Karst Topography

Karst Topography

This article deals with ‘Karst Topography.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Introduction

  • Limestone is sedimentary rock of organic origin . Chemically it is  Calcium Carbonate (but where Magnesium is also present , it is known as Dolomite).
  • Limestone is soluble in rain water with Carbon dioxide (weak acid.)
  • A region with large stretch of limestone therefore posses a very distinct topography termed as Karst (name derived from Karst District of Yugoslavia where such topography is particularly well developed)

There is absence of surface drainage as most of the surface water goes underground and form underground channels. When this water meets non-porous rocks, it re-emerges onto the surface as spring or resurgence.

Location

  • Karst region is in Dinarik Alps in Yugoslavia.
  • Such topography is also found in regions of Himalayas, Rockies, Andes, Atlas , Shan Plateau , Belo Horizonte etc.
  • In India this is found in Chirapoonji, Jammu-Kashmir, Himachal Pradesh, Panch Marhi (M.P.), Bastar (Chattisgarh and Coastal areas near Vishakhapatnam

Landforms

Erosional Landforms

1 . Lapies

  • Lapies are the irregular grooves and ridges formed when most of the surfaces of limestone are removed by solution process.

2. Swallow Hole /Sink Holes

  • A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom. 
  • On the surface of limestone, there are numerous small depressions carved out by solution at a point of weakness.  Holes size grow through continuous solvent action to form Sink Hole .

3. Limestone Gorge

  • When the roof of an underground tunnel collapses , a  limestone gorge is formed .
Limestone Gorge

4. Karst Window/ Karst fenster

  • It is spring that emerges from underground , discharge it’s water and then then abruptly disappears underground through a nearby sink-hole.

5. Doline

  • Due to high chemical activity on swallow holes, their size and depth increases. Its diameter may extend upto some kilometres and depth may run upto 100 meters.
  • It can be cylindrical, conical, bowl or dish shaped.
  • The name doline comes from dolina, the Slovenian word meaning valley.

6. Uvala

  • Series of smaller sinkholes coalesce into a compound sinkhole is called uvala.

7. Polze

  • Polje is an elongated basin having a flat floor and steep walls.
  • It is formed by the coalescence of several sinkholes. The basins often cover 250 square km and may expose “disappearing streams.”

8. Cave

  • In areas where there are alternating beds of rocks (shales, sandstones, quartzites) with limestones or dolomites in between or in areas where limestones are dense, massive and occurring as thick beds, cave formation is prominent.
  • Water percolates down  through the cracks and joints and moves horizontally along bedding planes. It is along these bedding planes that the limestone dissolves to form wide gaps called caves  .

9. Tunnel

  • Caves having openings at both the ends are called tunnels.

Depositional landforms

  • Where subterranean streams descend  to underground passages, the region may be honeycombed with caves
  • Most important features in limestone caves are Stalactites, Stalagmites and Pillars .

1 . Stalactites

  • Formed on roof of caves .
  • As rainwater seeps through the limestone, the water dissolves Calcium Carbonate in it. When from roof, water drips down, it leaves behind Calcium Carbonate (CaCO3) forming Stalactite .
  • They are thinner, long and pointed.
Stalactite

2. Stalagmite

  • They are formed on the floor.
  • All the dripping water has to land somewhere . When a drop finally hits cave floor , it deposits even more Calcite there in unassumed mound .
  • They are shorter , fatter and more round.
Stalagmite

3. Cave Pillars

  • Over a long time, stalactites hanging from roof is eventually joined to Stalagmite growing from floor to form pillar.

Fluvial Landforms

Fluvial Landforms

This article deals with ‘Fluvial Landforms.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Introduction

  • When rain falls , part of it sinks into ground , some of it is evaporated back into the atmosphere & rest runs off as rivulets , streams and tributaries of rivers . This running water is potent agent of erosion  .
  • The river performs three types of work. They are erosion, transportation and deposition.

Materials transported/carried by river

When a river flows , it carries eroded material in four forms

  • Solution – Material dissolved in water.
  • Suspension – Sand, Silt & mud  carried in suspended form.
  • Saltation: Some of the fragments of the rocks move along the bed of a stream by bouncing continuously.
  • Traction Load – This includes coarser materials such as pebbles , stones & boulders which are rolled along river bed .

Rivers carry great amount of material => Eg : Mississippi river removes 2 million tons to Gulf of Mexico daily .

River Erosion 

In rivers, erosion  comprise of following  processes :-

  • Corrasion / Abrasion : Mechanical grinding by river’s traction load against banks & beds of river . 
    • Lateral Corrasion : sideways erosion which widens  V-Shaped valley.
    • Vertical Corrasion : downward action which deepens the river channel.
  • Corrosion or Solution : chemical  action of water on soluble or partly soluble rocks. Eg Calcium Carbonate in limestone is  dissolved & removed in solution .
  • Attrition : This is wear and tear of transported material themselves when they  collide against one another. 
  • Hydraulic Action :  This refers to mechanical action of water. Eg when water splashes against river banks , surges into cracks & disintegrate the rocks .

River deposition

  • When the velocity of the stream decreases, the stream deposits sand, silt and other fragments
  • When a river moves in a gentle slope, its speed reduces and river begins to deposit its load.
  • The river starts depositing larger materials first and smaller and finer materials are carried further down to the mouth of the river.

Course of a River

Course of River

Upper or Mountain Course /Youthful Stage

In this stage

  • Predominant Work = Erosion
  • Predominant action = vertical corrasion.

Landforms formed in this stage

1. Valleys

1.1 V-Shaped Valley

  • In upper course, vertical corrasion is at work . Downward cutting takes place so rapidly that lateral corrasion can’t keep pace. After some time, the loosened material slowly creeps downward and takes shape of V.
  • The valley thus developed is deep , narrow & distinctively V-Shaped.

1.2 Gorges / I-shaped Valley

  • In some cases,  rocks are very resistant and hence afterward loosening don’t take place (because of resistant rock). The  valley formed is so narrow & sides are so steep that gorges are formed .
  • Eg : Indus Gorge in Kashmir .
Gorge

1. 3 Canyons

  • Canyons are extended form of gorges.
  • In arid regions, where there is little to widen the valley sides and river cuts deep into the valley floors , precipitous valleys called Canyons are formed.
  • Eg Grand Canyon of Colorado river in Arizona state of USA .
Grand Canyon

2. Falls

2.1 Rapids

  • These can form in any part of river course but are more numerous in mountain course.
  • They are formed when there are different layers of hard rock and soft rocks . Due to unequal resistance of hard & soft rocks , there is unequal erosion of both set of rocks . The hard rocks will make river to jump and fall down 
Rapids

2.2 Cataract

  • Falls similar to rapids but of greater dimensions are referred to as Cataracts.
  • There are 5 along the Nile that interrupt the smooth navigation.

2.3 Waterfalls

  • When rivers plunge down in a sudden fall of some height , they are known as waterfalls. Their force usually wears out a plunge pool beneath .
  • They can be formed in various ways
    • When a bar of resistant rock lies transversely across a river valley . Eg Niagara Falls in US
    • At fault line across river. Eg Victoria falls on River Zambezi
    • When river plunges down the edge of a plateau like River Congo .

3. Entrenched /Incised Meanders

  • These are formed when downcutting process is slow & river cause lateral erosion leading to asymmetric valley formation .
  • Note : These are different from meanders which occur due to both erosion and deposition. In this, only erosion  takes place. 
Incised Meanders

Middle or Valley Course /Mature Stage

In Middle Course,

  • In Middle Course, Erosion  , Transportation and deposition is done by the river .
  • But amount of erosion is very lower than Youthful stage and in that too, Lateral Corrasion tends to replace vertical corrasion.
  • The volume of the water increases with the confluence of many tributaries & this increases the river’s load.
  • Predominant work of the river =  predominantly transportation with some deposition (main deposition happen in Oldage Stage).

