Landforms are natural features on solid surface of the planet Earth that make up the terrain such as Oceans, Plains, Plateaus and Mountains.
Several related landforms together make
up landscapes, (large tracts of earth’s surface).
Each landform has its own physical shape,
size, materials and is a result of the action of
certain geomorphic processes and agent(s). Landforms once formed may
change in their shape, size and nature slowly
or fast due to continued action of geomorphic
processes and agents.
Due to changes in climatic conditions and
vertical or horizontal movements of landmasses, either the intensity of processes or the
processes themselves might change leading to
new modifications in the landforms.
A
landmass passes through stages of development
somewhat comparable to the stages of life —
youth, mature and old age.
There are major 3 types of Landforms:
A) First-Order Landforms (Continental & Oceans)
B) Second-Order Landforms (Plains, Plateaus & Mountains)
C) Third-Order Landforms (Volcanoes, Rivers, Glacier, Desert, etc.)
There are 2 types of Forces responsible for formation of Landforms:
a) Endogenic [Earthquake, Volcanic eruptions, Landslides, etc.]
b) Exogenic [Erosion, Winds, Sea Waves, etc.]
Endogenic and Exogenic forces are the geomorphic processes that bring changes in the earth’s surface internally and externally. Earth's surface is not flat as it goes through a lot of formation and deformation. The main reason why the earth's crust is so uneven is because of the influence of external and internal forces, known as exogenic and endogenic forces, respectively.
a) Endogenic Forces:
'Endo' is a prefix that means 'in'. Endogenic forces are the pressure whose origin is inside the
earth, which is why it is also regarded as internal forces. These internal forces lead to vertical and horizontal movements and result in subsidence, land upliftment, volcanism, faulting, folding, earthquakes, etc. These forces caused because of radioactivity, primordial heat, and tidal and rotational friction, and
they play a crucial role in the formation of the earth's crust.
Endogenic forces are also called constructive forces as they establish relief features on the
Earth’s surface. These exist in two forms: Slow movements (Diastrophic forces) and Sudden motions.
Diastrophic forces or Slow
movements are created by pressure caused by slow motion of the solid material on the surface of the earth which takes place over a long time and are non-noticeable. On the other hand, Sudden
movements such as earthquakes and volcanic eruptions are visible and cause a sudden change
in the landform.
There are 2 major movements in Diastrophism:
# Epeirogenic motions: or Vertical Movements responsible for creation of Plateaus and Continents of Earth. This is the process of warping or upliftment of large parts of the earth’s surface.
# Orogenic movements: or Horizontal movements that referred as mountain building that involves major folding, and affects the long as well as narrow belts of the surface. They can be categorized into two major pressures such as the pressure of tension and pressure of compression.
b) Exogenic Forces:
'Exo' is a prefix that means 'out'. Exogenic forces, also known as external forces, are the ones
that arise within the atmosphere of the earth. The result of exogenic forces destroys the earth's
surface by causing land to wear down, which is why they are regarded as "land-wearing forces".
Exogenic processes, such as weathering, deposition, erosion, etc., are the creators of exogenic
pressures.
All exogenic processes are referred to as denudation, which means “to peel away” or “to reveal.”
Geomorphic agents are natural elements capable of performing these exogenic processes (or exogenic geomorphic agents). For example, the wind, water, and waves.
Divisions in Landforms:
Geomorphology is the process-based study of landforms.
Geo-morph-ology originates from Greek: Geo meaning the “Earth”,
morph meaning its “shape”, and ology refers to “the study of”.
Scientists who study landforms are Geomorphologists.
Geomorphology defines the processes and conditions that influence
landform development, and the physical, morphological, and structural
characteristics of landforms.
A) 1st Order or Primary Landforms:
These consists of 7 Continents and 5 Oceans. The vast land masses on Earth are called Continents and huge water bodies are called Oceans.
Of the earth’s surface, an area of 148 million square km, i.e., 2/7th part, is covered by land and the remaining 361 million sq km, i.e., 5/7th part is covered by oceans.
