Types of Landforms in world
A mountain is defined as “a natural elevation of the earth surface rising more or less abruptly from the surrounding level and attaining an altitude which, relative to the adjacent elevation, is impressive or notable”. Mountains can be classified on the basis of their structure or their origin.
A. Structural classification:
I. Fold Mountains: These mountains have originated due to compressional tectonic forces and have been thrown up to form fold mountains e.g. Himalayas, Andes, Alps etc. The folds consist of two inclined parts called limbs, the upfold is called anticline and the downward portion is called syncline.
All young folded mountains have originated from geosynclines.
Geosynclines are long narrow and shallow water depressions characterized by sedimentation and the subsequent subsidence. The conversion of geosynclines into folded mountains requires geologically long time with definite phases of mountain building process-
(b) Orogenesis: After horizontal compression has completed its task, vertical uplift starts. This is the real stage of mountain building.
(c) Glyptogenesis: In this phase the characteristic land forms are sculptured by erosion.
On the basis of age the Fold Mountains can be grouped into:
(i) New or Young fold Mountains: Example: The Alps, the Himalayas, the Circum-Pacific oceanic Mountains, etc. The main features of these mountains are the complex folding of the rocks, faulting, volcanic activities, and the erosion caused by running water, ice, winds, etc.
(ii) Old Fold Mountains: Example: The Caledonian and Hercynian mountains ofcentral Europe, the Pennines, the Highland of Scotland, etc. These mountains were folded in very ancient times, and then subjected to denudation and uplift. Many faults were formed and the layers of the rock were wrapped. Many mountains exist as relicts due to erosion.
II. Block Mountains: They are originated by tensile forces leading to formation of rift valleys. They are also called horst mountains e.g. black forest, Vosges, Vindhya, Satpura, Sierra Nevada etc. When the crust cracks due to tension or compression faulting takes place. A section of the landform may subside or rise above the surrounding level giving rise to Rift valley or Graben and Block Mountains or Horst. The Block Mountains have a steep slope towards the rift valley but the slope on the other side is long and gentle.
III. Dome Mountains: They are originated by magmatic intrusion and upwarping of crustal surface e.g. lava domes, Batholith domes etc.
IV. Mountain of Accumulation: They are originated by accumulation of volcanic material e.g. cinder cones, composite cones etc. These are formed by the emission and deposition of lava and so they are also called volcanic mountains. The slope of the mountains becomes steep and the height increases due to the development of the cones of various types like Cinder cones, Composite Cones, Acid lava cones, Basic lava cones, etc. Some of the examples of this type are Popocatepetl of Mexico, Mount Rainier of Washington, Lessen Peak of California, the Vesuvius of Italy, the Fujiyama in Japan, the Aconcagua in Chile etc.
V. Circum Erosional or Relict Mountain: e.g. Vindhyachal ranges, Aravallis, Satpura, Eastern and Western Ghats, Nilgiris, Parasnath, Girnar, Rajmahal. Thesemountains have been subjected to weathering and erosion for a long time and lowered down. They represent the old stage of mountain life cycle.
B. Classification on the basis of Mountain Building periods
• Pre-Cambrian Mountains: Rocks of these mountains are older than the Cam- brian era, and are found in older stable blocks or old shields which are now metamorphosed. Some of those old shields are Laurentia, Fennoscandinevia (Europe), Angaraland (Asia), Gondwanaland (Asia), etc.
• Caledonian Mountains: (320 m.yrs.): Mountains of Scandinavia, Scotland, N. America, Aravallis, Mahadeo, Satpura fall under this category. This mountain building process started at the end of the Silurian period or at the beginning of the Devonian period.
• Hercynian Mountains: (240m.yrs.): These Mountains were formed during Permian and Permo-Carboniferous pe- riod. They include Appalachian in N. America, Meseta in Spain, Vosges and Black Forest in Germany, Harz, Donetz area of Ural , Altai, Kinghan ,Tien Shan, Alai, Nan-Shan, etc. Meseta Mountains in Morocco; the High Atlas Mountains also represent this category.
• Alpine Mountains (30m.yrs.): It started by the end of the Mesozoic era and con- tinued upto the Tertiary period. These are the highest mountains of the world. Being newer, the erosional forces could not erode them into a Peneplain like the Himalayas, the Alps, the Rockies, the Andes, the Atlas, etc. Stages of Mountains Building: The life history of mountains can be divided into youth, maturity and old stage. Following are the characteristics of mountains in different stages:-
A. The Youth Mountains:
1. The rivers are youthful and the valleys are deep and their flow is fast.
2. Landslide and volcanic activities are common.
3. The mountains are high.
4. The slopes are steep and the piedmont is bare.
5. The sky line is irregular.
B. The Maturity of Mountains:
1. The rivers are mature and many water- gaps exist in the area.
2. The height of the mountains is not much.
3. The peaks are rounded, generally covered by thick vegetation.
4. Landslides are uncommon and no earthquakes are experienced.
5. Slopes are not steep. Pebbles and rock fragments are accumulated in the piedmont area.
C. The Old-Age of Mountains:
1. The rivers have attained old age.
2. Monadnocks are found denuded and are a common sight.
3. The mountains are low. Peneplain condition seems imminent.
4. The area is broad, low and leveled which has wavy hills at some places.
Plateaus are extensive upland areas characterized by flat and rough top surface and steep walls which rise above the neighbouring ground surface at least for 300 m.
