Chapter 5: Landforms of the Earth
Stand anywhere in the Himalayas and you are standing on the floor of a sea that no longer exists. Much of the rock there is sedimentary rock — rock made of grains that once settled quietly as mud and sand on the bed of a shallow ocean called the Tethys. Nothing of that ocean is left now except the material it collected. Then the plates that carry the earth’s outer shell did what they always do. They moved, slowly and steadily, and they pushed towards one another. The sediments caught between them had nowhere to go but up. The layers wrinkled the way a rug wrinkles when you push it from both ends, and the wrinkles are still visible today as arches and troughs in the rock. The chain that rose is about 2400 km long, and its highest ranges carry snow through every month of the year. But nothing that is built stays built. From the moment the range rose, rain, rivers, snow and wind began taking it apart again. That work never stops. It is why some ancient highlands survive today only as low, rounded hills, worn down over an immense stretch of time.
Diagram
Fig. 5.A1 Caption: Forces inside the earth raise the land while forces on the surface wear it down, and every landform is the result of both. Brief: Three-panel cross-section of the same block of crust — flat layered sediments; then the layers folded into a high peaked range with upward arrows labelled endogenic forces and the arch and trough labelled; then the same range reduced to low rounded hills with downward and outward arrows labelled exogenic forces.
5.1 Causes Of Landform Development
The earth is made up of a series of concentric rock layers — the crust on the outside, the mantle below it and the core at the centre. Concentric means one layer wrapped around the next, the way the coats of an onion sit inside one another. Almost everything in this chapter begins with what that arrangement does to heat.
The interior of the earth is intensely hot, but that heat does not travel out to the surface, because rocks are bad conductors of heat. A bad conductor passes heat on slowly, so the heat stays trapped where it is. Trapped heat melts the rock around it, and it keeps the asthenosphere — the layer lying underneath the crust — in a semi-molten state, neither fully solid nor fully liquid.
A half-melted layer beneath the crust is what makes the crust above it movable. This leads to several conditions which cause landform development on the surface of the earth. A landform is a natural feature of the earth’s surface: a mountain, a plateau, a plain, a valley.
The forces that build landforms fall into two families, and Fig. 5.1 sets them out with the landforms each family produces.
Flowchart
Fig. 5.1 Caption: Forces causing the formation of landforms, and what each kind of force leaves behind on the surface. Nodes: forces causing landforms on the surface of the earth → branches into endogenic forces (internal processes) and exogenic forces (external processes); endogenic forces → diastrophic (large scale) forces → branches into epeirogenic (continent building) forces and orogenic (mountain building) forces; epeirogenic forces → block mountains, basins, rift valleys, plains; orogenic forces → fold mountains, faults, plateaus, landforms; exogenic forces → alluvial and erosional plains, valleys, glacial landforms
Endogenic forces are internal processes, working from deep inside the earth, and on the whole they build relief up. Exogenic forces are external processes, working on the surface, and on the whole they wear relief down. The chapter now takes the three causes of landform development in turn — two of them endogenic, one exogenic.
5.1.1 Plate Tectonics
The lithosphere, the rigid outer shell of the earth, is not one unbroken sheet. It is broken into several plates, some small and some very big. These plates float on the semi-molten asthenosphere below them, and that floating is what allows them to move at all.
Their movement is horizontal — sideways, across the surface — and it is very slow. Where two plates meet, they may push against one another, collide, or slide past each other. Each of these actions disturbs whatever lies in between.
What lies between two or more plates is often a thick deposit of sedimentary rock, and these deposits are the first thing to feel the movement. When plates move towards each other, the sedimentary rocks in between are compressed, squeezed from both sides at once. Rock under that kind of squeezing does not simply snap; it bends, and this bending is called folding of the rocks.
Almost all the fold mountain ranges of the world have been formed in this way. The forces responsible for the formation of fold mountains are called orogenic movements. The term ‘orogenic’ is derived from the Greek word meaning ‘mountain-building’.
These forces act at a tangent to the surface of the earth. A tangent runs along a curve rather than into it, so an orogenic force pushes sideways across the crust instead of lifting it from underneath. Such forces are primarily involved in plate tectonics.