Seven Continents are: Asia (Largest), Africa, North America, South America, Antarctica, Europe & Australia (Smallest).
Five Oceans of Earth: Pacific (Largest), Atlantic, Indian, Southern & Arctic (Smallest).
Land classification - Sangam period
1. Kurinji- Mountain and its environs
2. Mullai- Forest and its surroundings
3. Marutham - Agricultural land and its adjoining areas.
4. Neithal- Sea and its environs
5. Palai- Desert region
The major landforms of the continents are:
(a) Mountain belts
(b) Old stable shields.
The mountain belts include—the Himalayas, the Alps, the Rockies, the Andes, the Caucasus and the Eastern Indian mountain ranges.
The old shields of the world are—Canadian, Baltic, Brazil, Siberian (Angara), Gondwana, Australia, South Africa, Antarctica and China Shields. All the plateaus of the continents are located in these shields.
The landforms of the ocean floor consist of:
(a) Continental shelf
(b) Continental slope
(c) Deep sea floor
(d) Trench or canyons.
B) 2nd Order Landforms:
These are grouped into Mountains /Hills, Plateaus and Plains.
1) Mountains:
A Mountain is any natural elevation of the earth
surface higher than 900 meters. The mountains may have a small summit and
a broad base, considerably higher than the
surrounding area. Some mountains are even higher
than the clouds. As you go higher, the climate becomes
colder. In some mountains, there are permanently frozen
rivers of ice called glaciers. There are some
mountains you cannot see as they are under the sea. Mountains vary in their heights and shape.
Mountains may be arranged in a line known as
range. Many mountain systems consist of a series of
parallel ranges extending over hundreds of kilometers.
The Himalayas, the Alps and the Andes are mountain
ranges of Asia, Europe and South America, respectively.
There are 4 types of Mountains:
(a) Fold mountain,
(b) Block mountain,
(c) Volcanic / Mountain of accumulation, and
(d) Residual mountain.
a) Fold Mountains are formed because of collision or convergence of 2 tectonic plates due to compressional forces. These fold mountains are of 2 types - Old & Young / Alpine / Tertiary.
Examples of Old Fold Mountains:
The Aravalli range in India is one of the
oldest fold mountain systems in the world. The range
has considerably worn down due to the processes of
erosion. The Appalachians in North America and the
Ural mountains in Russia have rounded
features and low elevation. They are very old fold
mountains.
Examples of Young Fold Mountains:
The Himalayan Mountains in Asia and the Alps in Europe are young fold mountains with rugged relief and high conical peaks.
The Rocky Mountains of North America, or the Rockies, stretch from northern Alberta and British Columbia in Canada southward to New Mexico in the United States, a distance of some 3,000 miles (4,800 kilometers). In places the system is 300 or more miles wide.
The Andes Mountains are a series of extremely high plateaus surmounted by even higher peaks that form an unbroken rampart over a distance of some 5,500 miles (8,900 kilometers)—from the southern tip of South America to the continent's northernmost coast on the Caribbean.
Atlas Mountains, series of mountain ranges in northwestern Africa, running generally southwest to northeast to form the geologic backbone of the countries of the Maghrib (the western region of the Arab world)—Morocco, Algeria, and Tunisia. They extend for more than 1,200 miles (2,000 kilometres), from the Moroccan port of Agadir in the southwest, to the Tunisian capital of Tunis in the northeast.
b) Block Mountains are formed because of divergence as 2 tectonic plates are move in opposite direction due to tensile forces. These are
created when large areas are
broken and displaced
vertically. The uplifted blocks
are termed as horsts and the
lowered blocks are called
graben.
The Rhine valley, Black Forest and the Vosges mountain in
Europe are examples of Block mountain systems.
c) Volcanic Mountains are formed because of accumulation of acidic / andesitic lava.
Mt.Kilimanjaro in Africa and Mt.Fujiyama
in Japan are examples of such mountains.
2) Plateaus:
Plateaus are the elevated portions of the Earth extended from 300 mts to 900 mts that have flat surfaces bounded by steep slopes.