On the basis of mode of formation the plateaus can be classified into:
1. Plateaus Formed by Running Water: Many parts of the Deccan of India (Kaimur Plateau, Rewa Plateau, Rohtas Plateau, Bhander Plateau), Brazilian Plateau.
2. Plateaus formed by Glacial Erosion: Plateau of Greenland and Antarctica, Garhwal Plateau.
3. Plateaus formed by Glacial Deposition: Russian Plateau, Finland Plateau, Merg of Kashmir.
4. Aeolian plateaus: Loess Plateau of China, Potwar Plateau of Rawalpindi in Pakistan.
5. Plateaus formed by endogenic processes:
(a) Intermontane Plateaus: Tibetan Plateau, Bolivian Plateau, Peruvian Plateau, Columbian Plateau, Mexican Plateau.
(b) Piedmont Plateaus: Appalachian Plateau, Patagonian Plateau, Colorado Pla- teau.
(c) Dome Plateaus: Ozark Massif (USA), Chhotanagpur Plateau.
(d) Lava Plateaus: Columbian Plateau, Mahabaleshwar Plateau.
(e) Continental Plateau: Deccan plateau, Ranchi plateau, Shillong plateau, Colum- bia Plateau, Mexican Plateau etc. etc.
(f) Coastal Plateau: Coromandal coastal upland of India.
(g) Rejuvenated Plateau: Missouri Plateau (USA).
(h) Mature Plateau: Ranchi Plateau, Hazaribagh Plateau, Appalachian Pla- teau (USA).
(i)Young Plateau: Idaho Plateau (USA), Colorado Plateau (USA), Mahabaleshwar Plateau, Khandala up- land (Maharashtra).
Plains can be defined as flat areas with low height. They may be above or below sea level e.g. coastal plains of Netherlands.
The plains may be classified as under:
1. Formation of plain due to deposition of sediments over submerged coastlands e.g. Coromandal coastal plains.
2. River deposited plains e.g. north Indian plains
3. Piedmont alluvial plain e.g. Bhabar plain
4. Flood plains e.g. Khadar and Bhangar plains
5. Lava plains e.g. plains of New Zealand, Iceland etc.
6. Glaciated plains e.g. north west Eurasian plain.
A. Erosional Plains
1. Plains of Fluvial Erosion: The plains formed by river erosion have a lot of variation because of the stages of erosional development, the initial slope and the structure of basal rocks.
(a) The Dissected Plains of the Youth: The Colorado, Kansas, Nebraska, east of the Rockies belong to this category of plains. The broad water-divides, large valleys are the main characteristics of such plains. The drainage is dendritic in nature.
(b) The Dissected Plains of Maturity: Such plains are found in North Missouri, Southern Iowa and Eastern Nebraska of USA. Areas of gentle slope are very limited and plain areas are available more in the valleys and the water divides are reduced to small ridges.
(c) The Plains of Old Age: Peneplain and Panplains usually represent this stage of plains.
Peneplain: Very few areas like Guinea plain in the north-east S. America are fully developed peneplains. The Appalachian had developed into peneplains in the ancient times but was later uplifted again. Here the high summits are of equal heights.
Panplains: A plain formed of flood plains joined by their own strength. It is a product of lateral erosion by streams.
2. Glaciated Plains : When the ice sheet melted specially in N. America and W. Eurasia , the area eroded by ice was exposed . Here the rivers have adjusted themselves before the extension of ice sheet. Lakes, swamps, waterfalls and rapids are common.
3. Aeolian Plains: Winds blow the sand and starts the activities of deflection, abrasion, etc. The plains produced by the wind actions are Reg, Serir and Hamada.
4. Plains of Semi-arid Denudation : This type of plain includes the peneplains of USA and the pediplains of south-west of Africa.
5. Plains at Continental Edges: Theses have evolved at the sea coast by the action of waves and later uplifted. The flat plains situated at the coast of Norway fall into this category.
6. Karst Plains: They are found in limestone areas. The underground water removed the limestone layer by the process of solution. A large number of depressions are produced in these plains e.g. the coastal plain of Adriatic Sea and the Karst plain of Florida (USA).
B. Depositional Plains
1. Plains of Alluvial Deposition : The deposition of the sediments takes place in three areas – the floor , the mouth and the valley of the river where the slope suddenly decreases. The shape of such depositional plain changes according to the method and place of deposition and forms three types of plains.
