metres and they are filled.
- The gases accompanying the eruption of volcanoes of all types continue to emanates from the main vent and from fissure on the slope and at the foot of the volcano long after eruption proper. These are so called Fumargles and Solfataras gases.
- The Pacific zone is the largest volcanic zone. It include 60 percent of the all the volcanoes and the large number of those that have recently becomes extinguished. It extends across the Kamchataka Peninsula, the kuril Islands, the Islands of Japan, Philippines, New Guinea, New Zealand and the Solotnan islands. It also passes through the Antarctica and western coast of America.
- The Mediterranean and Indonesian zone extended from Alps across Apennines, causes, the mountains of Asia Minor and the islands of Malay Archipelago. This is the zone where largest volcanoes of Europe is situated.
- The ‘Atlantic Zone’ is associated with the central and most elevated part of the submarine mid-Atlantic ridge and the fault accompanying it. It include Canary Islands, Cape Verde, Azores etc.
- The ‘Indo – African Zone’ is mainly represented by those volcanoes that are found on the islands of the Indian Ocean, for example, Comoro islands, islands of Mauritius, Saint Paul island etc.
- Active Volcanos : The Volcanos which continue to erupt periodically are called active volcanoes. Mona Loa in Hawaii Island, Etna in Sicily and Vesuvius in Italy are example of active volcanoes. There are about 850 Active Volconoes out of which nearly 80 are in the oceans. Volcanoes are said to be active when eruption occurs frequently.
- Dormant Volcanos : It is the volcanos which have been quiescent for a long time but in which there is a possibility of eruption are called “Dormant Volcanoes”. Fujiyama of Japan and Karakota of Indonesia are examples of dormant volcanoes. It is dormant because no eruption has occured during historic time.
- Extinct Volcanos : It is the volcanoes in which the eruption has completely stopped and is not likely to occure are called Extinct Volcanoes. For example, Popa Mountain in Burma ( Myanmar ), Mt. Kilimanjaro in Africa and Mountains in Mouritius, Malagassy and several other islands in the Indian ocean.
Geographic Product of Volcano
- Solid Product : The solid products of volcanic activities are called pyroclastic, since they consists of fragmental material that emerged during volcanic explosion as a result of the ejection into atmosphere and dispersion of huge masses of lava, as well as the fragments of rocks which are exploded parts of the craters.
- Liquid Product : Liquid product of volcanic eruption are represented by lava. According to their chemical composition, Lava can be either acid, medium, basic or ultrabasic. The chemical composition of the lava determines their most important physical properties, viscosity and mobility which the characteristic of volcanic eruption.
- Gaseous Product : It is the volcanic product which are released from the vent, funnel, subordinate vents and numerous Fissure 60 – 90% of them consists of water vapour, during the condensation of which the atmosphere becomes very often characterised by heavy rains accompanying eruption of volcanoes. In addition to water vapours their composition includes H2S, So, Co, Co2, HF, NH4 Cl, NH3, H2, H2BO3 & other gases.
Exogenetic Forces Geography
- The exogenetic or geomorphic proceses or forces originate outside the earth’s crust, mainly from the atmosphere and therefore have been termed as exogenetic processes. These processes are continuously engaged in the destruction of the relief features created by the endogenetic forces. The major geomorphic processes include weathering, mass wasting and erosion.
- Weathering is disintegration and decomposition of rocks while erosion is the process of removed, transportation and deposition of the weathered particles. These two processes, together are known as “Denudation”. Weathering process brings mechanical disintegration and chemical decaying of rocks. Weather conditions are the most decisive phenomenon so the process is commonly known as weathering. However the type and rate of weathering are also influenced by rock structure, topography and vegetation. Weathering is a static process. It is also the process of soil genesis. It is of three types :
- Mechnical Weathering : When a region undergoes mechanical weathering, rocks are broken into small pieces. This mechanical disintegration takes place in different ways.
- Frost Action : in cold climatic region when water fills the pores, cracks and crevices in rocks and freezes, it expands and exert a brusting pressure. The rocks are ruptured and fragmented.
- Thermal Expansion and Contraction – In the area of hot deserts, the diurnal range of temperature brings the expansion and contraction of surface rocks, leading to their disintegration into smaller pieces.
- Exfoliation : This is the expansion by unloading process. Unloading occurs when large igneous bodies are exposed through the erosional removal of over-lying rock and the reduction in the pressure. On being exposed to the surface they expand slightly in volume. This lead to the beaking of thick shells like a onion’s layers from the parent mass just lying below.