Plateaus are of 4types:
a) Structural
b) Piedmont
c) Intermont
d) Lava
The causes of the formation of the plateaus are:
1. Thermal Expansion - Examples of the plateaus formed by the thermal expansion of lithosphere are: Yellowstone Plateau in the United State, Massif Central in France, and Ethiopian Plateau in Africa
2. Crustal Shortening - Examples of the plateaus formed by thrusting of one block of crust over another or
sliding of one block of crust over another, are: Plateaus found in North Africa, Turkey, Iran, Tibet.
3. Volcanism - Examples of the plateaus formed by the flood basalts are: India's Peninsular Plateau & The USA's Columbia Plateau.
Plateaus can also be formed by the erosional processes of glaciers on mountain ranges, leaving them sitting between the
mountain ranges. Water can also erode mountains and other landforms down into plateaus. Dissected plateaus are highly
eroded plateaus cut by rivers and broken by deep narrow valleys. Computer modeling studies suggest that high plateaus
may also be partially a result from the feedback between tectonic deformation and dry climatic conditions created at the
lee side of growing orogens.
Some of the famous plateaus across the globe are given below.
1. Deccan Plateau (India) - It extends over 8 Indian states, covering entire South India. It is bordered by 3
Mountain ranges.
2. Chotanagpur Plateau (India) - It has huge reserves of iron, coal, and manganese.
3. Tibetan Plateau (China) - It is the highest and biggest plateau on earth.
4. Katanga Plateau (Congo) - Famous for copper mines.
5. East African Plateau - Famous for Gold and Diamond mining.
Dharmapuri Plateau, Coimbatore Plateau and Madurai Plateau are found in Tamil Nadu.
a) Structural Plateaus are formed because of Epeirogenic movement which is upheavals or depressions of land exhibiting long wavelengths and little folding apart from broad undulations.
The plateaus are generally rich in minerals. The Chotanagpur Plateau is one of the mineral rich plateaus in India. Therefore, mining is one of the major activities of the people living here.
b) Piedmont plateaus are bordered on one side by mountains and on the other by a plain or a sea. The Piedmont Plateau of
the Eastern United States between the Appalachian Mountains and the Atlantic Coastal Plain is an example.
c) Intermontane plateaus are the highest in the world, bordered by mountains. The Tibetan Plateau is one such plateau. So, it is called as the ‘Roof of the world’. The flat topped part of the plateau is called Tableland. Therefore, mining is one of the major activities of the people living here.
d) Lava or volcanic plateaus are the plateau that occur in areas of widespread volcanic eruptions. The magma that comes
out through narrow cracks or fissures in the crust spread over large area and solidifies. These layers of lava sheets form
lava or volcanic plateaus. The Antrim plateau in Northern Ireland, The Deccan Plateau in India and the Columbia Plateau
in the United States are examples of lava plateaus.
There are two kinds of plateaus: dissected plateaus and volcanic plateaus.
A dissected plateau forms as a result of upward
movement in the Earths crust. The uplift is caused by the slow collision of tectonic plates. The Colorado Plateau, in the
western United States, has been rising about .03 centimeter (.01 inch) a year for more than 10 million years.
- A volcanic plateau is formed by numerous small volcanic eruptions that slowly build up over time, forming a plateau from the resulting lava flows. The Columbia Plateau in the northwestern United States of America and Deccan Traps are two such plateaus.
3) Plains:
Plains are a flat and relatively low-lying lands. Plains are usually less than 200 metres above sea level. Sometimes they may be rolling or undulating. Most plains are formed by rivers and their tributaries and distributaries. Generally, plains are very fertile. Construction of transport network is easy. These plains are used extensively for agriculture due to the availability of water and fertile soil. They are most suitable for human inhabitation. Some of the
largest plains made by the rivers are found in Asia and North America. For example,
in Asia, these plains are formed by the Ganga and the Brahmaputra in India and
the Yangtze in China.
a) Structural
b) Depositional
c) Erosional
a) Structural plains: These plains are mainly formed by the uplift of a part of the seafloor or continental shelf. These are located on the borders of almost all the major continents.
Example: The southeastern plain of the United States was formed by the uplift of a part of the Gulf of Mexico.
