Geomorphology: Fundamentals

Geomorphology: Scope and Significance

Geomorphology is the scientific study of landforms and the processes responsible for their creation, development, and eventual destruction. A landform is understood as an outcome of three interacting variables: structure, process, and time. Structure refers to the mineral composition, type, and arrangement of rocks, including whether they are jointed or compact. Process refers to the dominant geomorphic agent operating on the landform, such as glaciers, rivers, or wind. Time determines the stage of development, broadly classified as youthful, mature, or old age. Because endogenetic forces that raise the land surface and exogenetic forces that wear it down operate simultaneously, the height and character of any landform at a given moment reflects the net result of both sets of forces working together over geological time.

Endogenetic and Exogenetic Forces

Endogenetic forces originate from within the earth and are termed constructive forces because they increase the relief of the land. They are further divided into diastrophic forces, which operate very slowly and include the processes of folding and faulting, and catastrophic forces, which are sudden and include earthquakes and volcanic eruptions. Within diastrophic forces, epeirogenetic forces are responsible for continent building through broad vertical movements of emergence and submergence, while orogenetic forces produce mountains through folding and faulting.

Exogenetic forces originate at or near the earth's surface and are termed destructive forces because they continuously lower the height of landforms. These forces of denudation include weathering, mass movement such as landslides, and erosion by rivers, glaciers, wind, and waves. Together, weathering and erosion gradually reduce elevated landforms toward base level, producing a variety of characteristic landforms along the way. Applied geomorphology draws on this understanding for practical purposes including geohydrology, economic geology, and environmental management.

Origin and Evolution of the Earth's Crust

The earth formed approximately 4.5 billion years ago. In its earliest state it was dominated by hydrogen, helium, carbon dioxide, and nitrogen, with no free oxygen present. The geological time scale divides earth history into eons, which are subdivided into eras, which are further divided into periods and epochs. The Pre-Cambrian era saw the formation of oceans, the origin of the oldest fold mountains including the Aravallis, and the appearance of the earliest life in the form of anaerobic bacteria. Blue-green algae appeared around 2,500 million years ago, initiating photosynthesis and beginning the slow oxygenation of the atmosphere.

The Paleozoic era included the Cambrian period characterised by marine invertebrates, the Devonian period known as the age of fishes, and the Carboniferous period during which vast quantities of vegetation were buried and compressed to form most of the world's coal. Indian coal is younger, belonging to the Gondwana period. Coal is classified by age and quality from older anthracite through bituminous and lignite to the youngest peat. About 98 percent of India's coal is of Gondwana bituminous variety, found in the valleys of rivers such as the Damodar, Mahanadi, Godavari, and Subernarekha on the Chhotanagpur plateau.

The Mesozoic era is divided into the Triassic, Jurassic, and Cretaceous periods. The Cretaceous period was marked by large-scale volcanic activity that led to the extinction of dinosaurs through a combination of global cooling and, according to some accounts, meteor impact. The massive outpourings of basaltic lava during this period formed volcanic plateaus such as the Deccan Lava Plateau of India, the Siberian Plateau of Russia, and the Columbia Snake Plateau of the United States. The Cenozoic era includes the Tertiary period during which young fold mountains such as the Himalayas were formed, and the Quaternary period which includes the Pleistocene ice ages and the Holocene epoch leading to modern human evolution. The Anthropocene is defined as the last approximately two thousand years during which modern humans have substantially modified the atmosphere and climate through deforestation, agriculture, land use change, and the burning of fossil fuels.

Interior of the Earth

The earth's interior is divided into three concentric layers: the crust, the mantle, and the core. Direct study of the interior is limited by extreme pressure and temperature, with the deepest mines reaching only about 3.2 kilometres. Indirect evidence comes from the study of volcanoes, which originate from the asthenosphere at depths of around 200 kilometres, and from meteors, whose analysis suggests that the earth's interior is composed of increasingly denser materials. The study of seismic waves is the most reliable source of information about the earth's layered structure.

The crust is the outermost layer and is divided into the continental crust and the oceanic crust. The continental crust is composed of lighter materials dominated by silica and aluminium, giving it the chemical shorthand SIAL, and has an average thickness of around 35 kilometres, with considerably greater thickness beneath mountain ranges. The oceanic crust is denser, composed of silica and magnesium and iron, giving it the shorthand SIMA and SIFE, and has an average thickness of only about 5 kilometres, forming the floor of the ocean basins. The boundary between the crust and the mantle is called the Moho discontinuity.