- Chemical Weathering : It changes the basic properties of the rock. Principal processes of chemical weathering are :
- Solution : Here the rocks are completely dissolved. It leads to the evolution of karst topography where the water dissolve the rock structure of limestone, salt, gypsum, chalk etc.
- Oxidation : The presence of dissolved oxygen in water when comes in contact with mineral surface it leads to oxidation. Though it is an universal phenomenon but it is more apparent in rocks containing iron.
- Hydration : Most of the rock forming minerals absorbs water. This not only increase their volume but also produces chemical changes resulting in the formation of new minerals which are softer and more voluminous. For example this process converts hematite into limonite.
- Carbonation : Water combining with carbon-dioxide produces carbonic acid which dissolves several elements of minerals and the rock is weakened and broken into pieces.
- Biological Weathering : This type of weathering is performed by the tree roots, animals and human beings. As the plant roots grow, they wedge the rocks apart and causes the widening of joints and other fractures: Micro animals like earth worms, ants termites and other burrowing animals move materials to or near the surface where they are more closely subjected to chemical weathering.
- Physical weathering is more important in hot and dry climatic regions because of high diurnal range of temperature found there.
- Intense chemical weathering occurs in hot and humid regions.
- Chemical weathering is minimal in deserts and polar regions.
- The rocks in dry temperate climates are more susceptible to mechanical weathering due to frost action.
- Weathering is at its minimum in the polar regions due to permanent ice – cover.
- Carbonate rocks having more soluble minerals are easily affected by chemical weathering.
- Climate is thus, the single most important factor influencing weathering.
- Attrition : Rock fragments carried by the river strike and roll against each other.
- Corrasion / Abrasion : River, along with its bed wears its bed & banks.
- Corrosion : The river water dissolves the minerals in soluble rocks.
- Hydraulic Action : The sheer weight of the water itself wears away the bed and banks.
- Deep, narrow V – shaped valley : It is formed as the swift flowing river erodes its bed faster than the sides.
- Potholes : The grinding action of the pebbles caused by the swirling action of water deeper the circular depressions in the river bed forming potholes.
- Interlocking Spurs : It is caused by vertical river erosion where spurs alternate on each side of the river as if they are interweaving.
- Waterfalls & Rapid s: They are formed when the erosion caused by the river steepens its valley suddenly forcing the water to jump or fall over the steep slope or when river water plunges down the edge of a plateau, e.g. Angel fall on River Rio, Caroni in Venezuela ( highest in the world ), Niagra fall ( USA ), etc.
- Gorges & Canyons : These are deep, narrow I shaped valleys having very steep sides, formed due to vertical corrosion in the upper course of the river. Canyons are usually found in arid areas and are narrower and deeper than gorges, e.g. Grand Canyon of USA cut by River Colorado.
- River Capture or River Piracy : The river that is more powerful captures the headwaters of a weaker river by headward erosion, i.e. towards its source. In the given figure, due to capture of water of Cz by C1, the part of C2‘s valley that has become dry is called the wind gap, lying below the Elbow of Capture. C2 then becomes too small for its valley, hence called Misfit river.
- V – Shaped Valley : An open V – shape valley due to valley widening caused by reduced river gradient and velocity.
- Alluvial Fans : When river debouches from the mountains to the plains, steep fall in river gradient forces the river to deposit its sediment in a fan shape, called alluvial fans.
- Meanders : In the middle course, due to reduced slope and increased volume of water, the river resorts to pronounced meanders.
- OX – bow lakes : It is a crescent shaped lake, once been part of river – meander cut through by lateral erosion of the banks at the meander neck.
- Floodplain : A flat tract of land mainly in the middle and lower courses, consists of alluvium deposited by the river.
- Natural Levee : In times of flood, sediment is deposited along the banks and in the channels, elevating the channel & the bank. These raised banks are known as natural levee.
Features in the Lower Courses
- Braided Rivers : Due to reduced gradient and sediment – carrying capacity, large amount of deposited material on the river bed cause the river to divide and move around these barriers, resulting in braiding.
- When a bar of resistant rock lies transversely across the river valley, e.g. “Niagara Fall” ( USA ) and “Kaieteur Fall” ( Guyana ).
- When a fault line scarp caused by faulting lies across the river e.g. “Victoria Fall” ( on Zambazi river ).
- When water plunges down the edge of a plateau e.g. “Livingstone Fall” ( on river Zaire ).
- Glaciation produces hanging valley where tributary streams reaches the main V – shaped valley below as water fall, e.g. “Yosemite Fall” ( California ).
- “Gersoppa Fall” : In the Western Ghat of India is the greatest fall in the world in the wet season.
