This article deals with ‘Karst Topography.’ This is part of our series on ‘Geography’ which is an important pillar of the GS-1 syllabus. For more articles, you can click here.
Introduction
Limestone is a 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 rainwater with Carbon dioxide (weak acid.)
A region with a large stretch of limestone, therefore, posses a very distinct topography termed Karst (name derived from Karst District of Yugoslavia where such topography is particularly well developed)
There is the 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 a spring or resurgence.
Location
Karst region is in Dinarik Alps in Yugoslavia.
Such topography is also found in regions of the 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 the 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.
4. Karst Window/ Karst fenster
It is a spring that emerges from underground, discharge its water and then abruptly disappears underground through a nearby sinkhole.
5. Doline
Due to high chemical activity on swallow holes, their size and depth increases. Its diameter may extend up to some kilometres and its depth may run up to 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 ends are called tunnels.
Depositional landforms
Where subterranean streams descend to underground passages, the region may be honeycombed with caves
The 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.
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.
3. Cave Pillars
Over a long time, stalactites hanging from roof is eventually joined to Stalagmite growing from floor to form pillar.
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
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 .
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 .
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
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.
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
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.
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
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
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 .
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.
b. Arcuate shaped Delta
Delta look fan shaped with numerous distributaries
Examples are Amazon, Ganga & Mekong.
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.
d. Cuspate Delta
Have tooth like projection at
their mouth.
Example : Ebro of Spain.
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
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 :
Chemical
Physical or mechanical
Biological weathering processes.
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
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.
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.
If cooled slowly at great depths : Large grains
Sudden cooling (at the surface) : small grains.
Intermediate cooling : intermediate size of grains .
Granite, gabbro, pegmatite, basalt, volcanic breccia and
tuff are some of the examples of
igneous rocks.
Metamorphic rocks are formed when already consolidated rocks undergo reorganization in structure due to excessive pressure (through the process called Metamorphism)
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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
Sahara and Thar
Plains formed by filling of lakes in Kashmir and Manitoba (Canada)
Lava plains of Idaho (U.S.A.)
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.
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
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
Piedmont Plateau on the
border of Appalachian Mountains
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
4. Erosional Plateau
Form in semiarid regions where
streams have cut away portions of high lands.
Examples include
Allegany Plateau near New York
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
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
Laurentian Plateau of Canada
(North America)
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.
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
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
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
Landforms made by Normal fault are:
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
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.
2.2. Reverse Fault
Reverse fault is a horizontal displacement of the crust.
It is caused by compressional forces
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
Classification of Mountain Ranges of the world
Mountains can be categorised in different ways
1 . Classification of Mountains on the basis of height
2. Classification on basis of location
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 :-
Fold Mountains
Bock Mountains
Volcanic Mountains
Domed Mountains
1 . 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
Pre-Cambrian
Orogeny ( Laurentian , Algoma etc)
Caledonian (Aravallis,
Appalachian etc )
Hercynian Orogeny
(mountains include Mountains of Iberian Peninsula, Spanish Messeta
etc )
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.
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)
2. Block Mountains
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
Sierra Nevada mountains of California (USA)
Salt range of Pakistan,
Rhine rift valley in Europe
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.
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
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.
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.
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.
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).
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
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)
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 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 .
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
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 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.
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
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 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
Reservoir induced seismicity
Mining related seismicity
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.
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 :-
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.
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.
