The earth’s detailed form at any instant in time represents the net effect of surface, or exogenic process, and internal or endogenic process. Exogenic processes, including the action of water ice and wind, predominantly involve denudation, that is the removal of material, and thus generally lead to a reduction in elevation and relief. Exogenic processes derive their energy from atmosphere determined by the ultimate energy from the sun and also the gradients created by tectonic factors. Materials carried to the sea by rivers or transported by rivers transported by glaciers or the wind, experiences some degree of chemical decomposition or physical breakdown prior to being eroded. Weathering is the morphogenesis of exogenic geomorphic processes.
The tropical (or intertropical) zone lies between the tropics of Cancer and Capricorn and includes regions that have wide differences in climatic, biological, geological (structure and age), and geomorphological properties. All of these have combined to produce a wide variety of landforms.
The tropics
Tropical region is the region located between the tropics of cancer and tropic of Capricorn. The tropic of cancer and tropic of Capricorn lie between 231/2̊ N and 231/2̊ S of the equator respectively. The region is characterized by high temperature, low pressure and high rainfall usually around lower latitude which is decreasing as it approaching 231/2. The region comprises 40% of the earth surface, with 36% covered by land masses and 4% covered by water body. It geographical extent covered South Mexico and Central America, South America (both North and South of Amazon), West Africa, Central Africa, India, South East Asia, Northern Australia, Indonesia, Papua New Guinea. Temperature difference across the region is not too wide, while wide range of the amount of rainfall, 4,000 mm in the lower latitude and to about 500 mm around 23̊ N and S, lead to classify region in humid and arid tropics.
The Humid tropic is characterized by year-round rainfall (up to 2000–4000mm), a strong solar radiation year-round, and a nearly constant temperature (25–27◦C). Much of the sun’s energy is absorbed in the tropics, so it acts as a source for heat transfer to energy-deficit regions. There are three main climatic zones of the humid tropics: the equatorial trough zone; the monsoon areas; and the trade-wind zones. In the equatorial trough zone, temperatures are uniformly high, with a greater diurnal than annual variation. Precipitation occurs throughout the year, though most areas have a drier season. As a result of high temperature and rainfall, the humid tropics are associated with geomorphological processes of erosion and mass wasting caused by the higher activity rates of chemical processes at warmer temperatures when moisture is available. In regions of calcareous rock, solution is highly significant and produces distinctive landforms,
The Arid tropic is characterized by high temperature and low rainfall. Most of the arid tropic lie in the trade wind belt where the dry winds blow offshore between 15-23°N and 15-23°S. All months have mean temperatures above 18 ◦C. The mean temperature is lowest in the winter dry season. The annual precipitation ranges from 500 to 1500mm. As a result of low rainfall, in the arid tropic the geomorphological processes of erosion which is aided by wind derive form tropical continental air masses is more predominant.
Erosion
Erosion is the process of the weathering away of the land surface by natural agents such as running water, ice, wave action, wind and the transportation of the resultant rock debris. The process does not involve weathering of rocks in situ or mass movement. Processes of erosion include abrasion, corrassion, attrition, deflation, corrosion and plucking etc. Once weathering breaks down rock surfaces, the various agents of erosion are able to carry out their work easily. Water is the main agent of erosion at work in the world. The erosion process common in the tropics are: (i) fluvial erosion (tropical humid region). (ii) Aeolian erosion (arid tropical region).
Erosion in the humid tropics
The most common form of erosion common with the humid tropics is the fluvial erosion which is highly predominant in regions/areas close to the equator which is characterized by high precipitation (regions such as: the amazon basin, Congo basin and the island of Indonesia and Malaysia. When it rains, only a small proportion of rainfall reaches the river channel directly. A large part is held and stored in leaves and branches of plants, this is called the interception zone. The amount intercepted depends on the type and density of vegetation as well as rainfall intensity. Rain from heavy rainstorms saturates soil more easily than gentle drizzle. Before rainfall reaches the surface of the earth, some is evaporated into the atmosphere. The water that reaches the surface moves into the soil and this process is called infiltration. The soil ability to absorb rainwater is called infiltration capacity before saturation. The soil has a limit to absorb rainwater. The water which infiltrates flows by gravity into the soil to the water table. It may also flow as underground water and finally reach a river channel. Once it becomes part of a river, flowing water transports and deposits material far away from their original place. The water that exceeds the soil’s infiltration capacity flows on the surface of the earth as runoff, depending on its volume, slope and nature of the soil. Runoff erodes soil and at the same time forms small streams that join together to give larger streams, which subsequently join to form a river.
The amount of material carried by a river depends on steepness and resistance of the rocks, the amount of water flowing down into it and the material delivered down the valley slopes. The flow of a river depends on the energy provided by gravity which is also determined or related to the gradient of its bed and volume increases. There is a relationship between velocity of water in the river channel and the particle sizes, which can be eroded, transported and deposited. Lower speeds allow small particles to be moved on the channel bed while high speed can carry larger particles. Consequently, the larger the particles, the greater the velocity required to transport them. Fluvial erosion in the humid tropics takes place in various ways which includes:
Attrition: It is an action or process by which the load of the river gets broken down when particles rub against each other. Accordingly, as the load is transported, the fragments get smaller.
