Fundamental Concepts of Geomorphology: Meaning, Processes and Landform Evolution

Geomorphology Meaning, Definition, Nature and Scope

What Is Geomorphology? Meaning, Definition, Nature and Scope

The word geomorphology comes from three Greek words: ge, earth, morphe, form, and logos, study. Geomorphology is the scientific study of the origin and evolution of topographic and bathymetric features, the forms of the land and of the sea floor, produced by physical, chemical or biological processes at or near the earth's surface.

Geomorphology deals with the reconstruction of the history of the surface of the earth through a study of its forms, the materials of which it is made up and the processes that shape it. Its basic unit is the landform: a small to medium tract or parcel of the earth's surface. Several related landforms together make up a landscape, a large tract of the surface.

Geomorphology sits where the land meets the air, water and life. The earth's surface is modified by surface processes, the work of water, wind, ice, wildfire and life together with the chemical reactions that form soils, and by geologic processes that cause tectonic uplift and subsidence. The surface is thus an intersection of the lithosphere with the hydrosphere, atmosphere and biosphere, and geomorphologists work alongside physical geography, geology, geodesy, engineering geology, archaeology and climatology.

• Fluvial geomorphology: The work of rivers and running water; see riverine landforms.
• Glacial geomorphology: Glacial deposits such as moraines and eskers, and glacial erosional features.
• Aeolian geomorphology: The work of wind, most important in arid environments such as deserts.
• Hillslope and mass movement studies: How slopes form and change, and how debris moves down them.
• Tectonic geomorphology: How uplift and faulting shape the land; see plate tectonics.

Since the middle of the twentieth century the subject has become quantitative, measuring rivers and hillslopes directly. Terrain is now measured with differential GPS, remotely sensed digital terrain models and laser scanning.

Fundamental Concepts of Geomorphology

Uniformitarianism: The Present Is the Key to the Past

The starting point among the fundamental concepts of geomorphology is uniformitarianism: the processes working on the earth today also worked in the past, so the present is the key to the past. The idea began with the geologist James Hutton in the late eighteenth century, in contrast to catastrophism, was refined by John Playfair and popularised by Charles Lyell's Principles of Geology in 1830. William Whewell coined the name.

Uniformitarianism lets a geomorphologist read old landforms by watching present processes, but modern geologists no longer hold to a strict gradualism: processes need not have acted at the same rates in the past. Most geomorphic processes are slow and imperceptible and can be seen and measured only through their results, the landforms.

Endogenic and Exogenic Forces: Building Up and Wearing Down

The earth's surface is uneven because two sets of forces work on it at once. Endogenic forces come from within the earth, from energy generated mostly by radioactivity, rotational and tidal friction and primordial heat; they are mainly land building. Exogenic forces derive their energy from the atmosphere, ultimately from the sun, and from gradients created by tectonics; they are mainly land wearing.

• Diastrophism: All processes that move, elevate or build up the crust: orogeny (mountain building by severe folding), epeirogeny (uplift or warping of large parts of the crust), earthquakes and plate tectonics.
• Volcanism: The movement of molten rock onto or towards the surface, and the intrusive and extrusive forms it builds.
• Gradation: Exogenic forces wear down relief (degradation) and fill up basins (aggradation).

Because endogenic forces keep raising parts of the surface, exogenic processes never manage to even it out, and relief survives as long as the two opposing forces continue. How the crust itself floats and adjusts is explained in the theory of isostasy.

Geomorphic Processes and Geomorphic Agents

The endogenic and exogenic forces that cause physical stresses and chemical actions on earth materials and change the shape of the surface are called geomorphic processes. Diastrophism and volcanism are the endogenic processes; weathering, mass wasting, erosion and deposition are the exogenic ones. A geomorphic agent is a mobile medium, such as running water, groundwater, glaciers, wind, waves and currents, that removes, transports and deposits earth materials.

• Process and agent: A process is a force applied on earth materials; an agent is the mobile medium that carries the material.
• Gravity: It activates all downslope movement; without gravity and gradients there would be no erosion, transportation or deposition.
• Denudation: The general term for all exogenic processes, meaning to strip off or uncover; it includes weathering, mass wasting, erosion and transportation.