The mantle extends below the crust and is divided into the upper and lower mantle. The uppermost layer combining the crust and the top of the upper mantle is known as the lithosphere, which exists in solid form. Below the lithosphere and extending to a depth of about 200 kilometres is the asthenosphere, which is in a semi-plastic or jelly-like state and serves as the source of magma according to the plate tectonic theory. Lithospheric plates slide on this plastic asthenosphere. The lower mantle is in solid form due to the dominance of pressure over temperature at greater depths, as evidenced by the high velocity of both P-waves and S-waves passing through it.

The core is the innermost layer, composed of the densest materials, primarily nickel and iron. It is divided into the outer core and the inner core. The outer core is in liquid form because temperature increases outpace the pressure increase at those depths. The liquid outer core, combined with the rotation of the earth, generates charged particles that give the earth its geomagnetic properties. The inner core is in solid form due to the extreme pressures at the centre of the earth, a conclusion supported by the fact that P-waves achieve their highest velocities here. The boundary between the mantle and the outer core is called the Gutenberg discontinuity.

Seismic Waves and the Study of the Earth

Seismic waves are waves of energy released during earthquakes. They are divided into body waves, which travel through the interior of the earth, and surface waves, which travel along the earth's surface and cause the greatest damage. Body waves are of two types. Primary waves or P-waves are similar to sound waves and can travel through solids, liquids, and gases. They cause rocks to move parallel to the direction of wave propagation through compression and rarefaction, and they are the fastest body waves. Secondary waves or S-waves travel at approximately half the speed of P-waves and cause particles to move perpendicular to the direction of wave propagation. S-waves can pass only through solid material.

The behaviour of seismic waves as they pass through the earth's interior provides the clearest evidence for the layered structure of the earth. Waves change direction and velocity when they move from one medium to another of different density, a phenomenon known as refraction. The sudden changes in wave behaviour at the crust-mantle boundary and the mantle-core boundary define the Moho and Gutenberg discontinuities respectively. The fact that S-waves do not pass through the outer core conclusively proves that the outer core is in a liquid state. The shadow zone of S-waves, the region on the opposite side of the earth from an earthquake where S-waves cannot be detected, is larger than the shadow zone of P-waves for the same reason.

Continental Drift Theory

Alfred Wegener proposed the Continental Drift Theory in 1912. He argued that before the Carboniferous period all the continents were joined into a single supercontinent called Pangea, surrounded by a single super ocean called Panhalassa. During the Carboniferous period Pangea broke into two parts: the northern mass known as Laurasia or Angaraland, and the southern mass known as Gondwanaland. As drifting continued, Laurasia broke into North America and Eurasia while Gondwanaland broke into South America, Africa, Madagascar, Peninsular India, Australia, and Antarctica. The westward drifting of the Americas opened the Atlantic Ocean. The northward drifting of India and Africa reduced the Tethys Sea, whose remnant is the present Mediterranean Sea, and opened the Indian Ocean. According to Wegener, the folding of continental margins during drift created fold mountains such as the Rockies and the Andes.

Wegener proposed that two forces drove the drift: tidal forces from the gravitational pull of the sun and moon driving westward movement, and gravitational buoyancy forces of the earth driving northward movement. The theory is supported by several lines of evidence. The jigsaw fit of continental coastlines, particularly the matching profiles of Africa and South America, was the most visible evidence. Rocks of the same age and type are found on either side of the Atlantic, and the Appalachians of North America share characteristics with mountains of Ireland and Scandinavia, suggesting they were once joined. Gold-bearing rocks occur in both western Africa and eastern South America. Similar plant fossils of the Glossopteris flora are found in Brazil, Africa, Peninsular India, and Australia. Evidence of glaciation in all these regions in the distant past cannot be explained by their present positions. The behaviour of lemmings in Scandinavia, which run westward and fall into the sea, was cited as evidence of a westward continental memory.

Despite its insights, the theory faced several criticisms. Wegener could not adequately explain the forces responsible for the drift, since tidal forces are too weak to move entire continents. He also failed to explain the trigger mechanism for the initial breaking of Pangea and could not account for the detailed sequence of separation. Arthur Holmes subsequently proposed the Convection Current Theory, arguing that heat-driven convection currents in the mantle could both cause the break-up and drive the movement of continents, providing a mechanism that Wegener's theory lacked.