5.1.2 Uplift And Submergence
Another set of forces also operates inside the earth, but it works in a different direction. These forces act along the radius, the straight line running from the earth’s centre out to the surface, so they push and pull vertically rather than sideways.
Their signature is large-scale uplift or subsidence of a land area. Uplift raises a wide stretch of the surface; subsidence lets a wide stretch sink. These vertical movements are known as epeirogenic movements, or continent building movements. ‘Epeiros’ in Greek means ‘continent’, and ‘genic’ or ‘genetic’ means ‘building’.
Such movements are the result of faulting, or cracks in the earth’s surface, along which one mass of rock shifts relative to another. Block mountains, basins and rift valleys are produced by these movements. Thus the continent building activities occur on a very large scale — large enough to lift or drop a whole region rather than a single hill.
The Two Forces Taken Together
The above two forces, namely orogenic and epeirogenic, are together called diastrophic forces, which means forces originating deep inside the earth. Between them they act both horizontally and vertically, and Fig. 5.A2 shows the difference in direction.
Working together, they produce relief such as mountains, plateaus, plains, lakes and big faults. Relief is the difference in height between one part of the surface and another, so a landscape with high peaks and deep valleys is said to have strong relief.
Diagram
Fig. 5.A2 Caption: Orogenic forces act sideways along the surface, while epeirogenic forces act up and down along the earth’s radius. Brief: The same block of crust drawn twice — on the left, two large horizontal arrows pushing inward at a tangent to the curved surface, with the rock layers buckling upward between them; on the right, arrows along lines drawn from the earth’s centre, one area rising and the area beside it sinking, both labelled.
5.1.3 Exogenic Forces
Not every force that makes a landform comes from inside. There are forces operating on the surface of the earth, not inside the earth, which also cause the formation of landforms. These forces are called exogenic forces.
The external or exogenic forces are described as destructive forces, and the name is earned. They cause widespread destruction of existing landforms through weathering, which is the breaking down of rock where it stands, and erosion, which is the wearing away and carrying off of the broken material by agents such as running water, wind and ice.
But the word ‘destructive’ tells only half the story. Having stripped material off one place, these forces then form new landforms through both erosion and deposition. Erosion carves the land it works on; deposition sets the load down somewhere else, and what is set down is itself built into something new.
The surface of the earth is therefore worked on from two sides at once. Endogenic forces raise material up; exogenic forces take it down and lay it out again. Every landform in the rest of this chapter is the present state of that exchange.
5.2 Landforms
There are a large number of landforms on the surface of the earth. No two hillsides are exactly alike, and a complete list of them would never end. Geographers therefore work with a small group of major landforms — the large features that give a continent its broad shape, rather than the small ones you could walk across in an afternoon.
The major landforms are three: mountains, plateaus and plains. Each of them is taken up in turn in the rest of this chapter, and each is divided into types on the same principle — not by how high it stands or where it is found, but by the process that made it.
That principle is worth holding on to, because it explains some odd pairings. The same set of forces can raise a block mountain in one place and leave a plain in another, as Fig. 5.1 has already shown. Two highlands that look much alike from a distance may have completely different histories written into the rock beneath them.
5.3 Mountains
A mountain is described as a very steep land or hill rising to great heights above the land surrounding it. Two things are being said there, and both matter. It is steep, and it is high in comparison with whatever lies around it — height alone is not enough, which is why a flat upland far above sea level is not called a mountain.
Mountains are classified into three main types depending on the forces that have formed them. The three main types of mountains are fold mountains, residual mountains and block mountains. One is built by sideways squeezing, one is what is left after long wearing down, and one is a piece of crust that has been pushed up between cracks.
5.3.1 Fold Mountains
Fold mountains are the result of lateral compression of the earth’s crust. Lateral means sideways, so the crust here is being squeezed from the sides rather than pushed from below.
More fully, fold mountains are the result of large-scale earth movements caused by stresses in the earth’s crust. Such stresses may be caused by the weight of the overlying rocks, by movements in the mantle, or by the expansion or contraction of some part of the earth.
