Major Landforms of the Earth

The surface of the Earth is never uniform or static. About 29 percent of our planet's surface consists of continental landmasses, while 71 percent is covered by oceans. Even across the continents, the lithosphere displays dramatic vertical inequalities in elevation, slope, and ruggedness—collectively known in geography as Relief. Depending on height above Mean Sea Level (MSL), steepness of slope, and surface profile, the relief features of the continents are grouped into three major second-order landforms: Mountains (along with Hills and Valleys), Plateaus (Tablelands), and Plains (Lowlands). Every one of these landforms is the product of a continuous tug-of-war between two opposing sets of geomorphic forces: Endogenic (Internal) Forces and Exogenic (External) Forces. Endogenic or Internal Forces originate deep within the Earth's interior, driven by radio-thermal heat and convection currents in the mantle that move tectonic plates. Endogenic forces are called 'Landform Builders' because they uplift, warp, fold, fault, or pour molten rock onto the crust, creating vertical inequalities. They operate in two ways: first, Diastrophic (Slow) Movements, which include Epeirogenic (vertical continent-building uplift or subsidence) and Orogenic (horizontal mountain-building movements caused by either Compressional Forces that push rock layers together into wave-like folds, or Tensional Forces that pull rock layers apart until they fracture along fault lines); and second, Sudden Movements, such as volcanic eruptions and earthquakes. Working against these internal builders are Exogenic or External Forces, which operate on the Earth's surface powered by solar energy and gravity. Known as 'Landform Sculptors' or 'Gradational Forces', they continuously wear down high elevations through Degradation (Weathering—the in-situ breaking and decay of rocks—and Erosion—the wearing away and transportation of rock debris) and fill up low basins through Aggradation (Deposition of silt, sand, and gravel). The four principal agents of exogenic gradation are running water (rivers), moving ice (glaciers), wind, and sea waves. A Mountain is a landmass that rises prominently above its surrounding terrain—generally to an elevation of more than 600 metres above Mean Sea Level—and possesses steep slopes, a narrow summit or conical peak, and a broad base. An elevated feature that looks similar to a mountain but rises to less than 600 metres with gentler, rounded slopes is called a Hill. When mountains occur in a long, continuous line of peaks and ridges extending for hundreds or thousands of kilometres, they form a Mountain Range (such as the Himalayas or the Andes); a group of parallel ranges formed during the same geological period constitutes a Mountain System or Cordillera. Based on their mode of origin, geographers classify all mountains into four fundamental types: Fold Mountains, Block Mountains, Volcanic Mountains, and Residual (or Dissected) Mountains. Fold Mountains are the grandest, highest, and most extensive mountain ranges on Earth. They are formed by horizontal compressional forces when two tectonic plates collide, squeezing thick layers of sedimentary rocks deposited on ancient sea beds (geosynclines) into wave-like folds. In a folded rock structure, the upward-arched crest is called an Anticline (which forms a mountain ridge), while the downward-curved trough is called a Syncline (which forms a structural valley). Because they were uplifted from marine sea beds, marine fossils are frequently discovered high up on the peaks of fold mountains like the Himalayas. Fold mountains are further divided by geological age into Young Fold Mountains and Old Fold Mountains. Young Fold Mountains—formed relatively recently during the Alpine-Himalayan orogeny about 25 to 60 million years ago—are characterized by towering elevations, jagged snow-capped conical peaks, steep slopes, deep V-shaped gorges, and active tectonic crustal instability; prime examples include the Himalayas in Asia (containing Mount Everest at 8,848.86 metres, the world's highest peak), the Andes in South America (the world's longest continental mountain range stretching about 7,000 kilometres), the Rockies in North America, the Alps in Europe, the Atlas Mountains in North-West Africa, and the Great Dividing Range (partly old/rejuvenated) in Australia. Old Fold