Landforms formed in Middle Stage

1 . Alluvial Fans

  • Alluvial fans  are formed when streams flowing from higher levels break into foot slope plains of low gradient. Normally very coarse load is carried by streams flowing over mountain slopes. This load becomes too heavy for the streams to be carried over gentler  gradients and gets dumped and spread as a broad low to high cone shaped deposit called alluvial fan.
  • Examples : Alluvial fans are found in Kosi river when it enters Bihar just after exiting Himalayas
Alluvial Fans

2. Alluvial Cone

  • As the velocity of river decreases, its transportation capacity also decreases. Due to this decline deposition starts in Foot Hills. This deposition forms Alluvial Cones.
  • It is same as alluvial fan but slope is more (between 10 to 50 degree)

3. Flood Plains

  • Rivers in their course carry large quantities of sediments . During annual or sporadic floods, these materials are spread over the low lying adjacent areas.
  • A layer of sediment is thus deposited during each flood , gradually building up a fertile flood plain.
Flood Plains

4. Levees

  • With the continuous deposition of soil on the banks by the river, the level of banks rises and they look like natural dams known as levees . 
  • During flooding as the water spills over the bank, the velocity of the water comes down and large sized and high specific gravity materials get dumped in the immediate vicinity of the bank as ridges. They are high nearer the banks and slope gently away from the river
Levee

Lower or Plain Course /Old Age stage

In Lower/Oldage Course,

  • River moving downstream across a broad and level plain is heavy with debris brought down from the upper course . Hence, work of the river is mainly deposition, building up its bed & forming extensive flood plains.
  • Vertical corrasion has almost ceased though some lateral corrasion still goes on to erode its bank (like in Meanders).

Landforms formed in Later Stage

1 . Meanders

  • A meander is a winding curve or bend in a river.
  • Meanders are the result of both erosional and depositional processes.(explained in diagram below)
  • The irregularities of the ground , force the river to swing in loops.  Once the channel begins to flow in sinusoidal path , the amplitude & concavity of loop increases rapidly due to dense erosion occurring at the outside and deposition occurring inside .
  • Note : Meanders can be found in Middle as well as Later Stage
Meanders

2. Ox-bow Lakes

  • An oxbow lake is U-shaped body of water that forms when a wide meander from the main stream of river is cut off creating free standing body of water.
  • It has different nomenclature at different places – Billabong in Australia, Rasacas in Texas etc
  • Both meanders and ox bow lakes are formed both in middle and lower course. Ox Bow is more commonly found in lower course .
Oxbow Lake

3. Braided Streams

  • A braided stream is one which does not flow in a single definite channel but rather a network of everchanging, branching and reuniting channels.
  • Thread-like streams of water rejoin and subdivide repeatedly to give a typical braided pattern

4. Delta

  • When a river reaches the sea , the fine material it has not yet dropped are deposited at its mouth , forming a fan shaped alluvial area known as Delta .
  • This alluvial tract is , infact , a seaward extension of the flood plain.
  • Delta extend sideways and seaward at an amazing rate . The River Po extends its delta by over 40 feet a year
  • (GK) Ganges Brahmaputra delta is the largest delta in the world.

Deltas differ in their size , shape , growth & importance. A number of factors such as the rate of sedimentation , the depth of the river  & sea bed and character of  tides , currents & waves greatly influence formation of Delta.

Different type of Deltas

a. Bird’s foot Delta

  • Deposited alluvial material divides the river into smaller distributaries. Several distributaries look like the foot of a bird.
  • Example : Mississippi is example.
Bird Foot Delta

b. Arcuate shaped Delta

  • Delta  look fan shaped with numerous distributaries
  • Examples are Amazon, Ganga & Mekong.
Arcuate Delta

c. Estuarine Delta

  • When river has their deltas submerged in coastal waters (mainly due to submerged coast like India’s western coast) or don’t have much deposition to form delta.
  • Most of India’s west flowing rivers originating in Western Ghats.
Estuarine Delta

d. Cuspate Delta

  • Have tooth like projection at their mouth.
  • Example : Ebro of Spain.
Cuspate Delta

Conditions favourable for formation of deltas are

  • Active vertical & lateral erosion in upper course to provide  sediments to be eventually deposited as Deltas.
  • The sea adjoining the delta should be shallow or else the load will disappear in the deep waters.
  • The coast should be sheltered, preferably tideless.
  • There should be no large lakes in the river course to filter off the sediments.
  • There should be no strong current running at right angle to the river mouth.

Importance of Deltas

a. Ecological importance

  • Deltas absorb runoff from both floods (from rivers) and storms (from lakes or the ocean), filter water and thus reduces the impact of pollution flowing from upstream.
  • Deltas are also important wetland habitats. They support extremely diverse and specialized flora and fauna and are areas of dense forests.  

b. Economic importance

  • Deltas are important places for trade and commerce, and major ports.
  • Deltas due to rich accumulation of silt are fertile agricultural areas. World’s largest delta is the Ganges–Brahmaputra delta in India and Bangladesh,  is densely populated supporting livelihood of millions. Fish, other seafood, and crops such as rice and tea are leading agricultural products of the delta.
  • Deltas possess well sorted sand and gravel which is quarried.
  • Due to their diversity they are centres of tourism and recreation.

Threat to Deltas

  • Diversion of water for irrigation and creation of dams reduce sedimentation, which can cause delta to erode away.
  • Climate change and rising sea level – rising sea level flood deltas bringing in saline water and threatening wetland ecosystem. For example nearly 31 square miles of Sundarbans have vanished entirely due to sea level rise.
  • Use  of water upstream can greatly increase salinity levels as less fresh water flows to meet salty ocean water. 

While nearly all deltas have been impacted to some degree by humans, the Nile Delta and Colorado River Delta are some of the most extreme examples of ecological devastation

Weathering and Mass Movements

Weathering and Mass Movements

This article deals with ‘Weathering and Mass Movements.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Weathering

  • Weathering is the action of elements of weather  over earth materials  to reduce them to fragmental state.
  • Very little or no motion take place in them & process is in-situ.
  • There are three major groups of weathering processes :
    1. Chemical
    2. Physical or mechanical
    3. Biological  weathering processes.
Weathering

1. Chemical Weathering

a. Solution

  • When something is dissolved in water  , it is called solution.

b. Hydration

  • Hydration is the chemical addition of water. Minerals take up water and expand
  • Calcium sulphate takes in water and turns to gypsum, which is more unstable  

c. Oxidation & Reduction

  • Oxidation means a combination of a mineral with oxygen to form oxides or hydroxides.  Eg : Oxidation of  iron to form rust
  • When oxidised minerals are placed in an environment where oxygen is absent, reduction takes place. Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground. 

d. Carbonation

  • Carbonation leads to dissolution of Carbon Dioxide into water to form Carbonic Acids which will dissolve calcium and magnesium compounds

2. Physical Weathering

Physical Weathering is the disintegration of rock mainly induced by elements of weather and natural forces.

Physical weathering can be further divided into following categories :-

  • Gravitational forces such as overburden pressure, load and shearing stress.
  • Expansion forces due to temperature changes => Rocks expand during day and contract during night in arid and semi-arid regions=> the rocks  crack and eventually splits up.
  • Exfoliation : Rocks generally heat or cool more on the surface layers. The alternate changes in temperature could cause their outer layers to peel off from the main mass of the rock in concentric layers just as the skin of an onion.
  • Frost wedging : when water freezes, it expands. As water expands between the rock wedges expand, it puts great pressure on rocks resulting in weathering.
  • Water  pressures controlled by wetting and drying

3. Biological Weathering

Weathering due to growth or movement of organisms.

  • Burrowing &wedging by organisms like earthworms, termites, rodents etc. => exposing  new surfaces to chemical attack  
  • Human beings by disturbing vegetation, ploughing and cultivating soils=> this creates  new contacts 
  • Plant roots exert great pressure breaking rocks apart.