These stresses subject the rocks to compressive forces, which produce wrinkling or folding along the lines of weakness. A line of weakness is simply a place where the rock gives way more easily than the rock on either side of it, so that is where the bending begins. The raising of these folds makes the fold mountains, and Fig. 5.2 shows the sequence.
Diagram
Fig. 5.2 Caption: How fold mountains are formed. Brief: Two stacked block diagrams of the same layered crust — the upper block lying flat with large horizontal arrows labelled push coming in from the left and the right; the lower block with the layers wrinkled, the upward arch labelled anticline, the downward trough labelled syncline, and the surface wrinkles labelled as forming fold mountains.
Where The Fold Ranges Are
If you look at the physical map of the world you will find a number of arc shaped mountain chains — chains that curve across the map like a drawn bow rather than running dead straight. They are the Andes in South America, the Rockies in North America, the Alps in Europe, the Atlas in Africa, and the Himalayas, the Zagros, the Elburg and the Kunlun in Asia. More such mountain chains exist below the sea.
Almost all the high ranges, like the Himalayas and the Alps, have peaks covered with snow throughout the year. Many glaciers are found on these ranges. A glacier is a mass of ice that moves slowly downhill under its own weight, and it never entirely melts away in summer.
Those glaciers become the source of water for perennial rivers, which are rivers that flow all year instead of drying up between rains. Therefore these mountains give birth to active agents of denudation — the whole business of wearing away the land surface and carrying the loosened material off — and of glacial erosion in particular. A range, in other words, creates the very thing that will one day destroy it.
The Chief Characteristics
Fold mountains are the highest and the most extensive mountain ranges of the world. Their chief characteristics are these. (a) They form irregular wave-like formations called folds, and the two halves of a wave have their own names: the upfold rock strata in an arch-like shape are called anticlines or crests, and the down folded structures are known as synclines or troughs, as marked in Fig. 5.2. (b) Most of these mountains are constituted of sedimentary rocks, formed due to the deposition and consolidation of sediments in shallow oceans like the Tethys, in the case of the Himalayas. (c) Fold mountains have greater length but are comparatively smaller in width, which is why they appear on a map as long narrow belts; the Himalayas are about 2400 km in length. (d) Most fold mountains except the Himalayas are found on the margins of continents, and they are located in two directions — the Rockies and the Andes run north to south, while the Himalayas run west to east — so it is clear that all of them face the oceans. (e) They are associated with volcanic activity and have many active volcanoes, because the same plate movements that fold the rock also open paths for molten material to reach the surface. (f) They have parallel ranges, and between the parallel ranges are high plateaus known as intermontane plateaus, such as the Anatolian Plateau in Turkey and the Tibetan Plateau north of the Himalayas.
Fold Ranges Under The Sea
The fold mountain chains that exist beneath the sea have their tops revealed as islands, like those in the West Indies, the island arc that closes off the eastern side of the Caribbean Sea. So an island is not always a separate thing from a mountain; sometimes it is only the part of one that stands above water.
There are also oceanic trenches, long deep gashes in the ocean floor, which are the result of the same forces as fold mountains. These forces, in combination with volcanic mountain systems, lead to the formation of ridges — long raised chains running across the sea floor. The Mid-Atlantic Ridge, which runs down the middle of the Atlantic Ocean, rises 3 km above the floor of that ocean and joins the Indian Ocean Ridge.
Young And Old Fold Mountains
Not all fold mountains are at the same stage of their life, and there are two types of fold mountains. Table 5.1 sets them side by side.
Two terms in that table need unpacking. The agents of gradation are the natural agents that level the land — running water, wind, ice and the rest — and topography means the shape of the surface, so rough topography is a surface of sharp ups and downs. Of the old ranges named, the Appalachians run down the eastern side of North America, the Urals form the divide between Europe and Asia, and the Aravallis run across Rajasthan in north-western India.
5.3.2 Residual Mountains
Mountains which are subjected to weathering and erosion for a long time are lowered down, and are called residual or relict mountains. The name is exact: what you are looking at is the residue, the part that has not yet been taken away.
Denudation and erosion by natural agents such as winds, water and snow cause the mountains to be lowered, but some resistant areas may remain and form residual mountains. Resistant here means made of rock that the agents work through more slowly, so those patches are left standing while the softer rock around them goes.