Mountains—formed hundreds of millions of years ago during the Caledonian or Hercynian eras—have been continuously worn down by wind, rain, rivers, and glaciers so that they now exhibit much lower elevations, gently sloping sides, and smooth, rounded crests; classic examples include the Aravalli Range in India (one of the oldest fold mountain systems in the world, with Guru Shikhar at 1,722 metres), the Ural Mountains in Russia, and the Appalachian Mountains in North America. Block Mountains (also known as Horsts) are formed when horizontal tensional (or severe compressional) forces cause the brittle rocks of the Earth's crust to crack and fracture along parallel planes called Faults. When the central block of land between two parallel faults is pushed vertically upward—or when the side blocks on either side subside downward—the steep-sided, flat-topped uplifted block left standing is called a Block Mountain or Horst. Unlike fold mountains, block mountains have flat, table-like summits and steep, straight fault scarps rather than conical peaks. Famous examples include the Vindhya and Satpura Ranges in Central India, the Vosges in France, the Black Forest (Schwarzwald) in Germany, the Sierra Nevada in the USA, and the Salt Range in Pakistan. Closely associated with block mountains are Rift Valleys (also called Grabens). When the crustal block lying between two parallel faults sinks or subsides downward, it creates a long, narrow, steep-walled trough known as a Rift Valley. In India, the Narmada River and the Tapi River flow westward through classic rift valleys flanked by the Vindhya and Satpura block mountains. Globally, the Rhine Rift Valley in Europe (between the Vosges and Black Forest mountains), the Dead Sea Jordan Rift, and the Great East African Rift Valley (stretching nearly 6,000 kilometres from Syria and the Red Sea to Mozambique, housing deep lakes like Lake Tanganyika and Lake Malawi) are textbook examples. Alongside faulted rift valleys, rivers carve steep V-shaped valleys, gorges, and canyons (like the Grand Canyon of the Colorado River) in their youthful mountain stage, while moving mountain glaciers gouge out wide, flat-bottomed, steep-sided U-shaped valleys. Volcanic Mountains (also termed Mountains of Accumulation) are built up when molten rock (magma) from deep inside the Earth erupts through a narrow pipe or vent in the crust and piles up around the opening as successive layers of solidified lava, volcanic ash, cinders, and dust. They typically form symmetrical conical peaks topped by a funnel-shaped depression called a Crater. Famous volcanic mountains include Mount Fuji (Fujiyama) in Japan, Mount Kilimanjaro in Tanzania (the highest peak in Africa), Mount Vesuvius and Mount Etna in Italy, Mount Cotopaxi in Ecuador, Mount Mayon in the Philippines, Mauna Loa in Hawaii (USA), and Barren Island in India's Andaman and Nicobar Islands. The fourth category, Residual or Dissected Mountains (also called Relict Mountains or Mountains of Denudation), are not formed by fresh uplift; instead, they are the hard, resistant rock cores left standing after millions of years of exogenic weathering and erosion by rivers, wind, and rain have stripped away the softer surrounding rock of an ancient highland, plateau, or old fold mountain. Examples in India include the Nilgiri Hills, the Parasnath Hills, the Rajmahal Hills, the Girnar Hills, the Eastern and Western Ghats, and the eroded remnants of the Aravallis. The second major landform is the Plateau. A Plateau is an elevated tract of comparatively flat or gently undulating land that rises abruptly above the surrounding lowland on at least one side by steep cliffs or slopes called Escarpments, and has a broad, flat summit area. Because it resembles a raised dining table, a plateau is popularly known as a Tableland. Plateaus generally stand above 300 metres above Mean Sea Level, though some rise several kilometres high. Based on their geographical position and mode of formation, plateaus are classified into four types: (1) Intermontane Plateaus—meaning 'between mountains'—which are completely or partially enclosed by high fold mountain ranges and represent the highest and most extensive plateaus on Earth, such as the Tibetan Plateau (situated between the Kunlun Mountains to the north and the Himalayas to the south at an average elevation above 4,500 metres, earning it the title 'Roof of the