- Indus, Brahmputra, Ganga, Columbia rivers cut gorge across the mountain chain.
- Colorado rivers has cut gorge 1.6 km. deep and 480 km. long into the Colorado plateau and because of its size the gorge is called a Canyon.
- Canyons are usually formed in dry region where large rivers are actively eroding vertically and where weathering of the valley side is at a minimum, e.g. “Bryce Canyon” ( Utah USA ).
Condition necessary for delta formation are :
- The river must have a large load and this will happen when there is active erosion in upper coarse of the valley.
- The coast should be sheltered preferably tideless.
- The sea adjoining the delta should be shallow or else the load will disappear in sea.
- There should be no large lake in the river course to filter off the sediments.
- There should be no strong current running at right angle to the river mouth.
Types of 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”, “Hwang Ho”, “Niger”.
- Bird’s Foot / Degitate : It is composed of very fine sediment called silt. The river channel divides into few distributaries only and maintain clearly defined channels across the delta. The “Mississippi Delta” is one of the best example.
- Estuarine : Develops in the mouth of a submerged river. Rivers like “Amazon”, “Ebe”, “Ob”, “Vestula” form this type of delta.
- Cuspate : Only few rivers like Ebro of Spain form such type of delta. These have tooth – like projection.
- 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 ).
- Snowline : The ‘level above which there is a perpetual snow cover is called the snowline. The height of this range from sea level around the poles to 4800 metres in the mountains of E. Africa on equator and 9000 feet in the Alps and 17,000 feet at the Equator in general.
- Ice – sheets : Masses of ice which covers large areas of a continent are called ice – sheet as in Antarctica & Greenland.
- Valley Glaciers : those masses of ice which occupy mountain valley like in Himalayas, Andes, Alps and Rockies.
- Ice Shelves and Icebergs : When the ice – sheets reach right down to the sea they often extend outwards into the polar waters and float as ice shelves. When they break into individual blocks called ‘ice – bergs’.
- Piedmont Glacier : At the foot of the mountain ranges several glaciers may converge to form an extensive ice – mass called a piedmont glacier, like “Malaspina Glaciers” of Alaska.
- Plucking : The tearing away of blocks of rocks which have become frozen into the base and sides of a glader.
- Abrasion : By this process the glacier scratches, scrapes, polishes and scours the valley floor with the debris frozen into it.
- Landforms of Highland Glaciation : Corrie, Cirque or Cwm, Aretes and Pyramidal Peeks, Bergschrund, U – shaped Glacial Valley, Hanging Valleys, Rock Basins & Rock steps and Moraines.
- The downslope movement of a glacier from its snow covered valley – head tends to produce a depression where the ‘firm’ or ‘neve’ accumulates and this horse – shoe shaped basin is called ‘Cirque’ in French or “Corrie” in Scotland or a “Cwm” in Wales. When the ice melts the collected water form a Corrie Lake or Tan.
- When two Corries cut back on opposite sides of a mountain, knife – edged ridges are formed called aretes. Where three or more Cirque cut back together, their ultimate recession will form an angular horn or ‘Pyramidal Peak’.
- At the head of a glacier, where it begins to leave the snowfield of a corrie, a deep vertical crack opens up called a “Bergeschrund” or “Rimaya”. Further down where the glacier negotiates a bend more “Crevasses” or creeks are formed.
- The glacier on its downward journey begins to wear away the sides and floor of the valley and the interlocking spurs are thus blunted to form “Transcated spurs” and the floor of the valley is deepened as U – Shaped. After the disappearance of the ice, the deep narrow glacial troughs may be filled with water forming ‘ribbon lakes’ or ‘trough lakes’ or ‘finger lakes’.
- As the main valley contain much larger glader than the tributary valley it erodes much faster. After the melting of ice the tributary valley appears as hanging over the main valley so that its stream plunges down as a water fall. Such tributary valleys are termed “Hanging Valley”.
- A glacier erodes and excavates the bedrock in an irregular manner and this unequal excavation gives rise to many rocks basin later filled by lakes in the valley trough. Where a tributary valleys joins a main valley the additional weight of ice in the main valley cuts into the valley floor at the point of convergence forming a rock step.
- ‘Moraines’ are made up of pieces of rocks that are shattered by frost action and are brought down the valley. On the basis of place where the moraines are deposited the moraines are termed in different ways like medial moraine, ground moraine, terminal moraine etc.
- When the lower end of the trough is drowned by the sea it forms a deep, steep – side inlet called “fiord” as on the Norwegian and South Chilean coast.