They may also be formed on plateaus that are dissected by rivers — that is, cut into by rivers until the flat top survives only as separated pieces. The hills and valleys in the Deccan plateau of peninsular India are an example. In Fig. 5.3 the original highland, a plateau, is worn down by the down cutting action of rivers to form residual mountains.
Diagram
Fig. 5.3 Caption: Residual mountain. Brief: Cross-section with the flat original surface of the highland drawn as a dotted line across the top, and the present surface below it as an irregular wavy line, with downward arrows marking where rivers have cut the highland away and left the resistant humps standing between the cuts.
They are also called mountains of denudation, which names the process rather than the shape. Most residual mountains are characterised by low elevations; however, both gentle and precipitous relief can exist, depending on the degree of recent erosion. Precipitous means very steep, so a residual mountain that has been cut into lately may still have sharp faces even though it is low.
Examples of residual mountains are the Nilgiri Hills in the south of peninsular India, Parasnath in Jharkhand and the Rajmahal Hills in eastern India, and the Catskill range in New York State in the USA.
5.3.3 Block Mountains
The forces of faulting, also known as epeirogenic forces, lead to the formation of block mountains. This is the vertical family of forces at work, and the landform it makes is not bent but broken.
Due to tensional force — a pulling force, dragging the crust apart rather than squeezing it together — cracks or faults may occur on the earth’s crust. If two parallel faults occur, the land in between subsides, or sinks, to form a rift valley. If a block rises instead, it forms a horst or block mountain.
Block mountains which have flattened summits are also called horst mountains. Those which have tilted summits have one side steep and the other side very gentle, because the whole block has been tipped rather than raised level. Block mountains are often separated by intervening basins called graben, the sunken ground lying between two raised blocks, as Fig. 5.4 shows.
Diagram
Fig. 5.4 Caption: Rift valley. Brief: Block diagram of faulted crust with a raised block labelled horst on either side of a sunken strip labelled rift valley and graben, the fault faces drawn as steep scarps, and the rock layers drawn so that the same bands can be matched across the faults.
If there are two parallel faults, the valley floor may sink and a rift valley may be formed, as in Fig. 5.5. The most extensive system is the African Rift Valley, which runs for a great distance down the eastern side of Africa.
Diagram
Fig. 5.5 Caption: Formation of block mountain by tension after faults develop. Brief: Two stacked block diagrams — the upper block with horizontal arrows labelled pull drawing the crust apart and vertical cracks opening; the lower block with the central strip dropped and labelled rift valley (graben) and the raised blocks on either side labelled block mountain (horst), with small arrows on the fault faces showing which way each block moved.
Examples of block mountains are the Vosges in eastern France and the Black Forest mountains just across the border in Germany, which face each other across the Rhine valley. The Salt Range of Pakistan and the Sierra Nevada mountains of California in the USA are other examples of block mountains.
Fig. 5.6 shows the three types side by side as they actually look on the ground: a fold mountain in (A), a residual mountain in (B) and a block mountain in (C).
Diagram
Fig. 5.6 (A) Caption: Fold mountain. Brief: Photograph of a young fold mountain — a sharp pointed peak under snow, with steep rock faces and a deep valley falling away below it.
Diagram
Fig. 5.6 (B) Caption: Residual mountain. Brief: Photograph of low worn hills with gentle slopes, rounded tops and a broad even skyline, with no bare rock faces.
Diagram
Fig. 5.6 (C) Caption: Block mountain. Brief: Photograph of an upland whose front rises abruptly above flat lower ground in a long straight wall, with the top of the block level rather than peaked.
Set the three photographs against each other and the classification stops being a list of names. The peak in (A) is sharp because it is young and has been squeezed up; the hills in (B) are rounded because they are what erosion has left behind; and the front in (C) is straight because it follows a fault, and a fault is a crack, not a curve.
5.4 Plateaus
A plateau is defined as an area of high ground, 500 feet above the ground level. The measurement is taken against the land lying around it, not against the sea, so a plateau is high in the same relative sense that a mountain is.