Sea Floor Spreading

Harry Hess proposed the Sea Floor Spreading Theory, which became a crucial link between Wegener's Continental Drift Theory and the later Plate Tectonic Theory. Hess argued that the ocean floor behaves like a conveyor belt, being continuously created along mid-oceanic ridges through the upwelling of magma, and continuously destroyed at ocean trenches. Rising convective currents in the mantle break apart the oceanic crust at divergent boundaries, and as magma cools and solidifies it creates new ridge material. The oldest oceanic crust is found nearest the trenches, while the youngest is found at the ridges.

Evidence for sea floor spreading includes the existence of mid-oceanic ridges as the longest continuous mountain chain on earth at approximately 78,000 kilometres. Rocks at equal distances from mid-oceanic ridges have the same age, with younger rocks nearest the ridge and older rocks on either side. Greater ocean temperatures are recorded at the tops of mid-oceanic ridges due to ongoing volcanic activity. The continental crust is approximately 4.2 billion years old while the oceanic crust is only a few million years old, which would be inexplicable if the ocean floor had not been continuously renewed. The study of palaeomagnetism, which examines the frozen magnetic properties of older rocks, shows symmetrical magnetic stripes on either side of mid-oceanic ridges, recording repeated reversals of the earth's magnetic field as new ocean floor was formed.

Plate Tectonic Theory

The Plate Tectonic Theory synthesises the insights of continental drift and sea floor spreading into a comprehensive framework. The earth's lithosphere is divided into several large and small solid plates that ride on the semi-plastic asthenosphere. These plates are driven by convection currents in the mantle. The major plates are the North American, South American, Pacific, African, Eurasian, Indo-Australian, and Antarctic plates. The Pacific plate is the only major plate that is entirely oceanic. The Eurasian plate is largely continental. Important minor plates include the Nasca, Cocos, Juan de Fuca, Arabian, Philippine, and Burma plates. The interaction of the Burma and Indian plates was responsible for the 2004 Indian Ocean Tsunami.

Plate boundaries are of three types. Convergent or destructive boundaries occur where two plates move toward each other and are associated with fold mountains, ocean trenches, earthquakes, and volcanic eruptions. Divergent or constructive boundaries occur where plates move apart and are associated with mid-oceanic ridges, fissure volcanoes, and rift valleys. Transform or conservative boundaries occur where plates slide past each other and are associated with powerful earthquakes and faults such as the San Andreas fault system and the Great East African Rift Valley.

Continent-ocean convergence is the most common type of plate collision, occurring where a denser oceanic plate subducts beneath a lighter continental plate. The subducting plate descends into the mantle, melts, and the resulting pressure forces magma upward through volcanic eruptions. The sediments caught between the plates are folded upward to form fold mountains, often in the form of long parallel chains known as Cordillera systems. The subduction zone, known as the Benioff zone, forms the deepest trenches on earth. Examples include the Rockies formed by the collision of the Pacific and Juan de Fuca plates with the North American plate, and the Andes formed by the Nasca and South American plates. The Alps were formed by the convergence of the Eurasian and African plates.

Continent-continent convergence occurs where both plates are of similar density and subduction does not take place. Instead, the plate of slightly greater density or velocity slides beneath the other in a process called obduction, causing a doubling of the earth's crust, which explains the absence of volcanoes in these regions. The Himalayas are the classic example, formed by the collision of the Indian and Eurasian plates. The Indian plate separated from Gondwanaland in the Mesozoic era and drifted northward, with its western portion passing over the Reunion Hotspot and generating the Deccan Lava Plateau. The floors of the Tethys geosyncline were subducted to form the Trans-Himalayan ranges north of the main Himalayas. The collision took place in three phases, producing the three parallel ranges of the Himalayas. The Shivaliks, the youngest range, formed around 2 to 20 million years ago. The Himalayas continue to rise at a rate of 5 to 10 centimetres per year as the Indian plate continues to move northward beneath the Eurasian plate.