Example: The southeastern plain of the United States was formed by the uplift of a part of the Gulf of Mexico.
b) Depositional plains: These plains are formed by the deposition of sediments brought down by rivers, glaciers and winds.
Example: The Indo Gangetic plain
Example: The Indo Gangetic plain
c) Erosional plains: These plains are formed by the continuous and long time erosion of all sorts of upland. The surface of such plains is hardly smooth. These are therefore also called peneplains which means almost a plain.
Example: Canadian shield and the West Siberian plain
Example: Canadian shield and the West Siberian plain
C) 3rd Order Landforms:
These formed by Exogenic forces.
Exogenic forces are those forces which originate on the surface of the earth. These forces are:
1. Running water.
2. Wind.
3. Underground water.
4. Glacier.
5. Sea waves
Peaks, cols, cirques, gorge, moraines, alluvial fans, floodplains, ox-bow lakes, levees, deltas, ocean islands, volcanoes and ridges are some of the many features of third order landforms.
After weathering processes have had
their actions on the earth materials
making up the surface of the earth, the
geomorphic agents like running water, ground
water, wind, glaciers, waves perform erosion.
|
Third
Order Landforms |
||
|
Agents |
Erosional
|
Depositional |
|
Rivers |
V-shaped
Valleys, Waterfall, Gorges, Canyon |
Flood
Plains, Delta Plains, Meander, Ox-Bow Lakes |
|
Glacier |
Cirque,
Fjords, U-shaped Valley, Hanging Valley, Horns |
Moraines,
Drumlins, Eskers |
|
Underground
Water / Karst Topography / Limestone |
Sinkhole,
Caves, Doline, Uvalas |
Stalactite,
Stalagmite, Pillars |
|
Wind |
Mushroom
Rocks, Yardangs, Hamadas |
Sand
dunes, Loess, Barchans, Seif |
1. Running Water:
In humid regions, which receive heavy rainfall
running water is considered the most important
of the geomorphic agents in bringing about
the degradation of the land surface. There are
two components of running water. One is
overland flow on general land surface as a
sheet. Another is linear flow as streams and
rivers in valleys. Most of the erosional landforms
made by running water are associated with
vigorous and youthful rivers flowing over steep
gradients. With time, stream channels over
steep gradients turn gentler due to continued
erosion, and as a consequence, lose their
velocity, facilitating active deposition. There
may be depositional forms associated with
streams flowing over steep slopes.
2. Valleys start as small and narrow rills; the
rills will gradually develop into long and wide
gullies; the gullies will further deepen, widen
and lengthen to give rise to valleys. Depending
upon dimensions and shape, many types of
valleys like V-shaped valley, gorge, canyon,
etc. can be recognised. A gorge is a deep valley
with very steep to straight sides and a canyon is characterised by steep step like side slopes and may be as
deep as a gorge. A gorge is almost equal in
width at its top as well as its bottom. In
contrast, a canyon is wider at its top than at
its bottom. In fact, a canyon is a variant of
gorge. Valley types depend upon the type and
structure of rocks in which they form. For
example, canyons commonly form in
horizontal bedded sedimentary rocks and
gorges form in hard rocks.
3. Meanders: In large flood and delta plains, rivers rarely
flow in straight courses. Loop-like channel
patterns called meanders develop over flood
and delta plains. Meander is not a landform but is only a
type of channel pattern. This is because of
(i) propensity of water flowing over very gentle
gradients to work laterally on the banks;
(ii) unconsolidated nature of alluvial deposits
making up the banks with many irregularities
which can be used by water exerting pressure
laterally.
Normally, in meanders
of large rivers, there is active deposition along
the concave bank and undercutting along the
convex bank. The concave bank is known as
cut-off bank which shows up as a steep scarp
and the convex bank presents a long, gentle
profile. As meanders grow into
deep loops, the same may get cut-off due to
erosion at the inflection points and are left
as ox-bow lakes.