Importance of Weathering

Ecological Importance

  • Weathering is the initial stage in the formation of soil. It breaks down the initial rock mass into smaller fragments thus preparing the rock material for the formation of soil.
  • Trees are able to ‘mine’ essential nutrients such as calcium through their association with symbiotic mycorrhizae through small pores in the mineral soil, which is possible only due to weathering.
  • Erosion, with the aid of weathering, helps in mass wasting and reduction of relief. This leads to modifications in various landforms.

Economic Importance

  • It leads to the formation of various natural resources such as clay used in making bricks.
  • Placer deposits are formed due to weathering . These placer deposits are source of rare earth metals, thorium etc
  • It weakens the rocks, thus facilitating the mining and quarrying activities

Hence, we can say that although weathering is a disintegrating process yet it plays an integral role in sustaining life on earth.

Mass Movement

  • These movements transfer the mass of rock debris down the slopes under the direct influence of gravity ( happens only under influence of gravity & no other geomorphic agent is involved)
  • Weathering is not a prerequisite for mass movement although it aids mass movements. Mass movements are very active over weathered slopes .

Mass Movement can be grouped under two classes

1 . Slow Movement

  • Creep : Occur on moderately steep, soil covered slopes.  Movement of materials is extremely slow and imperceptible except through extended observation.
  • Solifluction : Slow downslope movement of soil mass  saturated  with water.  Quite common in moist temperate areas

2. Rapid Movement

Mostly prevalent in humid climatic regions with gentle to steep slopes.

  • Earthflow : Movement of water-saturated earth materials down hillsides. Arcuate scarps at  heads & accumulation bulge at the toe are observed in this.
  • Mudflow : Mudflow is a liquid mass of soil, rock debris and water that moves quickly down a well defined channel. Mudflow  originating on a volcanic slope is called a lahar.
  • Debris avalanche  : characteristic of humid regions with steep slopes. These are rapidly  churning mass of rock debris, soil, water, and air that moves down steep slopes. The trapped air may increase the speed of an avalanche by acting as a cushion between the debris and the underlying surface. They are much faster and deadlier than Earthflow & Mudflow
  • Rock falls : Rock falls occur when pieces of rock break from a cliff. It may result due to Frost wedging . Accumulation  of rock debris at the base of a steep slope is called talus.
  • Landslides : Landslides occur when a large piece of rock breaks off and slides down hill. It can be initiated by heavy rainfall or earthquake.
  • Slump : Great mass of bed rock moves downward by rotational slip from a high cliff
Mass Movement

Question – Why more Landslides & Debris Avalanches occur in Himalayas compared to Western Ghats?

  • There are many reasons for this. 
    • One, the Himalayas are tectonically active. 
    • They are mostly made up of unconsolidated and semi-consolidated deposits
    • The slopes are very steep.

Question : Compared to the Himalayas, the Nilgiris bordering Tamilnadu, Karnataka, Kerala and the Western Ghats along the west coast are relatively tectonically stable and are mostly made up of very hard rocks; but, still, debris avalanches and landslides occur although not as frequently as in  the Himalayas, in these hills.  Why? 

  • Many slopes are steeper with almost vertical cliffs and escarpments in the Western Ghats and Nilgiris.
  • Mechanical  weathering due to temperature changes and ranges is pronounced. 
  • They receive heavy amounts of rainfall over short periods. So, there is almost direct rock fall quite frequently in these places along with landslides and debris avalanches.

Rocks and Minerals

Rocks and Minerals

This article deals with ‘Rocks and Minerals.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Minerals

  • Naturally occurring organic and inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties.
  • Composed of two or more elements. But, sometimes single element minerals like sulphur, silver, gold, graphite etc. are found
  • Magma is the source of almost all minerals.

Types of Minerals

a. Metallic Minerals

These minerals contain metals and can be sub-divided into

Precious Metals gold, silver, platinum
Ferrous Metals iron and other metals often mixed with iron to form various kinds of steel.
Non-Ferrous Metals include metals like copper, lead, zinc, tin, aluminium etc.

b. Non-Metallic Minerals

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

Rocks

  • A rock is an aggregate of one or more minerals.
  • Rocks do not have definite composition of mineral constituents.
  • Petrology is the science of rocks.
  • The age of the rock is determined based on Carbon-14 dating.

Type of rocks

a. Igneous Rocks

  • Igneous rocks (Ignis in Latin means ‘Fire’) are formed when magma cools and solidifies.
  • They  are known as primary rocks
  • Igneous rocks are classified based on texture. 
    1. If cooled slowly at great depths : Large grains  
    2. Sudden cooling (at the surface) :  small grains.
    3. Intermediate  cooling : intermediate size of grains .
  • Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.

b. Sedimentary Rocks

  • Formed by lithification of sediments
  • They are also known as detrital rocks
  • Examples : sandstoneshale, loess , chalk, coal , limestone etc

c. Metamorphic Rocks

  • Metamorphic rocks are formed when already consolidated rocks undergo reorganization in structure due to excessive pressure (through the process called Metamorphism)
  • Eg : Gneiss, syenite, slate, schist, marble, quartzite, anthracite, diamond  etc.

Igneous  and metamorphic rocks together account for 95 percent of the earth while rest 5% are sedimentary rocks.

Rock Cycle

Rocks do not remain in their original form for long but may undergo transformation.  Rock cycle is a continuous process through which old rocks are transformed into new ones.

Rock Cycle

Volcanoes

Volcanoes

This article deals with ‘Volcanoes.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

What are volcanoes?

A volcano is an opening in the earth’s crust through which magma, gases and ash are released to the earth’s surface.

Related terminology

Magma The molten rock material found in the interior of the earth is called magma.
Lava When magma reaches the earth’s surface, it is known as lava.
Vent Vent is an opening or mouth of a volcano.
Fumaroles Fumaroles are the gushing fumes (fume = smoke) through the gap in the vicinity of volcano.
Fumaroles are often in the neighbourhood of volcanoes.
Crater Crater is a saucer shaped depression in the mouth of a volcano.
Caldera When the crater is widened, it is called as Caldera
Volcanic Ash Volcanic ash consists of fragments of pulverized rock, minerals and volcanic glass, created during volcanic eruptions.
Volcanoes

Causes of Volcanism

  • Weak Zones in the Earth Crust: The parts of the earth where two tectonic plates collide against or drift apart from each other are considered very weak. Volcanoes may erupt in such zones, for example, African and Eurasian plates.
  • Magma Saturated with Gases : The magma, in the interior of the earth, is often found saturated with gases like carbon dioxide, and hydrogen sulphide. These gases together with water vapour make the magma highly explosive. Magma is forced out as lava on the surface of the earth due to the pressure exerted by these gases.

Basaltic Eruption vs Andesitic Eruption

Basaltic Eruption Andesitic Eruption
Occur at mid oceanic ridge & Hotspot volcanism Occur at Volcanic Island arcs & volcanic mountains
Basalt is highly fluid & mobile Andesite is less fluid & less mobile
Spread across easily Solidifies at short distance
Quite eruption Explosive eruption
Form plateaus and island groups. Forms volcanic peaks

Types of Volcanoes

1 . Classification on basis of form developed

Volcanoes are classified on the basis of nature of eruption and the form developed at the surface.

a . Shield Volcanoes

  • These are made up of basalt, a type of lava that is very fluid when erupted. Since Basalt is very fluid , it make these volcanoes less steep.
  • Eg : Hawaiian volcanoes are the most famous examples.
  • They become explosive if water gets into the vent; otherwise, they are characterised by low-explosivity.
Shield Volcano

b. Composite Volcanoes

  • Composite volcanoes are  cone-shaped volcanoes composed of different layers of lava, ash and rock debris.
  • Magma which erupt in this case is Andesitic in nature . Hence, eruption is violent and explosive. Along with that, Andesitic lava is less fluid making the Composite volcanoes very steep.
  • Along with lava, large quantities of pyroclastic material also comes out.
  • Examples include Mt Stromboli , Mt Vesuvius, Mt St Helens, Mt Fuji etc.
Composite Volcanoes

c. Caldera

  • These are the most explosive of the earth’s volcanoes.
  • They are usually so explosive that when they erupt they tend to collapse on themselves .The collapsed depressions are called calderas.
Caldera

d. Flood Basalt Provinces

  • These volcanoes out-pour highly fluid lava that flows for long distances. 
  • There can be a series of flows with some flows attaining thickness of more than 50 m. 
  • Deccan Traps from India,  covering most of the Maharashtra plateau, are a  flood basalt province. 