Arid or Desert Landforms
- Most of the world’s deserts are located in latitudinal belt of 15° to 30° North and South of Equator. These mainly occur in the trade wind belt on western sides of continents. On – shore local winds do blow across these coast but they rarely bring rain because they have to cross cool currents which parallel the coast in these latitudes.
- ‘Continental Deserts’ occurs in the interior of continents where the traveling winds have lost much of their moisture as happens when winds blow over a dry land or over high mountains.
- Tropical Hot Desert or Trade Wind Desert : Sahara, Arabian, Iranian, Thar, Kalahari, Namib, Atacama, Australian Desert etc.
- ‘Continental Desert’ : Gobi, Turkistan, Arizona, Nevada Deserts.
- The works of winds and water in eroded elevated uplands, transporting the worn – off materials and depositing them elsewhere, has given rise to different types of desert landscape.
- Sandy Desert : Called ‘erg’ in Sahara and ‘Koum’ in Turkistan; there are sea of sand with undulating sand dunes in the heart of the deserts.
- Stony Desert : Called ‘reg’ in Algeria and ‘Serir’ in Libya and Egypt; surface is covered with boulders and angular pebbles and gravels.
- Rocky Desert : Called “Hamada” in Sahara; bare rock surface.
- Badlands : Develop in semi desert region mainly as a result of water erosion produced by violent rain storms. The land is broken by extensive gullies and ravines. Painted Desert of Arizona is the best example.
- Mountain Desert : On highlands such as mountain and plateaus. In the Sahara desert the Ahaggar Mountain and Tibesfi mountain are example of it.
Wind erosion is carried out in the following ways :-
( a ) Deflection : Lifting and blowing away of loose material from the ground. Deflection results in the lowering of the land surface to form large depression called “deflection hollows” like Qattara depression of the Sahara desert.
( b ) Abrasion : The sand blasting of rock surfaces by wind when they hurl sand particles against them. Abrasion is most effective at or near the base of rocks.
( c ) Attrition : When wind born particles roll against one another in collision they wear each other away so that their sizes are greatly reduced into smaller parts.
- ‘Rock pedestal’ are the result of abrasion effecting on any projecting rock mass formed by alternate layers of hard and soft rock. Soft rock layer is eroded much in comparison to hard rock. Such rock pillar is further eroded much near the base and gives mushroom shape to the rock.
- ‘Zeugens’ are tabular masses with layers of soft rocks lying beneath a surface of more resistant rocks. When the abrasion wears them the “ridge and furrow” landscape develops. The hard rock then stands above the furrow as ridge or zeugen.
- ‘Yardangs’ are another type of ridge and furrow landscape and develops when bands of hard and soft rocks lie parallel to the prevailing winds.
- ‘Mesa’ is a flat table like landmass with very resistant horizontal top layer and very steep sides. Continuous denudation through the ages may reduce mesas in the area so that they become isolated flat – topped hills called “buttes”.
- ‘Inselbergs’ are isolated residual hills rising abruptly from the ground level with very steep slope and rounded tops. They are often composed of granite and gneiss.
- ‘Depression’ are produced by wind deflection and reach down to water bearing rocks, developing a swamp or an oasis. Quttara Depression is the best example.
- ‘Dunes’ are gentle ripples or sandy ridge which are produced by the deposition of sand grains brought by wind eddies from the neighbouring desert region.
- ‘Barchans’ or ‘Barkhan’ are crescent – shaped dunes lying at right angels to the prevailing winds. The crest of the barchans moves forward as more sand is accumulated by the wind.
- ‘Seif’ is a narrow ridge of sand lying parallel to the direction of the prevailing winds.
- The wind blows fine particles out of deserts each year. Some of them are blown into the sea, the rest are deposited on the land where they accumulate to form loess. There are extensive loess in Northern China, blown out of the Gobi Desert to the West.
Water Action in Desert
- The rare but heavy rainstorm gives birth to rushing torrents on steep slopes and sheet flood water on gentle slopes. The run off on steep slopes in usually via ‘rills’ ( shallow groves ) which lead into “gullies”, which, in turn, connects with steep sided deep and often flat floored valley called “Wadis or Chebka”.
- In intermontane desert basins, intermittent rivers drain into the centre of the basin and the alluvial fans build up around the edge of such a basin may eventually join together to form a continuous depositional features, sloping gently to the centre of the basin. This feature is called “Bahade or Bajada”.
- As the edge of desert and semi desert highlands gets pushed back by erosion and weathering, a gentle sloping platform develops called “pediments”. The slope of the land changes abruptly where a pediments joins the highland mass.