4. Groundwater / Karst Topography: Here the interest is not on groundwater as a
resource. Our focus is on the work of
groundwater in the erosion of landmasses and
evolution of landforms. The surface water percolates well when the rocks are permeable,
thinly bedded and highly jointed and cracked.
After vertically going down to some depth,
the water under the ground flows horizontally
through the bedding planes, joints or through
the materials themselves. It is this downward
and horizontal movement of water which
causes the rocks to erode. Physical or
mechanical removal of materials by moving
groundwater is insignificant in developing
landforms. That is why, the results of the work
of groundwater cannot be seen in all types of
rocks. But in rocks like limestones or
dolomites rich in calcium carbonate, the
surface water as well as groundwater through
the chemical process of solution and
precipitation deposition develop varieties of
landforms. These two processes of solution
and precipitation are active in limestones or
dolomites occurring either exclusively or
interbedded with other rocks.
Any limestone
or dolomitic region showing typical landforms produced by the action of groundwater
through the processes of solution and
deposition is called Karst topography after the
typical topography developed in limestone
rocks of Karst region in the Balkans adjacent
to Adriatic sea.
The karst topography is also characterised
by erosional and depositional landforms.
5. Pools, Sinkholes, Lapies and
Limestone Pavements: Small to medium sized round to sub-rounded
shallow depressions called swallow holes form
on the surface of limestones through solution.
Sinkholes are very common in limestone/
karst areas.
A Sinkhole is an opening more or
less circular at the top and funnel-shaped
towards the bottom with sizes varying in area
from a few sq. m to a hectare and with depth from a less than half a metre to thirty metres
or more. Some of these form solely through
solution action (solution sinks) and others
might start as solution forms first and if the
bottom of a sinkhole forms the roof of a void
or cave underground, it might collapse leaving
a large hole opening into a cave or a void
below (collapse sinks). Quite often, sinkholes
are covered up with soil mantle and appear
as shallow water pools. Anybody stepping
over such pools would go down like it
happens in quicksand in deserts. The term
doline is sometimes used to refer the collapse
sinks. Solution sinks are more common than
collapse sinks. Quite often the surface run-off simply goes down swallow and sink holes
and flow as underground streams and re-emerge at a distance downstream through a
cave opening.
When sink holes and dolines
join together because of slumping of materials
along their margins or due to roof collapse of
caves, long, narrow to wide trenches called
valley sinks or Uvalas form. Gradually, most
of the surface of the limestone is eaten away
by these pits and trenches, leaving it extremely
irregular with a maze of points, grooves and
ridges or lapies. Especially, these ridges or
lapies form due to differential solution activity
along parallel to sub-parallel joints. The lapie
field may eventually turn into somewhat
smooth limestone pavements.
6. Caves:
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 either
through the materials or through cracks and
joints and moves horizontally along bedding
planes. It is along these bedding planes that
the limestone dissolves and long and narrow
to wide gaps called caves result. There can
be a maze of caves at different elevations
depending upon the limestone beds and
intervening rocks.
Caves normally have an
opening throug`h which cave streams are
discharged. Caves having openings at both
the ends are called tunnels.
Many depositional forms develop within the
limestone caves. The chief chemical in
limestone is calcium carbonate which is easily
soluble in carbonated water (carbon dioxide
absorbed rainwater). This calcium carbonate
is deposited when the water carrying it in
solution evaporates or loses its carbon dioxide
as it trickles over rough rock surfaces.
7. Stalactites, Stalagmites and Pillars: Stalactites hang as icicles of different
diameters. Normally they are broad at their
bases and taper towards the free ends
showing up in a variety of forms. Stalagmites
rise up from the floor of the caves. In fact,
stalagmites form due to dripping water from
the surface or through the thin pipe, of the
stalactite, immediately below it.
Stalagmites may take the shape of a
column, a disc, with either a smooth, rounded
bulging end or a miniature crater like depression. The stalagmite and stalactites
eventually fuse to give rise to columns and
pillars of different diameters.