e. Mid Oceanic Ridge Volcanoes

  • These volcanoes occur in the oceanic areas at points where Oceanic – Oceanic tectonic plates diverges. 
  • There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins.
  • The central portion of this ridge experiences frequent but peaceful eruptions.
Mid Oceanic Ridge

2. Classification on Basis of Periodicity of eruption

a. Active Volcanoes

  • Volcanoes which erupt frequently.
  • Generally, their vent remains open.
  • Examples : Mount Etna of Italy, Cotopaxi in Ecuador (highest volcano) and many others situated in Pacific ring of fire .

b. Dormant Volcanoes

  • These volcanoes may not have erupted in the recent past but there is a possibility of eruption at any time.
  • Examples : Mt. Vesuvius of Italy , Mt. Fujiyama of Japan and Mt Kilimanjaro in Tanzania.

c. Extinct Volcanoes

  • These volcanoes have exhausted their energy and have  not erupted during the known geological period.
  • Their Crater is generally filled with water making it a lake.
  • Examples : Popa in Myanmar and Mt. Kenya in eastern Africa

Recent Volcanic Activities

Barren Island Barren Island is in Andaman .
It again became active in 2017
Earlier, it became active in  1991 and 1995.  
Anak Krakatau   It is in Indonesia
Latest eruption happened in April 2020
Note : greatest volcanic explosion known to humans is  Mt. Krakatau in August 1883.  
Vulcan de Fuego Volcanic eruption happened in June 2018
Here , Cocos plate is subducting under Caribbean plate.
It lies in the Pacific Ring of Fire in Guatemala    
Kilauea Volcano Kilauea Volcano is situated in Hawaii .
It erupted in May 2018  
Mount Agung and Mount Sinabung – They are in Bali
– Eruption happen due to Oceanic -Oceanic Convergence of Indo-Australia and Sunda Plates
– Volcanic eruptions happened in whole of 2018

Distribution of Volcanism

a. Ring of Fire

  • Pacific Ring of Fire is the Circum-Pacific region that has the greatest concentration of active volcanoes. Two-third of world’s volcanoes lie here.
  • Volcanoes are found here due to Oceanic -Oceanic & Oceanic-Continental Plate convergence .
Oceanic – Oceanic Convergence Results in formation of Volcanic Island Arcs.
Oceanic -Continental  Convergence. Volcanoes are formed

b. Mid Atlantic Region

  • Formed due to Oceanic -Oceanic plate divergence .
  • Basaltic Eruptions  happens here which  are peaceful in nature .
  • Mid Oceanic Ridge Volcanoes are found here.
Mid Oceanic Ridge

c. Great Rift Valley

  • In Africa some volcanoes are found along the East African Rift Valley. 
  • Kilimanjaro and Mt. Kenya are extinct volcanoes. The only active volcano in West Africa is Mt. Cameroon.
Great Rift Valley

d. Mediterranean Volcanism

  • Volcanoes of the Mediterranean region are mainly associated with the Alpine folds.
  • Examples : Mt. Vesuvius,  Mt. Stromboli (known as the Light House of the Mediterranean Sea

e. Hotspot  Volcanoes

  • These have to do with plate tectonics .
  • In these volcanoes, magma from  the deep mantle come outside directly through plume  .
  • Lava is of Basaltic nature => Shield Volcanoes are formed in this activity which are not explosive.
  • Their location remains fixed but plates on them can move leading to formation of island arcs or plateau depending upon conditions .
In Oceans Island Arc
On Continents Volcanic Plateau 
  • Eg Hawaii, Reunion Island , Kurile, Aleutian island , Iceland , Yellowstone (US Continental)

Lava / Volcanic Plateaus

  • When Lava  is basaltic (like in case of hotspots) , it can  flow easily . It will keep on forming layer above layer.
  • When this process goes on for large amount of time ,Lava Plateau is created .
  • Eg: Deccan Plateau was formed in this way when Indian plate passed over Reunion Island hotspot during the cretaceous period.
Location of Volcanic Plateaus

Side Topic : Formation of Deccan Plateau

  • During Cretaceous Period , Indian Plate was moving northward and passed over Reunion islands (near Madagascar island in Indian Ocean)
  • Hotspot  volcanism was active over there which resulted in outpour of highly basaltic lava at that point . As the plate movement was extremely slow, India remained over the Reunion hotspot for a considerable long time which led to Basaltic deposits in Deccan area  . Hence , Deccan Basaltic Plateau formed.
  • 16-17 percent of India is under Deccan traps.

Intrusive Volcanic Landforms

We have studied about various landforms made by the volcanoes on the surface of earth. But volcanoes make large number of landforms inside the earth’s crust. These are called Intrusive Volcanic Landforms.

Intrusive Volcanic landforms are formed when magma  fails to come out and solidifies under the Earth’s crust . Some of the landforms formed are as follows :-

a. Batholiths

  • Batholiths are large rock domes formed due to cooling and solidification of hot magma inside the earth.
  • They appear on the surface only after the denudational processes remove the overlying materials
  • They are granitic in origin.

b. Laccoliths

  • Laccoliths are formed when magma solidifies in cracks of sediments and take concave shape/ dome like shape
  • Karnataka plateau is spotted with dome hills of granite rocks. These are exposed Laccoliths .

c. Phacoliths

  • Phacoliths are formed when magma is filled in anticlines and synclines of folded mountains.

d. Sills

  • Sills are parallelly solidified lava layers in  sedimentary rocks in the interiors of Earth.

e. Dykes

  • Longitudinally solidified magma in rocks is known as dyke.

Importance of Volcanic Activity

  • Volcanism creates new landforms (all the landforms we have seen above)
  • Volcanic rocks yield very fertile soil upon weathering and decomposition. For example black soil of Deccan Plateau in India is made for volcanic rocks and it is best suited to grow cotton .
  • Source of Diamonds :  Eg – Kimberlite rock of South Africa, the source of diamonds, is the pipe of an ancient volcano.
  • Geothermal Energy : In the vicinity of active volcanoes, waters in the depth are heated from contact with hot magma giving rise to springs and geysers. The Puga valley in Ladakh region and Manikaran (Himachal Pradesh) are promising spots in India for the generation of geothermal electricity.
  • Volcanism has also helped in the formation of atmosphere in the past. (How? – explained below)

Importance of Volcanic Activity in atmosphere formation in the past

  • Degassing : Volcanic activity released essential gasses such as water vapor, carbon dioxide, methane, ammonia and very little free oxygen from the interior of the earth through a process called degassing.  
  • Volcanic eruptions injected ash and sulphur-rich aerosol clouds into the atmosphere which shaded sunlight and reduced the amount of solar radiation reaching the Earth’s surface thus cooling the planet. As the earth cooled, water vapor condensed to form rain dissolving carbon dioxide and other gases.

Side Topic : Impact of Volcanic eruptions on Climate Change

  • Volcanic eruptions pour sulfur dioxide and other particles into the stratosphere. Gases react with water to form aerosols that linger in the stratosphere, reflecting sunlight and heat from the sun and thus lowering temperatures in the troposphere
  • Intense volcanism has significantly increased the amount of carbon dioxide in the atmosphere and causes global warming. Volcanic eruptions produce more than 100 million tons CO2 each year.   For Example: The 1980 eruption of Mount St. Helena vented approximately 10 million tons of CO2 into the atmosphere in only 9 hours. 
  • Dark lava flow absorbs more of the solar energy (low albedo) , so a large enough lava flow could warm a local region. 

—> Volcanoes can have both a cooling and warming effect on climate. However, in the long term frequent volcanic eruptions will have a net effect of cooling the earth and counter global warming.