These uplands have a steep slope on one side. The upper part is generally flat and is called a tableland, which is exactly what the shape suggests — a raised flat top with an edge that drops away.
That flat top is what separates a plateau from a mountain. A mountain narrows upward to a peak or a crest and is steep on more or less every side. A plateau does not narrow; it keeps its width all the way to the top, and only one side falls away sharply. What the two share is height above the surrounding land, and often the very forces that raised them.
Plateaus are also classified in several ways. In the case of mountains we have used the criteria of mode of formation, and we will use the same criteria in the case of plateaus. So once again the question is not how a plateau looks but what made it.
5.4.1 Intermontane Plateaus
These are the highest and most extensive types on the globe. The plateaus surrounded by hills and mountains on all sides are called intermontane plateaus, and the name says where they sit: ‘inter’ between, ‘montane’ mountains.
They are formed along with fold mountains, which explains the ring of high ground around them. When compression throws up parallel ranges, the ground caught between those ranges is carried up too, but it is not folded into peaks in the same way, so it arrives at height still comparatively flat.
Diagram
Fig. 5.A3 Caption: An intermontane plateau is the high flat ground trapped between parallel fold ranges. Brief: Cross-section across two parallel fold mountain ranges with folded strata drawn in each, and between them a broad block of high ground with a level top labelled intermontane plateau, the surrounding lowland drawn well below on both outer sides for comparison.
The Tibetan plateau is the world’s highest as well as the most extensive intermontane plateau. Other examples are the Columbian Plateau, the North American Cordillera — the wide belt of ranges running down the western side of North America — and the Bolivian, Peruvian and Mexican plateaus.
5.4.2 Volcanic Plateaus
Volcanic plateaus are formed by numerous volcanic eruptions that slowly build up over time, forming a plateau. Nothing is folded and nothing is faulted here. Lava spreads out across the surface, cools and hardens, and the next eruption lays a fresh sheet on top of the last one until the whole surface stands high.
Diagram
Fig. 5.A4 Caption: A volcanic plateau is built upward, one lava flow resting on the last. Brief: Cross-section showing several thin horizontal lava sheets stacked one above another on an original low surface, each sheet drawn as a separate band with a slightly ragged edge, the finished flat top labelled volcanic plateau and the stepped edge labelled where the layers are exposed.
The Plateau of Peninsular India, especially the north western Deccan, is an example of a lava plateau. Other examples are the South African plateau, the Columbian Plateau in the north-western United States and the Ethiopian plateau in the highlands of eastern Africa.
The layers left behind are the evidence for all this. The Peninsular plateau of India has an enormous accumulation of basaltic rock, basalt being the dark heavy rock that forms when lava cools at the surface. The Columbian Plateau has about 20 layers of lava, suggesting that 20 different volcanic eruptions have formed this plateau.
A plateau of this kind is therefore a record as well as a landform. Each band in its edge is one event, and the sequence is preserved in order, oldest at the bottom.
5.5 Plains
A plain is defined as an extensive area of lowland with a level or gently undulating surface. Undulating means gently rising and falling, like a very shallow wave, so a plain need not be perfectly flat — it only has to be low and even.
The formation of plains is one of the most intriguing landform development processes on the surface of the earth. There are seemingly endless variations in plains, and they are an imprint of past and present processes alike. For example, a sudden heavy downpour may wash away a standing landform, or fill up a depression, to form a plain.
A plain is seldom formed by a single process, and that is worth remembering before any classification is set out. However, three types of plains can be distinguished: structural plains, erosional plains — which are worn flat by the agents that strip material away — and depositional plains. Fig. 5.7 gathers all three, and everything else in this chapter, into a single chart.
Flowchart
Fig. 5.7 Caption: The major landforms and the types within each of them. Nodes: landforms → branches into mountains, plateaus and plains; mountains → fold mountains, residual mountains, block mountains; fold mountains → branches into old fold mountains and new fold mountains; plateaus → intermontane plateau, volcanic plateau; plains → structural, erosional, depositional; depositional → branches into river, wind, marine
5.5.1 Structural Plains
The plains that are formed due to uplift or subsidence of land are called structural plains. These are the epeirogenic forces at work again, this time producing lowland rather than a block mountain.