8. Glaciers: Masses of ice moving as sheets over the land
(continental glacier or piedmont glacier if a
vast sheet of ice is spread over the plains at
the foot of mountains) or as linear flows down
the slopes of mountains in broad trough-like
valleys (mountain and valley glaciers) are
called glaciers. The movement of
glaciers is slow unlike water flow. The
movement could be a few centimetres to a
few metres a day or even less or more. Glaciers
move basically because of the force of gravity.
9. Cirques are the most common of landforms
in glaciated mountains. The cirques quite
often are found at the heads of glacial valleys.
The accumulated ice cuts these cirques while
moving down the mountain tops. They are
deep, long and wide troughs or basins with
very steep concave to vertically dropping high
walls at its head as well as sides. A lake of
water can be seen quite often within the
cirques after the glacier disappears. Such
lakes are called cirque or tarn lakes. There
can be two or more cirques one leading into
another down below in a stepped sequence.
e.g. The highest peak in the Alps, Matterhorn
and the highest peak in the Himalayas,
Everest are in fact horns formed through
headward erosion of radiating cirques.
10. Horns and Serrated Ridges: Horns form through head ward erosion of
the cirque walls. If three or more radiating
glaciers cut headward until their cirques meet,
high, sharp pointed and steep sided peaks
called horns form. The divides between cirque
side walls or head walls get narrow because
of progressive erosion and turn into serrated
or saw-toothed ridges sometimes referred to
as arêtes with very sharp crest and a zig-zag
outline.
11. Glacial Valleys/Troughs:
Glaciated valleys are trough-like and U -
shaped with broad floors and relatively
smooth, and steep sides. The valleys may
contain littered debris or debris shaped as
moraines with swampy appearance. There
may be lakes gouged out of rocky floor or
formed by debris within the valleys. There
can be hanging valleys at an elevation on one
or both sides of the main glacial valley. The
faces of divides or spurs of such hanging
valleys opening into main glacial valleys are
quite often truncated to give them an
appearance like triangular facets. Very deep
glacial troughs filled with sea water and
making up shorelines (in high latitudes) are
called fjords/fiords.
12. Moraines:
They are long ridges of deposits of glacial till.
Terminal moraines are long ridges of debris deposited at the end (toe) of the glaciers. Lateral
moraines form along the sides parallel to the
glacial valleys. The lateral moraines may join a
terminal moraine forming a horse-shoe shaped
ridge. There can be many lateral
moraines on either side in a glacial valley. These
moraines partly or fully owe their origin to
glacio-fluvial waters pushing up materials to
the sides of glaciers. Many valley glaciers
retreating rapidly leave an irregular sheet of
till over their valley floors. Such deposits varying
greatly in thickness and in surface topography
are called ground moraines. The moraine in the
centre of the glacial valley flanked by lateral
moraines is called medial moraine. They are
imperfectly formed as compared to lateral
moraines. Sometimes medial moraines are
indistinguishable from ground moraines.
13. Eskers: When glaciers melt in summer, the water flows on the surface of the ice or seeps down along the margins or even moves through holes in the ice. These waters accumulate beneath the glacier and flow like streams in a channel beneath the ice. Such streams flow over the ground (not in a valley cut in the ground) with ice forming its banks. Very coarse materials like boulders and blocks along with some minor fractions of rock debris carried into this stream settle in the valley of ice beneath the glacier and after the ice melts can be found as a sinuous ridge called esker.
14. Outwash Plains: The plains at the foot of the glacial mountains
or beyond the limits of continental ice sheets
are covered with glacio-fluvial deposits in the
form of broad flat alluvial fans which may
join to form outwash plains of gravel, silt,
sand and clay.
15. Drumlins are smooth oval shaped ridge-like
features composed mainly of glacial till with
some masses of gravel and sand. The long
axes of drumlins are parallel to the direction
of ice movement. They may measure up to 1 km in length and 30 m or so in height. One
end of the drumlins facing the glacier called
the stoss end is blunter and steeper than the
other end called tail. The drumlins form due
to dumping of rock debris beneath heavily
loaded ice through fissures in the glacier. The
stoss end gets blunted due to pushing by
moving ice. Drumlins give an indication of
direction of glacier movement.