Plains

Plains

This article deals with ‘Plains.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Introduction

  • Plain is an extensive tract of flat and or a gently undulating terrain without prominent hills or depressions.
  • They are formed both by internal forces of the Earth and by external processes of aggradation and degradation.
  • They range in size from very small to a very large areas.
  • The great centres of population of the world are on plains. Development of means of communications and transportation facilities is easier in the plains. Hence, these are best known areas for human habitation. 

Types of Plains

Plains are best classified according to their origin

1 . Diastrophic Plains

  • Plains formed on regions that were once submerged under ocean or sea. 
  • Example : Great plains of U.S.A which extend till Canada were  formed due to upliftment submerged landmasses under epicontinental seas and were uplifted at the end of Cretaceous period to due tectonic movements . They have deposits of horizontal thick beds of Marine sediments

2. Peneplains

  • Undulating  surface of low relief, interspersed with occasional residual hills and claimed to have been formed due to erosion by rivers and rain.
  • Example include East Central Africa

3. Flood Plains

  • Flood Plain is that part of river valley adjacent to the channel, over which a river flows in times of floods.
  • Floodplain is composed of Alluvium
  • Examples : Indo-Gangetic plain and the plains of Mississippi, Amazon, Nile, Hwang-Ho, Yangtze Ob, , Lena, Volga rivers 
Plains

4. Delta Plains

  • As rivers draw near seas to disappear in them, their flow goes dead slow. It necessities the waters to deposit all types of materials being carried by it. Such depositions are made in triangular shape which resembles to Greek word ‘Delta’.
  • These plains are the most fertile plains of the world.
  • Examples : Sundarbans of Ganga and Brahmaputra, Deltas of Nile and Mississippi etc.

5. Aeolian Plains

  • Aeolian plains are plains formed by either erosive or depositional action of winds
  • Examples
    1. Sahara and Thar
    2. Plains formed by filling of lakes in Kashmir and Manitoba (Canada)
    3. Lava plains of Idaho (U.S.A.)
    4. Plains of Mecca & Medina

Importance of Plains

  • Plains are the ‘cradles of civilizations’ and the ‘food baskets’ of the world with 80% of population living in plains, i.e., Prairies (U.S.A), Steppes, Pustaz (Europe), Veld (South Africa), Great Indo-Gangetic Plains of India, Downs (Australia), Canterbury plains of New Zealand
  • Undulating and fertile land of plains is beneficial for conducting agricultural activities and irrigation
  • Developing means of transport like building roads, lying railways, preparing air strips etc. is  easy in plains.
  • Industry and other commercial activities are more easy to be carried out in plains.

Plateaus

Plateaus

This article deals with ‘Plateaus.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Introduction

  • Plateau is an elevated tract of relatively flat land , limited on atleast one side by steep slope falling abruptly
  • Eg: Tibetan plateau.

Reasons for formation of Plateaus

There are many reasons for the formation of Plateaus

  • When two mountain ranges are forming, then landmass in between them rise too &  Plateaus are formed .
  • Deposition from lava – if lava is basaltic it will spread easily &  form flat elevated surface
  • Deposition from wind over long time => After compression & solidification of deposited material plateau will form.
  • When upland of any surface is eroded due to glaciers, plateau is formed.

Types of Plateaus

1 . Intermontane plateau

  • Intermontane plateaus are  highest, largest & most complex plateaus of world.
  • Intermontane plateaus are enclosed and surrounded by mountain ranges from different sides.

Examples include

a. Tibetan Plateau

  • Stretches 1000 km north to south &  2500 km east to west & average elevation is 4500m  => called Roof of the world .
  • Bounded by Kunlun Mountains in the north & Himalayas in the south. 
  • Many major rivers of Asia like Indus, Brahmaputra etc rise here & also holds constellation of salt & freshwater lakes .

b. Plateau of Bolivia

  • Lies largely  in Bolivia
  • It has average elevation of 1350 m.
  • Highland  was uplifted during tertiary period when the Andes were formed
  • Contrary to Tibetan Plateau, it is very dry and  has no exterior drainage

c. Plateau of Mexico

  • Plateau stands between the eastern and western Sierra Madre Mountains.
  • It has average elevation between 1800 meters to 2300 meters
  • Large  parts of this Plateau are very dry.

d. Colorado Plateau

  • Situated in USA between Cascade Range and Rocky Mountains
  • It is situated at elevation

e. Anatolia Plateau

  • Lies in Turkey between Taurus & Pontic Range
  • It’s elevation is cause of cool weather of Turkey 
Intermontane Plateaus

2. Piedmont or Border Plateau

  • These  Plateaus border mountain ranges and owe their present position to the same uplifts that raised the mountains. 
  • Examples include
    1. Piedmont Plateau on the border of Appalachian Mountains
    2. Patagonia Plateau in South America

3. Volcanic Plateau

  • Volcanoes form variety of plateaus. 
    • Larger : built by BASALTIC lava flow.
    • Small : formed by resistant lava caps that aren’t eroded & maintain its elevation after surrounding land has been worn away.
  • Examples are
    • Columbia Snake Plateau .
    • Deccan Plateau , India.
    • Shan Plateau , Myanmar
    • Katanga Plateau, Congo
    • North Island in New Zealand
Volcanic Plateaus of the world
Volcanic Plateaus of the world

4. Erosional Plateau

  • Form in semiarid regions where streams have cut away portions of high lands.
  • Examples include
    1. Allegany Plateau near  New York 
    2. Cumberland Plateau near Appalachians in USA

5. Depositional Plateau

  • Formed due to depositional action of wind
  • Examples include Loess Plateau in China => It is formed due to process of deposition of sediments carried from the desert  by the Anticyclonic  winds which develop over Russia and come to China shedding their load in this area
Loess Plateau

6. Dome Plateau

  • These plateaus are uplifted by folding and faulting processes  into a broad dome.
  • Entrenched Meanders are feature of these plateaus
  • Examples include Ozark Plateau of USA

7. Glacial Plateau

  • These are formed due to  erosional action of glaciers.
  • Examples include
    1. Laurentian Plateau of Canada (North America)
    2. Garhwal Plateau of India

Importance of Plateaus

  • Plateaus have large amount of   mineral wealth like Gold, Iron, Copper, Diamond, Manganese, Mica , Granite etc. which forms industrial base of any economy. Eg : Katanga Plateau of Congo is very rich in copper and Deccan Plateau of India is very rich in resources.
  • Plateaus are have more plain regions as compared to pure mountainous regions which helps in development of means of transport. Rail and road transport is lesser costly in plateau regions as compared to mountainous regions.
  • Plateau regions have abrupt slopes which are beneficial for setting up hydroelectrical centres thus helping in overall development of the region
  • Plateaus greatly effect the climate of region . For example Tibet plateau divides western Jet Stream in two parts while in summer and helps to create low pressure over Indian subcontinent which results into attraction for Monsoons 
  • They are important agriculturally as well . Eg : Deccan Plateau in India has black soil which is unmatchable for production of Cotton and Sugarcane.
  • Plateaus especially Intermontane Plateaus are great source of water resources as well. Eg: Large number of rivers like Indus , Brahmaputra etc originates from Tibet plateau.

Mountains

Mountains

This article deals with ‘Mountains.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Folding and Faulting

Process of mountain formation involves concepts of Folding and Faulting. Hence, we will first learn about these concepts.

1 . Folding

  • Folding is the bending of rock strata due to compression.
  • Folding on a large scale results in mountain building referred to as orogeny
  • Up thrown part of a fold is called anticline. Down thrown part of a fold is syncline. The side of the fold is a limb
Folding

Types of folding

Symmetrical fold When compressional force is equal from both sides, the angle of the limb is same on both sides.
Asymmetrical fold When compressional force is more from one end, one limb is steeper than the other.
Isoclinal folds similar to symmetrical folds, but these folds both have the same angle and are parallel to each other
Over turned fold When one limb of the fold is pushed over the other limb of the fold, it is called as over turned fold.
Recumbent fold When one side of the fold is pushed so much that it lies positioned over the other
A. Open (Symmetrical) 
D. Recumbent 
B. Asymmetrical 
E. Isoclinal

2. Faulting

A fault is a break in earth’s crust where blocks of rock crust slide past each other.