- Sometimes water collected in the depression or a desert basin does not completely disappear by evaporation or sleep age and a temporary lake is formed called “playas” “Salinas” or “Salars”.
- The coastline is the margin of the land. This is also the “Cliff line” on rocky coast.
- The water throws up the beach by breaking waves is called “Swash” and when the swash drains back down the beach it is called “backwash”.
- When waves break at the rate of ten or less a minute, each breaking wave is able to run its course without interfering the wave behind it. These waves are called “constructive waves”. When waves break more frequently than the backwash of a wave it runs into the swash of the wave behind. These waves remove pebbles and sands from a coast. They are destructive waves.
- Corrosive Action : Boulders, pebbles and sand are hurled against the base of the cliff by breaking waves.
- Hydraulic Action : Breaking waves causes air in the cracks and crevices to become suddenly compressed. After the retreat of waves air expands often explosively and causes the rocks to shatter as the cracks become enlarged.
- Attrition : Boulders and pebbles dashed against the shore are themselves broken into finer and finer particles.
Coastal Features of Deposition
- Constructive waves deposit pebbles, sand and mud along a coast and form a gentle sloping platform called a “beach”. Beaches usually lie between high and low water level but storm waves along some coast throw pebbles and stones well beyond the normal level reached by waves at high level tide and the material deposited in this way is called “storm beach”.
- Material which is eroded from a coast may be carried along the coast by long shore drift and deposited further along the coast as a spit. This is likely to happen along indented coast and the coast broken by river mouths.
- Bay – bar formation starts as a spit growing out from a headlands but ultimately it stretches across the bay to the next headland. Along the coast of Poland the bay bars are called “Nehrungs”.
- When a bar links an inland to the mainland it is called “tombolo”.
- Tides tends to deposits fine silts along gently shelving coast, especially in bays and estuaries. The deposition of these silts together with river alluvium, results in building up of a platform of muds called “mud-flat”.
- In the tropical region, mud flats often become “mangrove swamps”.
Types of Coast
Coastal region may be either submerged or become uplifted by change, in the land or sea levels. It can be further divided into highlands or lowland types. The types of coast can be divided into –
- Ria Coast : When a highland coast is submerged the lower parts of its river valley becomes flooded. These submerged parts of the valley are called “rias” e.g. in S.W. Ireland, S.W. England, N. W. Spain, Brittany etc.
- Longitudinal Coast : When a highland coast whose valley are parallel to the coast is submerged, some of the valley are flooded and separating mountain range becomes chain of Islands. These types of coast occurs in Yugoslavia along part of pacific coast of North and South America.
- Fiord Coast : When glaciated highland coast become submerged the flooded lower parts of the valley are called “fiords”. Fiord coast are common in south Island of New Zealand, Greenlands, Norway and British Columbia.
- Submerged Lowland Coast : A rise in sea level along a lowland coast causes the sea to penetrate inland along the river valley. The flooded parts of the valley are called “estuaries”. The Baltic coast of Poland and Germany and the Dutch coast are good examples of estuarine coasts.
- Emerged Highland Coast : An old sea beach backed by a sea cliff lying from 7.5 metres to 30 metres above sea levels is often characterised by this type of coast. The main cause of its formation is the change in either sea level or the level of the land. Raised beaches are common in Western Scotland.
- Emerged Lowland Coast : This forms when a part of continental shelf emerges from the sea and forms a coastal plain. This coast has no bays or headlands and deposition takes place in the shallow water off-shore, producing off-shore bars, lagoons, spits and beaches. S.E. coast of USA, North coast of Gulf of Mexico are such examples.
- Knick Point : The point where the old and rejuvenated profiles meet is called the knick point or “Rejuvenated Head”. It is the major split line where the courses of the river get changed. There may be circumstances of the emergence of knick points.
- Valley – in – Valley : When the old course pf valley is upliftted and a greater slope is attained the old V – shaped open valley begins to perform downward cutting with greater speed than Sideward. This lead to the development of deep cut narrow valley within the old valley, it is known as ‘valley in valley’.
- Erosional Surface : Erosional surface emerges when the peneplain is uplifted, an uplifted peneplain makes a flatter summit of the plateau and that becomes the source region of several rivers. That plateau is basically an Erosional surface or eroded plain. For example Chotanagpur plateau and Applachain plateau.
- Incised Meanders : Incised Meanders emerges in the meanders of old course when the meandering course of the rivers is upraised & deepening begins then a typical deep cut narrow valley emerges with the old meander. This time Ox-bow lakes are not made. Whenever the valley becomes straight due to erosion at the pressing point detached land looks like a Ii hill or hillock. Colorado river present a good site for incised meander.
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