Factors Controlling Landform Development: Structure, Climate and Time

The same process does not make the same landform everywhere. Climatic factors being equal, the intensity of exogenic processes depends on the type and structure of rocks: folds, faults, the dip of beds, joints and bedding planes, the hardness of minerals and permeability. A rock may resist one process and yield to another, so processes act at different rates and create differences in topography.

• Climate: Temperature and precipitation control the exogenic processes, which therefore vary from one climatic region to another, and even with altitude and slope aspect.
• Independent controls: The stability of sea level, the tectonic stability of landmasses and climate influence landform evolution; a disturbance in any of them can upset the sequence of stages.
• Time: Each landform has a beginning and a history of development; it may change slowly or fast as processes continue.
• Equifinality: Similar landforms may arise from quite different sets of processes, so a landform alone does not always reveal its origin.

Exogenic Geomorphic Processes: Weathering, Mass Movement, Erosion and Deposition

Weathering: Chemical, Physical and Biological

Weathering is the mechanical disintegration and chemical decomposition of rocks by the elements of weather and climate. Very little material moves, so it is an in-situ, on-site process. Climate is of particular importance: both the processes and the depth of the weathering mantle change from one climate to another.

Chemical weathering needs water and air with heat. Carbon dioxide from the air and soil is absorbed by water to form carbonic acid, which dissolves limestone; in oxidation, minerals such as iron combine with oxygen, and red iron turns brown or yellow. Weathering prepares the way for soils, erosion and mass movement, and it enriches ores of iron, manganese, aluminium and copper; see leaching of soil and soil formation.

Mass Movement and Landslides: Types and Causes

Mass movements transfer rock debris down slopes under the direct influence of gravity. No agent such as running water or ice carries the debris; instead the debris may carry air, water or ice with it. So mass movements are not counted as erosion, though weathering aids them, and they are most active on weathered slopes.

The speeds differ enormously. Creep is so slow that it shows only in fence posts and telephone poles leaning downslope; solifluction is the slow flow of soil saturated or lubricated with water. Rapid movements are commonest in humid regions: mudflows occur frequently on erupting or recently erupted volcanoes, and a debris avalanche can be much faster than a mudflow.

• Activating causes: Removal of support from below, steeper and higher slopes, overloading by material or heavy rain, earthquakes and explosions, excessive seepage, rapid drawdown of reservoirs and removal of natural vegetation.
• Falls: Abrupt movements of rocks and boulders that become detached from steep slopes or cliffs, separating along fractures, joints and bedding planes.

Landslides in the Himalayas and Western Ghats: Why They Differ

Debris avalanches and landslides occur very frequently in the Himalayas. The range is tectonically active, made mostly of sedimentary rocks and unconsolidated and semi-consolidated deposits, and its slopes are very steep. The Western Ghats and the Nilgiris are tectonically stable and made mostly of very hard rocks, yet they also have landslides, though less often.

The Ghats have their own reasons: many slopes are steep, with almost vertical cliffs and escarpments; mechanical weathering by temperature change is pronounced; and they receive heavy rain over short periods, so rock falls come often along with landslides and debris avalanches. About 0.42 million sq km, or 12.6 per cent of India's land outside snow-covered areas, is prone to landslides, most of it in the Himalaya.

Erosion, Transportation and Deposition

Erosion is the acquisition and transportation of rock debris by the agents: running water, groundwater, glaciers, wind and waves. It wears the landscape down, and it is erosion that is largely responsible for the continuous changes on the earth's surface. Weathering aids erosion but is not a precondition for it. The processes are covered in detail in processes of erosion.

Erosion can be defined as the application of the kinetic energy of an agent to the land surface along which it moves, where kinetic energy is half the mass times the square of the velocity. So a glacier, though slow, is a powerful eroder because of its great mass, while wind, being a gas, is the least effective.