Types of Faults

2.1 Normal Fault

  • Vertical displacement of the crust is called a normal fault.
  • Normal fault is caused by tensional forces where plates diverge.
  • One block lies above and  other block lies below the fault
Normal Fault

Landforms made by Normal fault are:

  1. Rift Valley or Graben : When a narrow block of land drops or subsides between two parallel normal faults, rift valley (Graben) is formed. Eg : River Rhine Rift valley between Black Forest and Vosges, Narmada Rift Valley between Satpura and Vindhya and Great African Rift Valley
  2. Horst : When a block of land between two faults is pushed up, block mountain or horst is formed. In this case, the central block is not only up thrown but the side blocks are also relatively downthrown . Eg : Mountains Vindhya and Satpura.
Horst and Graben
Horst and Graben

2.2. Reverse Fault

  • Reverse fault is a horizontal displacement of the crust.
  • It is caused by compressional forces
Reverse Fault

2.3 Shear Fault

It is created by shearing along transform boundaries. Rocks on either side of fault slip past each other sideways with little up or down motion

Shear Fault

Classification of Mountain Ranges of the world

Mountains can be categorised in different ways

1 . Classification of Mountains on the basis of height

Mountains

2. Classification on basis of location

Mountains

3. On basis of period of formation

We have to note the fact that Mountains are born & have finite life span like

Young mountains High, steep & growing upward (like Himalayas and other Alpine mountains).
Middle aged mountains Cut by erosion
Old mountains Deeply eroded & often buried (like Aravalli, Appalachians etc)

Types of Mountains on basis of formation

 Based on difference in process of their formation, there are following types of mountains :-

  1. Fold Mountains
  2. Bock Mountains
  3. Volcanic Mountains
  4. Domed Mountains

1 . Fold Mountains

Fold Mountains

Folded mountains are formed due to folding of crustal rocks by compressive forces generated by the convergence of tectonic plates. Eg :

  • Convergence of Indo-Australian and Eurasian plate leads to the formation of Himalayas.
  • Convergence of American and Pacific plate leads to formation of Rockies
  • Convergence of South American and Nazca  plate leads to formation Andes 

Process of their formation is known as Orogeny. It is not a continuously happening process in the geological past but it happens intermittently. In whole of the geological past, total 9 Orogenies have happened of which last four are important for us

  1. Pre-Cambrian Orogeny ( Laurentian , Algoma etc)
  2. Caledonian (Aravallis, Appalachian etc )
  3. Hercynian Orogeny (mountains include Mountains of Iberian Peninsula, Spanish Messeta etc )
  4. Alpine Orogeny (they are the youngest and are still rising. Mountains include Andes, Rockies, Himalayas, Alps, Atlas etc)

Characteristics of Fold Mountains

  • Extensive mountain chain spread over  large area .
  • They are of great height .
  • Formed along unstable parts of earth and plates are active there . Hence, earthquakes are quite common in this region.
  • Sedimentary deposits of marine origin are also found in this    .

Fold Mountains also have age – 

  • Himalayas are one of the youngest ranges & that is why they are so high .
  • Aravalli is one of  oldest mountain range . After million of years Aravalli is still standing , this vouches for its great heights during youthful stage ( which might be even higher than Himalayas ) 

Side Topic : Phases in formation of Mountain Ranges

1st Stage : Oceanic-Continental Collision

  • Convergence of Ocean & Continental Plate.
  • This will lead to formation of mountains on the Continent-Ocean margin.
  • Examples include Andes Mountain at convergence of Nazca and South American plate.
Oceanic-Continental Collision

2nd Stage : Development of Geo-Syncline

  • This is developed between Mountains & Trench .
  • In this , sediments from river as well as from ocean keep on accumulating
  • And geo-syncline is formed

3rd Stage : Continental -Continental Collision

  • Ultimately whole of ocean plate will be subsumed . 
  • Then continental continental plates will collide  and compressive forces cause folding of  Continental Crust along with  squeezing and folding of sediments and material at the Geo-Syncline (reason why Marine Sediments are found in Fold mountains)
Continental -Continental Collision

2. Block Mountains

Block Mountain formation
  • Block mountains are formed due to faulting in the ground surface. In this case, the central block is not only up thrown but the side blocks are also relatively downthrown
  • Block Mountains represent the Horst with Rift valley or Graben on either sides.
  • Examples include
    1. Sierra Nevada mountains of California (USA)
    2. Salt range of Pakistan,
    3. Rhine rift valley in Europe
    4. Vindhya and Satpura in India

3. Volcanic Mountains

  • Volcanic Mountains are formed due to Volcanic Activities
  • Examples include Mount Kilimanjaro in Africa, Mount Fujiyama etc
  • They are formed with the consolidation of Andesitic  magma coming out of earth’s crust

4. Upwarped (domed) Mountains

  • Formed by  upwarding of surface due to pressure on crust from below
  • Example : Adirondack mountains of New York.
Upwarped Mountains

Side Topic : Isostasy

It is seen that heavily snow covered glacial regions in the Polar belts (eg Norway, Greenland) tend to rise up over long period with the melting of the snow (rebound). Why?

Isostasy is the phenomenon of rebound of the earth’s crust in regions where elevation is reduced due to degradation and relative erosive processes. Here , the rebound compensates reduction in height

Since, Tectonic Plate is floating on Asthenosphere , when mountain is eroded or snow melts there is lowering of mass and hence rebounding of the tectonic plate upwards happen

Isostacy

Importance of Mountains

  • Mountains cover 27% of earth’s continents and 20% of population resides here making it an important part of human civilization
  • Mountains are great source of resources like Hydroelectricity, Wood, Medicinal plants, wild animals/insects, fruits etc.
  • Most of the perennial rivers of the world originate in mountains like Ganga, Brahmaputra, Rhine, Hwang Ho , etc.  Human civilizations were made possible by the rivers originating in mountains since most of the old civilizations like Indus valley civilization, Mesopotamian Civilisation (Euphrates) , Egyptian Civilization (Nile) etc flourished on the banks of these rivers
  • Mountains are source of attraction for tourism related activities. Large number of people visit mountainous regions as tourists and also helps in generation of employment in those regions.
  • They have religious and cultural significance as well. For example, Himalayas are abode to many Hindu gods like Shiva.
  • Mountains have their lasting effect over climate of any region. In India, the Himalayas contribute very importantly for rainfall not only through Monsoon winds but through cyclones also.

Continental Drift and Plate Tectonic Theory

Continental Drift and Plate Tectonic Theory

This article deals with ‘Continental Drift and Plate Tectonic Theory.’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Continental Drift Theory

  • Given by Alfred Wegener (German Meteorologist)  in  1912   . It speaks about rifting and drifting of continents .
  • According to Wegener, about 250 million years ago all the continents formed a single continental mass and mega ocean surrounded the same. 
    • Super continent was named PANGAEA, which meant all earth.
    • Mega-ocean was called PANTHALASSA, meaning all water.
  • He argued that, around 200 million years ago, the super continent, Pangaea, began to split
    • Pangaea broke to Laurasia/ Angaraland (forming Northern Continents) and Gondwanaland (Southern Continents) with Tethys Sea between them. 
    • Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today. (Note – India was part of Gondwanaland.)
  • Interesting theory but was scrapped saying it GEO-POETRY because he wasn’t able to explain forces of movement.