Diastrophic forces may cause the uplift of a portion of land lying beneath the ocean water, or they may cause the submergence of coastal land under ocean water. Either movement leaves a plain, but from opposite directions — one brings a sea floor up into the air, the other takes dry land down under water.
The Great Plains of the USA were formed due to the uplift of land that had been submerged under water. The Great Plains are surrounded by the Mississippi-Missouri Plains in the east and the Rockies in the west. The fact that they were submerged under water is responsible for the existence of many lakes in the vast lowland.
The Gulf Coast, the low southern coast of the USA along the Gulf of Mexico, is another example of an emerged coastal plain. Emerged means the land has come up out of the water rather than the water having gone down.
On the other hand, the Coromandel plains along the east coast of southern India are the result of mild subsidence followed by sedimentation, which is the settling of material out of water. Thus the forces of structural development as well as deposition were active in the formation of these plains, which is a reminder of what was said at the start of this section: one process is rarely the whole story.
5.5.2 Depositional Plains
The plains that are formed by the deposition of materials which have been brought by various agents of transportation are called depositional plains. Here the material did not begin where it now lies. Something carried it, ran out of energy, and set it down, and the plain is the pile that was left.
The chapter takes the three carrying agents in turn.
River Deposition
The most widespread of these are the alluvial plains. Alluvium is the load of sand, silt and clay that a river drops when its flow slows, so an alluvial plain is a plain built out of river deposits. The Northern Plains of India are alluvial plains; the flood plains like the Mississippi Plains in the USA are another form; and there are deltaic plains in Egypt and India, built where a river splits and drops its load as it meets the sea. At the foothills of mountains are formed piedmont alluvial plains, piedmont meaning at the foot of the mountains, where a river leaves a steep slope for a gentle one and lets go of the coarsest part of its load.
The flood plains described above are also alluvial plains. They are found along rivers like the Ganga, the Yang-tze and the Mekong in China, and the Salveen in Myanmar, and they are regarded as the granaries of the world, because their renewed soil grows grain in quantities that feed far beyond their own borders.
In India, flood plains are divided into two by age of deposit. (a) Khadar plains are made of new deposits, laid down fresh every year when the river floods, so their material is young and constantly renewed. (b) Bhangar plains are made of old deposits that are not renewed every year, because they lie above the reach of the ordinary flood.
Diagram
Fig. 5.A5 Caption: One river builds several kinds of plain between the mountains and the sea. Brief: Long cross-section running left to right from mountain foothills to coast, labelling in order the piedmont alluvial plain at the foot of the slope, the flood plain along the channel with khadar marked on the low ground beside the river and bhangar on the slightly higher ground behind it, and the deltaic plain where the channel splits at the sea.
Wind Deposition
Most such plains are sandy deserts, like the Sahara in northern Africa and the Thar desert of north-western India. They have an irregular and undulating surface made by the existence of sand dunes and hallows — the mounds the wind piles up and the hollows it scoops out between them. Because wind can lift only fine material, these plains are sorted by grain size in a way that river plains are not.
Loess plains in China are formed from deserts. Loess is the very fine dust that the wind carries far beyond the desert edge before dropping it in thick soft blankets.
Marine Deposition
These plains are found in coastal regions, where the sea itself is the agent that brings and lays down the material. Examples are the east coast of India, the estuarine banks in the Netherlands, and the coasts of Germany and Denmark. An estuary is the widened mouth of a river where it meets the sea, and the banks that build up there are made of material worked over by both river and tide.
In Denmark we find polders and dykes. A polder is a piece of land reclaimed from the sea or a lake. The submerged land is surrounded by an embankment, called a dyke, and is then drained by pumping the water into canals until the floor is dry enough to farm.
Diagram
Fig. 5.A6 Caption: A polder is made by walling the sea out and then pumping the water away. Brief: Cross-section with the sea on the left held back by a raised earth embankment labelled dyke, the reclaimed floor on the right lying below sea level and labelled polder, a drainage canal along the inner side of the dyke, and an arrow showing water being pumped from the polder up into the canal.