16. Coastal processes are the most dynamic and
hence most destructive. Some of the changes along the coasts take
place very fast. At one place, there can be
erosion in one season and deposition in
another. Most of the changes along the coasts
are accomplished by waves. When waves
break, the water is thrown with great force
onto the shore, and simultaneously, there is
a great churning of sediments on the sea
bottom.
Other than the action of waves, the coastal
landforms depend upon (i) the configuration
of land and sea floor; (ii) whether the coast is
advancing (emerging) seaward or retreating
(submerging) landward.
Assuming sea level
to be constant, two types of coasts
are considered to explain the concept of
evolution of coastal landforms: (i) high, rocky
coasts (submerged coasts); (ii) low, smooth and gently sloping sedimentary coasts
(emerged coasts).
Along the high rocky coasts, the rivers appear
to have been drowned with highly irregular
coastline. The coastline appears highly indented
with extension of water into the land where
glacial valleys (fjords) are present. The hill sides
drop off sharply into the water. Shores do not
show any depositional landforms initially.
Erosion features dominate.
Along high rocky coasts, waves break with
great force against the land shaping the hill
sides into cliffs. With constant pounding by
waves, the cliffs recede leaving a wave-cut
platform in front of the sea cliff. Waves
gradually minimize the irregularities along
the shore.
17. Cliffs, Terraces, Caves and Stacks
Wave-cut
cliffs and terraces are two forms
usually found where erosion is the dominant
shore process. Almost all sea cliffs are steep
and may range from a few m to 30 m or even
more. At the foot of such cliffs there may be
a flat or gently sloping platform covered by
rock debris derived from the sea cliff behind.
Such platforms occurring at elevations above
the average height of waves is called a wave-cut terrace. The lashing of waves against the
base of the cliff and the rock debris that gets
smashed against the cliff along with lashing
waves create hollows and these hollows get
widened and deepened to form sea caves. The
roofs of caves collapse and the sea cliffs recede further inland. Retreat of the cliff may leave some remnants of rock standing isolated as
small islands just off the shore. Such resistant
masses of rock, originally parts of a cliff or
hill are called Sea stacks.
DEPOSITIONAL LANDFORMS
18. Beaches and Dunes
Beaches are characteristic of shorelines that
are dominated by deposition, but may occur
as patches along even the rugged shores. Most
of the sediment making up the beaches comes
from land carried by the streams and rivers
or from wave erosion. Beaches are temporary
features. The sandy beach which appears so
permanent may be reduced to a very narrow
strip of coarse pebbles in some other season.
Most of the beaches are made up of sand
sized materials. Beaches called shingle
beaches contain excessively small pebbles and
even cobbles.
Just behind the beach, the sands lifted and
winnowed from over the beach surfaces
will be deposited as Sand dunes. Sand dunes
forming long ridges parallel to the coastline are
very common along low sedimentary coasts.
Dry hot deserts are good places for Sand dune
formation. Obstacles to initiate dune formation
are equally important. There can be a great
variety of dune forms.
Crescent shaped dunes called Barchans
with the points or wings directed away from
wind direction i.e., downwind, form where the
wind direction is constant and moderate and
where the original surface over which sand is
moving is almost uniform.
Parabolic dunes
form when sandy surfaces are partially covered
with vegetation. That means parabolic dunes
are reversed barchans with wind direction
being the same.
Seif is similar to barchan with
a small difference. Seif has only one wing or
point. This happens when there is shift in
wind conditions. The lone wings of seifs can
grow very long and high.
Longitudinal dunes form when supply of sand is poor and wind
direction is constant. They appear as long
ridges of considerable length but low in
height.
Transverse dunes are aligned
perpendicular to wind direction. These dunes
form when the wind direction is constant and
the source of sand is an elongated feature at
right angles to the wind direction. They may
be very long and low in height. When sand is
plenty, quite often, the regular shaped dunes
coalesce and lose their individual
characteristics.
19. Bars, Barriers and Spits
A ridge of sand and shingle formed in the sea
in the off-shore zone lying approximately
parallel to the coast is called an off-shore bar.