World'), the Bolivian Plateau (Altiplano) in the Andes, the Mexican Plateau, the Columbian Plateau, the Anatolian Plateau in Turkey, the Plateau of Iran, and the Ladakh Plateau in India; (2) Piedmont Plateaus—located at the foot of a mountain range on one side and bordered by a plain or sea on the other side, such as the Patagonian Plateau in Argentina (at the foot of the Andes facing the Atlantic Ocean), the Piedmont Plateau of the eastern USA (at the foot of the Appalachians), and the Malwa Plateau in India; (3) Volcanic or Lava Plateaus—formed when highly fluid basaltic lava erupts quietly through long crustal fissures (cracks) and spreads out in thick horizontal sheets over vast areas, such as the Deccan Trap (Deccan Plateau) of Peninsular India (whose basaltic rocks weather into moisture-retentive Black Regur Soil ideal for cotton cultivation), the Columbia-Snake Plateau in the USA, the Antrim Plateau in Northern Ireland, and the Ethiopian Plateau; and (4) Continental Plateaus—vast ancient crystalline shields uplifted by slow vertical continental movements far away from fold mountains and rising abruptly from coastal plains or seas, such as the Chota Nagpur Plateau in eastern India (famous as the 'Mineral Heartland of India' for its rich reserves of coal, iron ore, manganese, bauxite, and mica), the Ranchi Plateau, the Arabian Plateau, the Plateau of South Africa, the Western Australian Plateau, and the Canadian (Laurentian) Shield. The third major landform is the Plain. A Plain is an extensive tract of flat or gently rolling lowland, generally having an elevation of less than 200 metres above Mean Sea Level and a very gentle gradient. Occupying nearly 55 percent of the Earth's continental land area, plains are classified by their mode of origin into three major types: (1) Structural Plains—formed by endogenic diastrophic movements, either representing structurally depressed horizontal rock strata of continental interiors (such as the Great Central Plains of the USA, the Russian Platform, and the Central Lowlands of Australia) or formed by the gentle uplift and emergence of the shallow continental shelf above sea level (such as the Gulf Coastal Plain of the USA and the Coromandel Coastal Plain of eastern India); (2) Erosional Plains—formed when rivers, glaciers, and wind wear down elevated highlands over millions of years into an almost flat undulating surface called a Peneplain ('almost a plain') or wind-planed Pediplain, such as the Canadian Shield, the Baltic Shield, the West Siberian Plain (the world's largest unbroken lowland plain), and the Lorraine Plain in France; and (3) Depositional Plains—formed by exogenic aggradation when rivers, wind, glaciers, lakes, or sea waves deposit thick layers of fertile sediment in low depressions. Depositional plains include Alluvial and Deltaic Plains built by rivers (such as the Indo-Gangetic-Brahmaputra Plain in India and Bangladesh, the Hwang-Ho and Yangtze Plains in China, the Nile Valley and Delta in Egypt, the Mississippi Plain in the USA, and the Po Valley in Italy), Aeolian or Loess Plains built by wind-blown yellow silt (such as the Loess Plains of North-West China and the Pampas of Argentina), Glacial Drift or Outwash Plains deposited by melting ice sheets (such as the North German Plain and prairies of North America), Lacustrine Plains formed when ancient lake beds are silted up (such as the Valley of Kashmir and the Imphal Basin in India), and Coastal Plains formed by marine wave action and coastal rivers. Finally, each landform profoundly governs human habitation and economic activity. Mountains serve as climatic barriers, sources of perennial glacier-fed rivers, hubs of hydroelectric power, timber, medicinal herbs, terrace farming, and tourism, yet remain sparsely populated due to steep slopes, harsh cold climates, thin soils, and difficult transport. Plateaus are the world's great storehouses of metallic and fuel minerals, generate hydroelectricity at steep escarpment waterfalls (like Jog Falls and Hundru Falls), and support cotton farming on black lava soils and pastoral grazing. Plains, blessed with deep fertile alluvial soils, abundant river water, a flat surface for effortless canal irrigation and road-rail construction, and congenial climates, are the 'Food Baskets' and 'Cradles of Human Civilization', supporting more than 80 percent of the world's human population.