Continental Drift theory was based on following clues

a. Continental fit / Jig Saw Fit

  • Continental lands can be joined together like jig saw puzzle.

b. Rocks of same age across oceans

  • Belt of ancient rocks of 2,000 million years from Brazil coast matches with  western Africa. 
  • Geological Structure of Appalachian Mountains matches with Morocco and Algeria in North Africa.

c. Fossils

Mesosaurus Freshwater reptile found in Africa & South America.
Glassopteris Fern found on all southern continents.
Lemur Found in India, Africa and Madagascar.

d. Placer Deposits

  • The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region => gold bearing veins are in Brazil => Ghana & Brazil Plateau used to lay side by side.

e. Tillite

  • Tillite are the sedimentary rock formed out of deposits of glaciers.
  • Gondwana system of sediments from India has counter parts in six different landmasses of Southern Hemisphere.
Pr

Forces for Drifting

  • Wegener suggested that movement responsible for drifting of continents was caused by pole-fleeing force and tidal force.
  • Polar-fleeing force relates to the rotation of the earth.
  • Tidal force—is due to the attraction of the moon and the sun that develops tides in oceanic waters.
  • Wegener believed that these forces would become effective when applied over many million years. However, most of scholars considered these forces to be inadequate 

Post Drift Studies

  • It is interesting to note that for continental drift, most of the evidences were collected from the continental areas .
  • Number of discoveries during the post-war period added new information to geological literature. Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.

Convectional Current Theory

  • Wegener wasn’t able to explain the  force  behind Continental Drift.
  • Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion. These currents are generated due to radioactive elements causing thermal differences in the mantle portion. Holmes argued that there exists a system of such currents in the entire mantle portion.
  • These convection currents are nothing but molten rocks
    • Rising limb :  it will pressurise crust in such a way that crust will break .
    • Diverging limbs :  take crust away from each other .
    • Descending limbs : make two crusts to collide (like Indian & Eurasian plate ).
  • According to Holmes , these Convection Currents are the Driving Force . This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.
Convectional Current Theory

Ocean  Seafloor Spreading Theory

Post war studies using which Ocean Map was prepared showed that ocean floor is not just a vast plain but it is full of relief.

Mapping of the ocean floor and palaeo-magnetic studies of rocks from oceanic regions revealed the following facts :

  • It was realised that all along the mid-oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava
  • Ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years.
  • Age  of the rocks increases as one moves away from  crest.
  • Sediments  on the ocean floor are unexpectedly very thin => nowhere was  sediment column found be older than 200 million years.
  • Deep trenches have deep earthquake occurrences while in mid-oceanic ridge areas, earthquake foci have shallow depths
  • Concept of zebra strip / Magnetostratigraphy : rocks equidistant on either sides of mid-oceanic ridges show  similar magnetic properties

This led Hess (1961) to propose his hypothesis, known as the “sea floor spreading”

  • Constant eruptions at  crest of oceanic ridges cause  rupture of the oceanic crust and  new lava wedges into it, pushing the oceanic crust on either side. The ocean floor, thus spreads.
  • Ocean floor that gets pushed due to volcanic eruptions at crest, sinks down at the oceanic trenches & gets consumed (Seafloor Spreading Theory).
Ocean  Seafloor Spreading Theory

Plate Tectonic Theory

  • Given in 1967 by McKenzie & Parker
  • Tectonic plate ( lithospheric plate) is a massive slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere . Its thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas.
  • A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion 
    • Pacific plate is largely an oceanic plate
    • Eurasian plate may be called a continental plate.
  • Theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates.
  • Young Fold Mountain ridges, trenches etc are formed due to movement and interaction of these plates

The major plates are

  • Antarctic and the surrounding oceanic plate
  • North American
  • South American
  • Pacific plate.
  • India-Australia-New Zealand plate .
  • Africa with the eastern Atlantic floor plate .
  • Eurasia and the adjacent oceanic plate.

Some important minor plates are

  • Cocos plate : Between Central America and Pacific plate .
  • Nazca plate :Between South America and Pacific plate .
  • Arabian plate : Mostly the Saudi Arabian landmass.
  • Philippine plate : Between the Asiatic and Pacific Plate .
  • Caroline plate : Between the Philippine and Indian plate (North of New Guinea) .
  • Fuji plate : North-east of Australia.
  • 2017 update – Zealandia is now considered separate Continent/Plate

These plates have been constantly moving over the globe throughout the history of the earth.

  • All the plates, without exception, have moved in the geological past, and shall continue to move in the future as well.
  • Pangaea of Wegner was also result of convergence of continental masses
Movement of Continents in history

Plate Boundaries

There are three types of plate boundaries according to Plate Tectonic Theory (these three types of plate boundaries are discussed in detail below)

Type of Boundaries

Rate of Plate Movement

  • Strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help the scientists to determine the rates of plate movement.
  • These rates vary considerably.
    • Arctic Ridge: slowest rate (less than 2.5 cm/yr),
    • East Pacific Rise in the South Pacific : fastest rate (more than 15 cm/yr).

Force for the Plate Movement

  • Convectional Currents which were first explained by Hess in his Convectional Current Theory was the main force behind plate movement

Side Topic : Palaeomagnetism & how it prove Plate Tectonics

Palaeomagnetism is the study of the record of the Earth’s magnetic field in rocks and sediments

How it proves Plate Tectonic Theory

  • Magnetostratigraphy, with rocks equidistant on either sides of mid-oceanic ridges show  similar magnetic properties
  • Polar wandering – Magnetic minerals formed at same time but on different continents points have different orientation .  So, there were either multiple north poles during the same time period or that the continents moved in relation to a single north pole. Geophysicists concluded that the magnetic poles remained stationary, and the continents moved
  • Palaeomagnetism is also used to match once joined landmasses that are now separated. For example, the orientation of magnetic minerals along the eastern coast of South America very closely matches that of similar minerals on the western coast of Africa. 

Convergent Plate Boundaries

Convergent plate boundary is the margin where two plates collide with one another.

Convergent plate boundary

Convergent Plate Boundaries can be of three types :-

1 . Ocean Ocean Convergent Boundary

  • Denser of the two oceanic plates is subducted   . It goes to Asthenosphere & generate new Magma .
  • Andesitic Magma will from in this case . Andesitic Magma is less  mobile and solidifies quickly. As a result,  underwater Volcano or Volcanic island arc will form in this case.
  • Characterised by  Trenches , Underwater Volcanoes , Volcanic Island Arc and Earthquakes .
  • Island arc  (and not single island) will be formed because they will be formed on whole boundary where Ocean-Ocean plate is converging. All these islands will be volcanic islands .
Ocean Ocean Convergent Boundary

2. Ocean – Continental Convergence Boundary

  • Oceanic Plate is denser than Continental Plate . Hence, Ocean plate will be subducted into Asthenosphere &  melt down there. But at the same time, due to the great compressional force between two converging plates, folding will happen on the Continental Plate , resulting in formation of Marginal Fold Mountains . As we know, folding happens along the zones of  weakness , hence, when Magma of the subducted  Ocean plate will rise , it will  come out of the crust through these weak zones  resulting in formation of Volcanic Peaks .  
  • Hence, Volcanic mountains ranges, Trench and earthquakes  are common on boundaries
  • Examples of such volcanic mountain ranges are
Andes  mountains South America
Rockies North America
Atlas Africa
Ocean - Continental Convergence Boundary

3. Continental – Continental Convergent Boundary

  • When continental plate converges into continental plate , crust at  both sides is tool light  & buoyant to be subducted. Both are  compressed against each other and folding happens. Hence, in this case Fold mountain Belt forms.
  • No Volcanism but powerful earthquakes are created in this region .
  • Himalayas & Urals  are formed in this way due to convergence of Indo-Australian and Eurasian plates.
  • Pressure between plates is so high that metamorphic rocks form there.