• Climate-controlled agents: Wind, running water and glaciers, the three states of matter.
• Agents not controlled by climate: Waves, set by the coast, and groundwater, set by rock type; karst develops only where rocks are permeable and soluble.
• Deposition: A consequence of erosion; agents lose velocity on gentler slopes, dropping coarser material first and finer later, and filling depressions.

Landform Evolution: Davis's Cycle of Erosion and Its Rivals

Davis's Cycle of Erosion: Youth, Maturity and Old Age

The best-known model of landform evolution is the geographical cycle, or cycle of erosion, developed by William Morris Davis between 1884 and 1899. It pictures a single uplift followed by a long wearing down, in which a landmass passes through stages comparable to the stages of life: youth, maturity and old age.

1. Youth: Few, poorly integrated streams in shallow V-shaped valleys with little or no floodplain; waterfalls and rapids where hard rock is exposed.
2. Maturity: Many well-integrated streams in deep V-shaped valleys with wider floodplains; stream divides turn sharp and waterfalls disappear.
3. Old age: Few tributaries with gentle gradients; streams meander over vast floodplains with natural levees and oxbow lakes, and most of the land is at or just above sea level.

The end of the cycle is a peneplain, an almost plain, a lowland of faint relief with a few resistant remnants called monadnocks. Erosion works down towards the base level, a term introduced by John Wesley Powell in 1875: the ultimate base level is sea level projected under the land, while a lake or basin far from the sea can act as a local base level.

Penck, King and Gilbert: Alternative Models of Landform Evolution

Davis's model did not go unchallenged. In the 1920s Walther Penck argued that uplift and denudation go on together, at gradual and continuous rates, rather than one uplift followed by decay. His Morphological Analysis of Landforms was published after his death, in 1924. In his model the steepness of valley slopes depends on the rate of uplift, not on the stage reached, and slopes retreat parallel to themselves.

• Lester Charles King: In South African Scenery (1942) he described the pediplain, an extensive plain formed by the coalescence of pediments, and held that peneplains do not exist.
• Grove Karl Gilbert: His monograph on the Henry Mountains (1877) and later work began the measured study of processes that grew into modern quantitative geomorphology.

Why Geomorphology Matters: Applications in India

The land surface is sensitive, and humans depend on it for their sustenance. Most of it was shaped over very long periods, and misuse diminishes its potential fast, so understanding the processes that shape it is the first step to using it without disturbing its balance.

• Landslide hazard mapping: Under the National Landslide Susceptibility Mapping programme, begun in 2014-15, the Geological Survey of India has mapped 4.3 lakh sq km of landslide-prone areas, including about 42,093 sq km in Himachal Pradesh.
• Satellite inventories: The Landslide Atlas of India (2023) maps about 80,000 landslides from 1998 to 2022 and ranks 147 districts in 17 states and 2 Union Territories by their exposure.
• Soils and resources: Weathering forms regolith and soils and enriches ores of iron, manganese, aluminium and copper.
• Biodiversity: Forests, and so biomes and biodiversity, depend on the depth of the weathering mantle.

Sources

• NCERT: Fundamentals of Physical Geography (Class XI), Geomorphic Processes
• NCERT: Fundamentals of Physical Geography (Class XI), Landforms and their Evolution
• ISRO / NRSC: Landslide Atlas of India, 2023
• PIB (Ministry of Environment, Forest and Climate Change): National Landslide Susceptibility Mapping, 18 December 2023
• USGS: Landslide Types and Processes (Fact Sheet 2004-3072)
• Wikipedia: Geomorphology
• Wikipedia: Uniformitarianism
• Wikipedia: William Morris Davis
• Wikipedia: Walther Penck
• Wikipedia: Base level
• Wikipedia: Peneplain
• Wikipedia: Pediplain
• Wikipedia: Grove Karl Gilbert
• Wikipedia: Equifinality
• Wikipedia: Denudation
• Wikipedia: Weathering
• Wikipedia: Mass wasting
• Wikipedia: Landslide
• Wikipedia: Erosion
• Wikipedia: Landform
• UPSC: Civil Services Examination question papers