5.6 Importance Of Landforms
The landforms on the surface of the earth are symbols of the energy of the earth. They are basically a result of the enormous heat generated inside the earth and of the climate factors operating on its surface — the same two sources this chapter opened with.
The earth releases its energy through tectonic activity, earthquakes and volcanoes. Tectonic activity is the movement of the plates themselves, and each of these three is a way of letting out energy that has been building up below. These internal forces, in addition to the surface weathering and erosional forces, become the dominant factors in the sculpturing of land.
Diagram
Fig. 5.A7 Caption: The earth’s energy makes relief, and relief goes on to shape climate, rivers, forests and where people find what they need. Brief: Branching chart running left to right — heat inside the earth leading to tectonic activity, earthquakes and volcanoes, these leading to a single box labelled relief features of the earth, and that box branching into five outcomes labelled climate and winds, snow and perennial rivers, forests and biodiversity, fishing grounds, and minerals and fossil fuels near the surface.
The landforms have, thus, the following advantages. (a) Mountains, plateaus, plains and valleys are beautiful sculptures of nature on the surface crust of the earth, and they add to the beauty of the earth. (b) Landforms represent the relief features of the earth, and they have an impact on climate, on the direction of winds, on precipitation and on other atmospheric processes; the Himalayas act as a climatic divide, shaping the distinctive climate of the Indian subcontinent. (c) The mountain ranges of the world are the abode of snow and glaciers, which release their water gradually, and they are thus the source of perennial rivers. (d) Many kinds of forests are found on the slopes of mountains, hills and plateaus, and these are home to different species of plants and animals, so the biodiversity of life on earth — the sheer variety of living things — can also be attributed to relief. (e) Since such relief features also exist in the oceans, the island groups and coastal regions built on them are ideal fishing sites. (f) Major landforms are created by disturbances resulting from the forces operating in the interior of the earth, and most mineral and fossil fuels come near the surface of the earth due to these same disturbances.
Read that list again and a pattern shows through it. Relief is not simply scenery. Where water is stored, where rain falls, where forests grow, where fish gather and where minerals lie close enough to reach are all decided, in part, by the shape of the surface. A landform is where the earth’s energy is spent, and much of what lives on the surface is arranged around the result.
5.7 Summary
- A landform is a natural feature of the earth’s surface, such as a mountain, a plateau, a plain or a valley.
- The earth is made of concentric layers — crust, mantle and core. Rocks are bad conductors of heat, so the internal heat stays trapped, melts rock and keeps the asthenosphere below the crust in a semi-molten state.
- Two families of forces make landforms, and every landform is the result of both acting on the same ground.
- Endogenic forces work from inside the earth and, on the whole, build relief up.
- Exogenic forces work on the surface and, on the whole, wear relief down.
- Plate tectonics is the first cause of landform development. The lithosphere is broken into plates that float on the asthenosphere and move horizontally, compressing the sedimentary rocks between them and folding them.
- The forces that raise fold mountains are called orogenic, or mountain-building, movements, and they act at a tangent to the surface.
- Uplift and submergence is the second cause. These forces act along the earth’s radius, producing large-scale uplift or subsidence through faulting.
- They are called epeirogenic, or continent building, movements, and they produce block mountains, basins, rift valleys and plains.
- Orogenic and epeirogenic forces together are called diastrophic forces, and they produce relief such as mountains, plateaus, plains, lakes and big faults.
- Exogenic forces are the third cause. They are called destructive forces because they wear existing landforms away by weathering and erosion, but they then form new landforms through both erosion and deposition.
- The major landforms are three: mountains, plateaus and plains. Each is classified by its mode of formation, not by its appearance.
- A mountain is a very steep land or hill rising to great heights above the land surrounding it, and there are three main types.
- Fold mountains are the result of lateral compression, which wrinkles the rock into anticlines and synclines; they are the highest and most extensive ranges, mostly made of sedimentary rock, long but narrow, and often bordered by volcanic activity.
- Young fold mountains such as the Himalayas, Andes, Alps and Rockies are high and sharp-peaked, while old fold mountains such as the Appalachians, Urals and Aravallis have been eroded to gentler slopes and rounded tops.