• Flood Plains: Here the river deposits its sediments by meandering through its course. The flood plains of Mississippi, Ganga, Indus and Nile are good ex- amples.
• Deltaic Plains: When the river termi- nates in the sea or lake, the deltas are formed due to deposition. The deltaic plains resemble flood plains but the ex- istence of large number of distributaries provides them with a distinction. Marshes and natural levees are common here. The Deltaic plains of the Ganga, the Indus, the Nile and the Mississippi are famous.
• Piedmont Alluvial Plains: The piedmont alluvial fans combine together and form a plain. Rough particles are found at the apex but the particles of debris get finer as we move towards the periph- ery.
2. Plains of Glacial Deposition: These are found in N. America and Europe, in areas which were affected by glacial action. The surface is slightly undulating and has low and broad ridges and depressions.
3. Desert Plains of Wind Erosion: The Loess Plain of China was formed by the windblown deposition of Gobi desert, situated west of it. Some other examples of such plains are the Sahara of Africa, the Koum of Russian Turkistan, the north- central Nebraska, etc.
4. Plains of Marine Deposition: They develop near the coast of shallow sea. Sand, alluvium, vegetation, etc. are deposited at the coastal areas of Netherlands, Germany,
Denmark, The Gulf of Mexico in U.S.A., etc.
Deserts are classified by their geographical location and dominant weather pattern as trade wind, midlatitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in nonarid environments are paleodeserts, and extraterrestrial deserts exist on other planets.
Trade wind deserts
Africa’s Sahara Desert The trade winds in two belts on the equatorial sides of the Horse Latitudes heat up as they move toward the Equator. These dry winds dissipate cloud cover, allowing more sunlight to heat the land. Most of the major deserts of the world lie in areas crossed by the trade winds. The world’s largest desert, the Sahara of North Africa, which has experienced temperatures as high as 57° C, is a trade wind desert.
Midlatitude deserts occur between 30° and 50° N. and S., poleward of the subtropical highpressure zones. These deserts are in interior drainage basins far from oceans and have a wide range of annual temperatures. The Sonoran Desert of southwestern North America is a typical midlatitude desert.
Coastal deserts generally are found on the western edges of continents near the Tropics of Cancer and Capricorn. They are affected by cold ocean currents that parallel the coast. Because local wind systems dominate the trade winds, these deserts are less stable than other deserts. Winter fogs, produced by upwelling cold currents, frequently blanket coastal deserts and block solar radiation. Coastal deserts are relatively complex because they are at the juncture of terrestrial, oceanic, and atmospheric systems. A coastal desert, the Atacama of South America, is the Earth’s driest desert. In the Atacama, measurable rainfall–1 millimeter or more of rain–may occur as infrequently as once every 5-20 years.
“Monsoon,” derived from an Arabic word for “season,” refers to a wind system with pronounced seasonal reversal. Monsoons develop in response to temperature variations between continents and oceans. The southeast trade winds of the Indian Ocean, for example, provide heavy summer rains in India as they move onshore. As the monsoon crosses India, it loses moisture on the eastern slopes of the Aravalli Range. The Rajasthan Desert of India and the Thar Desert of Pakistan are parts of a monsoon desert region west of the ranqe.
Polar deserts are areas with annual precipitation less than 250 millimeters and a mean temperature during the warmest month of less than 10° C. Polar deserts on the Earth cover nearly 5 million square kilometers and are mostly bedrock or gravel plains. Sand dunes are not prominent features in these deserts, but snow dunes occur commonly in areas where precipitation is locally more abundant. Temperature changes in polar deserts frequently cross the freezing point of water. This “freeze-thaw” alternation forms patterned textures on the ground, as much as 5 meters in diameter.
The Dry Valleys of Antarctica have been ice-free for thousands of years
Data on ancient sand seas (vast regions of sand dunes), changing lake basins, archaeology, and vegetation analyses indicate that climatic conditions have changed considerably over vast areas of the Earth in the recent geologic past. During the last 12,500 years, for example, parts of the deserts were more arid than they are today. About 10 percent of the land between 30? N. and 30 S. is covered now by sand seas. Nearly 18,000 years ago, sand seas in two vast belts occupied almost 50 percent of this land area. As is the case today, tropical rain forests and savannahs were between the two belts.
Fossil desert sediments that are as much as 500 million years old have been found in many parts of the world. Sand dune-like patterns have been recognized in presently nonarid environments. Many such relict dunes now receive from 80 to 150 millimeters of rain each year. Some ancient dunes are in areas now occupied by tropical rain forests.
The Nebraska Sand Hills is an inactive 57,000square kilometer dune field in central Nebraska. The largest sand sea in the Western Hemisphere, it is now stabilized by vegetation and receives about 500 millimeters of rain each year. Dunes in the Sand Hills are up to 120 meters high.