Ocean-ocean convergence occurs where both plates are oceanic. The denser or faster-moving plate subducts and the sediments of both plates are folded into submarine mountains. As lava builds up, these mountains project above the ocean surface forming island arcs or archipelagos. The Caribbean Island Arc was formed by the subduction of the North American plate beneath the Caribbean plate, with the Puerto Rico Trench marking the subduction zone. The Japanese Island Arc formed from the interaction of the Eurasian, Pacific, and Philippine plates. The Philippine archipelago was created by the subduction of the Pacific plate beneath the Philippine plate, forming the Mariana Trench, which is the deepest point on earth. The Indonesian Island Arc sits at the trijunction of the Indo-Australian, Pacific, and Asian plates, making it tectonically among the most unstable regions on earth. The Java or Sunda Trench, where the Pacific plate subducts beneath the Indian plate, is the deepest point in the Indian Ocean.

Folding and Faulting

Folding occurs when compressive forces from within the earth cause rock layers to buckle and warp rather than break. It produces anticlines, which are upward arches or crests, and synclines, which are downward troughs. A structure where a large anticline carries numerous smaller anticlines and synclines within it is called an anticlinorium, and the corresponding syncline-dominated structure is a synclinorium. Fold types range from symmetrical folds where both limbs are inclined equally, to asymmetrical folds where the limbs are inclined at different angles, to monoclinal folds where one limb is steep and the other gentle. Isoclinal folds have both limbs running nearly parallel to each other due to intense compression. Recumbent folds develop in complex mountain building zones such as the Himalayas, where both limbs become parallel to the ground surface. When the central point of a recumbent fold breaks under further compression, it becomes an overthrust fold, and when the broken limbs are displaced several kilometres apart, the displaced portion is called a nappe.

Faulting is caused by tensional or compressive forces that fracture rocks along lines or zones of weakness called joints. Normal faults are caused by tensional forces pulling rocks apart, resulting in elongation of the crust and producing horst and graben structures. A horst is an uplifted block that forms a block mountain, while a graben is a downthrown block that forms a rift valley. Reverse faults are caused by compressive forces and result in crustal shortening. The Vindhyas and Satpuras of India are examples of block mountains formed by reverse faulting. Step faults occur when displacement takes place in the same vertical direction in successive steps, as seen on the eastern side of the Western Ghats, which are also known as Deccan Traps because the word traps in Swedish means staircase. Lateral faults produce horizontal movement either to the right, called dextral movement, or to the left, called sinistral movement, and are associated with long narrow rift valleys.

The Great Rift Valley is the world's longest rift valley, extending from Syria through the Dead Sea, the Red Sea, Lake Tanganyika, and Lake Nyasa. Lake Baikal in Russia, the world's deepest lake and the largest freshwater lake by volume, occupies a rift valley. The Rhine Rift Valley in Europe lies between the Vosges Mountains of France and the Black Forest of Germany. In India, the Narmada and Tapi rivers flow through rift valleys between the block mountains of the Vindhyas and Satpuras, originating at Amarkantak Plateau and Multai respectively. The rivers Son and Damodar flow through rift valleys in their upper courses.

Geomorphic Cycles and Landscape Development

Landscape development is understood as a progressive sequence of change over geological time. The concept of a geomorphic cycle describes how an uplifted landmass is progressively reduced by the agents of erosion. In the youthful stage, rivers cut deeply into the landscape producing V-shaped valleys and steep gradients. In the mature stage, lateral erosion widens valleys, slopes become more gentle, and divides between river basins are reduced. In the old age stage, the landscape is worn down to a nearly flat surface close to sea level known as a peneplain. Rejuvenation, caused by tectonic uplift or a fall in base level, restarts the cycle and introduces features such as incised meanders and river terraces.

Isostasy

Isostasy refers to the condition of gravitational equilibrium between the earth's crust and mantle. The concept holds that the lithosphere floats on the denser asthenosphere in a state of balance, and that any change in loading or unloading of the crust triggers a compensatory vertical adjustment. Where the crust is thickened by mountain building, its roots extend deeper into the mantle, much as a floating iceberg has most of its mass below the water surface. When erosion removes material from a mountain range, the reduced load causes the crust to rise isostatically. Conversely, when sediment is deposited in a basin, the added weight causes the crust to subside. The melting of ice sheets at the end of an ice age removes a superincumbent load from the crust, triggering isostatic rebound that is still measurable in formerly glaciated regions such as Scandinavia and Canada. Isostasy is also relevant to understanding how geomagnetic equilibrium is maintained and why the Himalayas experience a doubling of the earth's crust that precludes volcanic activity.