Divergent plate Boundaries

  • Divergent plate boundary is the margin where two plates move apart. For instance, African plate and South American plate
  • Divergent plate boundary is termed as the constructive plate boundary as it leads to the formation of new lithosphere .
Divergent plate boundary

Divergent plate boundaries are of two types :-

1 . Ocean – Ocean Divergent Boundary

  • Creation of new crust takes place at submarine mountain ridge . Ocean crust is rifted apart & basaltic magma wells up to fill the opening.
  • Basaltic Magma will come out from the Mantle. Since basaltic magma  is very mobile, hence it will spread out. This magma hardens & forms igneous rock . Since magma is basaltic, hence ocean crust is basaltic in nature .
  • Mid-Atlantic Ridge is an ideal example of a submarine mountain ridge in the Atlantic Ocean. It is the longest mountain ridge in the world. It extends for about 16,000 km, in a ‘S’ shaped path, between Iceland in the north and Bouvet Island in the south
  • Water from hydrothermal vents (along the submarine ridges) is rich in dissolved minerals and supports organisms like chemo-autotrophic bacteria.

2 . Continental – Continental Divergent Plate

  • Rift  valley along with block mountains are formed when two continental  plates move apart.
  • Initially it leads to the development of a small body of water . But if rifting continues , body of water becomes bigger to juvenile ocean and consequently to Large Ocean.
  • Example : The Great Rift of Africa

Side Topic : Great Rift of Africa

  • In Great African Rift Valley, Continental Continental Divergence is observed leading to formation of Rift Valley.
  • Almost all the lakes in Africa are in the Rift Valley generated by diverging of  continental Plates except Lake Victoria.
Great Rift of Africa

Transform Plate Boundary

  • Where two plates are sliding past each other.
  • They are  under shear stress.
  • The lithosphere is neither destroyed nor created by the transform plate boundary. Hence , it is called Conservative or passive plate boundary.
  • Earthquakes are common & Volcanoes are not formed at Transform boundaries. Whenever plate boundary is active, Earthquakes are also experienced in that region.

  • Example –San Andreas Fault in USA (Pacific Plate & North American) , Chile etc
San Andreas FAult

Earthquakes

Earthquakes

This article deals with ‘Earthquakes .’ This is part of our series on ‘Geography’ which is important pillar of GS-1 syllabus . For more articles , you can click here

Introduction

Sudden release of energy in Earth’s crust, which leads to series of motions due to  waves created by the released  energy is called Earthquake. Hence , it is nothing but release of energy.

Terminology

Hypocentre/ Focus Point inside surface where earthquake is generated by first rock displacement & fault is created.
Epicentre Point on earths surface which is directly above hypocentre. Most destruction occurs here.
Earthquakes

Earthquakes occur in three forms of clusters

Foreshocks Occur before a larger one at same location.
Mainshocks Are of highest magnitude & occur within an hour of foreshock.
Aftershocks Are smaller quakes that occur at same general geographic location for days & even years after the larger main shock

Types of Earthquake

1 . Types  based on causes

a. Tectonic Earthquake

  • Most common type are the tectonic earthquakes.
  • These are generated due to sliding of rocks along a fault plane.

b. Volcanic Earthquake

  • Due to volcanic eruption.
  • But confined to areas having active volcanoes .

c. Collapse Earthquakes

  • In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors.

d. Explosion Earthquake

  • Ground shaking may also occur due to the explosion of chemical or nuclear devices.

e. Human Induced Earthquake

  • Earthquakes that occur due to human activities
    1. Reservoir induced seismicity
    2. Mining related seismicity
    3. Groundwater extraction related earthquake

Types on basis of depth

a. Shallow focus

  • Hypocentre is upto 70 km
  • Not felt away from epicentre
  • Cause maximum destruction in the region near epicentre (energy released is close to surface)

b. Medium focus

  • Hypocentre is from 70 to 300 km
  • Distance of impact and destruction potential between deep focus and shallow focus.

c. Deep Focus

  • Hypocentre is below 300 km
  • Felt upto large distance from epicentre
  • Cause low destruction

Side Topic : Why maximum destruction is near the epicentre?

As one moves away from the epicentre, wavelength of surface wave will increase. Hence, building situated near the epicentre will fall on both crest and trough of wave causing building to collapse while building situated away from the epicentre will either fall entirely on crest or trough not doing much damage.

maximum destruction near epicentre

Effects of Earthquake

Earthquake is a natural hazard. The following are the immediate hazardous effects of earthquake:

  • Ground Shaking
  • Land and mud slides.
  • Soil liquefaction.
  • Ground lurching .
  • Avalanches.
  • Ground displacement
  • Floods from dam and levee failures .
  • Fires.
  • Structural collapse.
  • Falling objects
  • Tsunami.
  • Change in course of river
  • Human and property loss
  • Cracks in building

Earthquake Belts in World

Three major earthquake belts in this world :-

Earthquake belts of the world

a. Circum-Pacific  Belt

  • Along a patch surrounding the Pacific ocean
  • Region of great seismic activity eg Japan, Philippines, Chile etc lies here
  • Coincides with Pacific Ring of Fire.

b. Alpine Himalayan Belt

  • Runs through mountainous region that flank Mediterranean Sea extend through Iran & Himalayan mountains
  • Frequent & destructive earthquakes occur here.

c. Other areas

  • Northern Africa
  • Rift Valley areas of the Red Sea and the Dead Sea.

Earthquakes In India

  • India has high frequency of great earthquakes (greater than 8.0) .

Reason

  • Tectonic setting of India .
  • Indian plate is moving at a speed of one cm/ year towards the north & north-eastern direction and this movement of plates is being constantly obstructed by the Eurasian plate from the north. As a result of this, both the plates are said to be locked with each other resulting in accumulation of energy .  Excessive accumulation of energy results in building up of stress, which ultimately leads to the breaking up of the lock & sudden release of energy causing earthquake

Region has remained seismically quiet for more than 600-700 years => enormous buildup of strain => Earthquake of magnitude 8.5 or more can hit region in near future

Some Great Earthquakes occurred in India.

1819:Gujarat 8.3  
1897:Assam 8.7 Extensive liquefaction in alleviated plains of Brahmaputra.
1934:Bihar-Nepal 8.4 Extensive liquefaction —-> buildings tilted & slumped bodily into ground.
1967:Koyna 6.5 – 1962 —-> Koyna Dam built —> earlier area was aseismic but after this seismic activity increased—-> dam induced earthquake
– Revision of Indian Seismic was done and in Zone map, Koyana was moved from Zone I to  zone IV & Bombay to Zone III.

Question : What are the reasons for occurrence of earthquakes in geologically inactive regions like Peninsular India ?

  • It is possible that collision of Indian and Eurasian plate has generated stresses not only at boundaries but also inside plate. As a result zones of weakness have formed on the plate.  There is possible breaking up of Indian plate, which is most evident along river Bhima near Latur and Osmanabad, regions experiencing disturbances in the past.
  • Peninsular India is home to some grand dams and reservoirs which have resulted in reservoir induced earthquakes (eg. Koyna Dam).

Measuring the Earthquake

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

1 . Richter Scale

  • Concept of Earthquake magnitude was developed by Richter who invented Richter scale   .
  • It is Base 10 logarithmic scale obtained by calculating logarithm of shaking amplitude of largest displacement from zero Anderson Torsion seismometer at 100 kms from epicenter.
  • Increase in 1 means 10 times more shaking amplitude.
  • Earthquakes with magnitude more than 6 are destructive

2.  Mercalli Scale 

  • Intensity scale is named after Mercalli, an Italian seismologist.
  • Intensity scale takes into account the visible damage caused by the event.
  • The range of intensity scale is from 1-12.
Indian Earthquake Zones

Side Topic: Shindo scale

  • Known as Japanese Meteorological Agency(JMA) seismic intensity scale.
  • Used in Japan & Taiwan.
  • JMA scale tells us about degree of shaking at a point on earths surface .
  • Ranges between Shindo 0(no shaking)  to Shindo 7(most devastating).
  • Same earthquake has different Shindo number at different locations.

EARTHQUAKE EARLY WARNING SYSTEM

  • Earthquake Early warning system will issue warnings 1-40 seconds before earthquakes 
  • Based on detection of  waves generated during an earthquake.
    • P wave is harmless but travels faster than the Surface and S waves which cause maximum destruction
    • This system works on detection of P wave  for advance warning.
Earthquakes
EARTHQUAKE EARLY WARNING SYSTEM

India has already installed it in Dehradun .