- Residual or relict mountains are what is left when weathering and erosion lower a highland over a long time, so they are also called mountains of denudation; Nilgiri, Parasnath and the Rajmahal Hills are Indian examples.
- Block mountains are produced by faulting under tensional force, where a raised block forms a horst and the sunken ground between blocks forms a graben or rift valley.
- A plateau is an area of high ground, 500 feet above the ground level, with a steep slope on one side and a generally flat upper part called a tableland.
- Intermontane plateaus are surrounded by hills and mountains on all sides and are formed along with fold mountains; the Tibetan plateau is the highest and most extensive.
- Volcanic plateaus are built up by numerous eruptions over time, as in the north western Deccan, and the layers can be counted, the Columbian Plateau showing about 20.
- A plain is an extensive area of lowland with a level or gently undulating surface, and a plain is seldom formed by a single process.
- Structural plains are formed by the uplift or subsidence of land, as in the Great Plains of the USA and the Coromandel plains of India.
- Erosional plains are worn flat by the agents that strip material away.
- Depositional plains are built by material carried in and dropped, by rivers, by wind or by the sea.
- Landforms matter beyond their shape. They add beauty; they shape climate, winds and precipitation; they store snow that feeds perennial rivers; they carry forests and biodiversity; they create fishing grounds; and the disturbances that make them bring minerals and fossil fuels near the surface.
Flowchart
Fig. 5.A8 Caption: Rock is raised, worn down, carried away and raised again — the chapter’s whole argument runs as one cycle. Nodes: heat trapped inside the earth keeps the asthenosphere semi-molten → plates move on it → endogenic forces act; branch one: orogenic forces push sideways → sedimentary rock is compressed and folded → fold mountains; branch two: epeirogenic forces push along the radius → faulting → block mountains, rift valleys and structural plains; both branches → raised land is exposed to exogenic forces → weathering and erosion lower it, leaving residual mountains → the loosened material is carried away by river, wind and sea → deposition builds depositional plains, and sediment settles in shallow oceans → the sediment hardens into sedimentary rock between the plates → back to compression by orogenic forces
What This Chapter Cannot Tell You
The chapter gives almost no time. It says that old fold mountains formed long ago and that a volcanic plateau built up slowly, but no date, no rate and no length of time is attached to any of it, so nothing here lets you say how fast a range rises or how long it takes to wear down.
The classifications are also tidier than the ground. A plain is seldom formed by a single process, as the chapter itself admits, yet every chart in it puts each landform in exactly one box. The Coromandel plains, made by subsidence and then by deposition, are the warning against reading those boxes too strictly.
And the examples fix a landform in place without fixing it in time. Naming the Nilgiri Hills as residual tells you what erosion has already done there; it tells you nothing about what is happening on that slope now.
5.8 Check Your Concepts
Look at rows 3 and 4 of Table 5.1. Old fold mountains have gentler slopes, rounded tops and lower heights than young fold mountains. Explain why a single cause is enough to account for all of these differences, and name that cause.
Intermontane plateaus are not found just anywhere on the earth’s surface. State where they are found, and explain why they occur in that position and not elsewhere.
The Columbian Plateau has about 20 layers of lava.
- What does this number allow us to say about the plateau’s history?
- Name one thing about that history which counting the layers cannot tell us.
A river carries loose material from a mountain slope down to the sea.
- Name, in order, the three kinds of depositional plain it can build along the way.
- Explain what makes the river set down part of its load at each of those three places.
Briefly describe how the Great Plains of the USA were formed. They are examples of which type of plains?
Distinguish between fold mountains and block mountains.
How are various landforms formed on the surface of the earth?
Give a reason for each of the following.
- The asthenosphere, though it lies underneath the crust, is in a semi-molten state.
- Exogenic forces are also called destructive forces.
- Residual mountains are also called mountains of denudation.
On an outline map of the world, show the major fold mountains, plateaus and plains with the help of symbols.
5.9 Objective Questions
- Look again at Fig. 5.2. In the lower diagram, one part of the folded rock arches upward and another part bends downward. Taking A as the upward arch and B as the downward bend, identify A and B.
- A: Anticline B: Archline
- A: Syncline B: Anticline
- A: Anticline B: Syncline
- A: Syncline B: Archline