An off-shore bar which is exposed due to
further addition of sand is termed a barrier
bar. The off-shore bars and barriers commonly
form across the mouth of a river or at the
entrance of a bay. Sometimes such barrier bars
get keyed up to one end of the bay when they
are called Spits. Spits may also
develop attached to headlands/hills. The
barriers, bars and spits at the mouth of the
bay gradually extend leaving only a small
opening of the bay into the sea and the bay will eventually develop into a Lagoon. The
lagoons get filled up gradually by sediment
coming from the land or from the beach itself
(aided by wind) and a broad and wide coastal
plain may develop replacing a lagoon.
20. Winds
Wind is one of the two dominant agents in hot
deserts. The desert floors get heated up too
much and too quickly because of being dry
and barren. The heated floors heat up the air
directly above them and result in upward
movements in the hot lighter air with
turbulence, and any obstructions in its path
sets up eddies, whirlwinds, updrafts and
downdrafts. Winds also move along the desert
floors with great speed and the obstructions in
their path create turbulence. Of course, there are storm winds which are very destructive.
Winds cause deflation, abrasion and impact.
Deflation includes lifting and removal of dust and
smaller particles from the surface of rocks. In
the transportation process sand and silt act as
effective tools to abrade the land surface. The
impact is simply sheer force of momentum which
occurs when sand is blown into or against a rock
surface. It is similar to sand-blasting operation.
The wind action creates a number of interesting
erosional and depositional features in the
deserts.
EROSIONAL LANDFORMS
21. Pediments and Pediplains
Landscape evolution in deserts is primarily
concerned with the formation and extension
of pediments. Gently inclined rocky floors
close to the mountains at their foot with or
without a thin cover of debris, are called Pediments. Such rocky floors form through
the erosion of mountain front through a
combination of lateral erosion by streams and
sheet flooding.
Erosion starts along the steep margins of
the landmass or the steep sides of the
tectonically controlled steep incision features
over the landmass. Once, pediments are
formed with a steep wash slope followed by
cliff or free face above it, the steep wash slope
and free face retreat backwards. This method of erosion is termed as parallel retreat of slopes
through backwashing. So, through parallel
retreat of slopes, the pediments extend
backwards at the expense of mountain front,
and gradually, the mountain gets reduced
leaving an inselberg which is a remnant of the
mountain. That’s how the high relief in desert
areas is reduced to low featureless plains called Pediplains.
22. Playas
Plains are by far the most prominent
landforms in the deserts. In basins with
mountains and hills around and along, the
drainage is towards the centre of the basin
and due to gradual deposition of sediment
from basin margins, a nearly level plain forms
at the centre of the basin. In times of sufficient
water, this plain is covered up by a shallow
water body. Such types of shallow lakes are
called as playas where water is retained only
for short duration due to evaporation and
quite often the playas contain good deposition
of salts. The playa plain covered up by salts
is called alkali flats.
23. Deflation Hollows and Caves
Weathered mantle from over the rocks or bare
soil, gets blown out by persistent movement
of wind currents in one direction. This process
may create shallow depressions called
deflation hollows. Deflation also creates
numerous small pits or cavities over rock
surfaces. The rock faces suffer impact and
abrasion of wind-borne sand and first
shallow depressions called blow outs are
created, and some of the blow outs become
deeper and wider fit to be called caves.
24. Mushroom, Table and Pedestal Rocks
Many rock-outcrops in the deserts easily
susceptible to wind deflation and abrasion
are worn out quickly leaving some remnants
of resistant rocks polished beautifully in the
shape of mushroom with a slender stalk and
a broad and rounded pear shaped cap above.
Sometimes, the top surface is broad like a
table top and quite often, the remnants stand
out like pedestals.
- When soft rocks covered by hard rocks are eroded by winds, hard rocks left behind looks like table and known as ‘Zeugen’. Their length may vary from 1 meter to 30 meters. Along with winds, rainfall and weathering also help in formation of ‘Zeugen’.
Geomorphic Terminology:
1) Isthmus: A narrow strip of land which connects two large landmasses or separate two large water bodies.
No comments:
Post a Comment