Corrasion (Abrasion): It is a process of mechanical erosion of a rock surface by the friction of rock material with the surface. The rock material can be moved under gravity or by running water. Corrasion wears away the bed of a river’s channel, which causes the load to increase. This consist of wearing away or detachment of the bedrock by particles moved by the water flow. The particles involved can be of any size that can be transported at prevailing flow velocities, and large boulders several metres across may be in motion in fast-flowing, deep river channels. The effectiveness of abrasion depends on the concentration, hardness and kinetic energy of the impacting particles and the resistance of the bedrock surface. Since the kinetic energy is proportional to the square of velocity, rates of abrasion increases rapidly as velocity flows increases.
Corrosion: Corrosion is the chemical weathering of minerals in contact with stream water and the removal of soluble products downstream. The key main factors of corrosion are bedrock mineralogy, the solute concentration of the stream water, the stream discharge and velocity of the flow. Maximum rates of corrosion are achieved where fast-flowing under saturated stream water pass over lithologies with high proportion of reactive minerals: for instance, corrosion is an important process, in the bedrock channels in mountainous limestone terrains in humid environment. It involves transportation of material which has dissolved in water. Only a few minerals directly dissolve in water. However, some, particularly calcium carbonate dissolve freely in water containing Carbon dioxide. Rainwater dissolves both carbon dioxide and oxygen in the air such that it reaches the ground as a weak carbonic acid. This is able to turn many insoluble minerals into soluble minerals that can be carried away in solution. In humid tropics, so often the majority of minerals dissolve in water with the exception of iron and aluminium. These accumulate in the top layers of the soil through leaching thus developing lateric soils.
Hydraulic Erosion: Refers to the force of moving water that is able to remove loose material such as gravel, sand and silt. The action is able to weaken solid rock when it enters into cracks of a rock. Nonetheless, hydraulic action leads to little erosion if the river has little or no load.
Transportation of eroded materials
The load of a river is transported by traction, saltation, and suspension and by solution. Traction refers to the dragging of large material such as pebbles along its bed. Saltation is the bouncing of smaller pieces down its bed. Under suspension, light materials, such as silt and mud are carried in water as the river flows. Finally those minerals that dissolve in water are transported by solution. A river transports its load until when it has insufficient energy to transport it any further. When this condition happens, the river deposits its load.
Deposition of materials
The laying down of material, particularly of debris transported mechanically by running water, waves and wind is called deposition. It takes place each time the velocity of a river decreases. Deposition can occur on the insides of meanders where the gradient decreases, and where there is high evaporation. It also decreases when a river enters the sea and thus builds a delta. Deposition is a continuous process, in that the material is constantly being deposited and then picked up again and transported to another part of the channel where it is once again deposited. Deposition is by selection. Heavier materials such as boulders and pebbles are deposited first whilst the fine sediments such as silt are deposited last.
Erosion in the arid tropics
Aeolian activities are found in regions of the Earth where erosion and deposition by wind are the dominant geomorphic forces shaping the face of the landscape. Regions influenced by wind include most of the dry climates of the Earth. According to the Köppen Climate Classification System, this would include regions of the world that are classified as arid deserts (BW) and semiarid steppe (BS). Wind can also cause erosion and deposition in environments where sediments have been recently deposited or disturbed.
Wind action is important in arid and semi-arid regions due to absence or little vegetation cover. Most of the arid regions lie in the trade wind belt where the dry winds blow offshore between 15-30°N and 15-30°S. Major deserts include the Sahara, Arabian, Iranian, Thar, the Kalahari, the Namib, Great Australian desert and the Atacama deserts. Transport and deposition are the most significant processes of wind erosion. Wind blows away fine particles over long distances while the coarser particles bounce over the surface. The bouncing movement is called saltation. Similar to other agents, Wind erosion engages two processes: (i) deflation and (ii) abrasion.
Deflation is the removal of loose particles by the wind. Smaller sedimentary particles are more susceptible to wind erosion than larger particles. Particles of about 100 micrometres diameter are the most vulnerable to wind erosion. Above that size, increasingly higher velocities are needed to entrain increasingly large grains and to keep them airborne. Below that diameter, and especially for clay particles, greater wind velocities are needed to surmount the cohesional forces binding individual grains together. Deflation of sand-sized particles is localized, and it takes a long time to move sand great distances. Silt and clay, on the other hand, are far more readily lifted by turbulence and carried in suspension in the atmosphere, the finest material being transported great distances. The world’s hot deserts are a leading source of atmospheric dust. It has being estimated that between 130 metric tonne and 800 metric tonne of materials are deflated from the continents annually, with Sahara alone contributing between 60 and 200 metric tonne. Deflation is temporally and spatially concentrated process. Even in the arid tropics it is largely restricted to areas where surface conditions and wind speeds are particularly favorable. Soil erosion by wind is well documented and well known Wind without grains is an impotent geomorphic agent; wind armed with grains may be a powerful erosive agent.
Abrasion is the cannonading (colliding) of rock and other surfaces by particles carried in the wind – a sort of natural sandblasting’. Rocks and boulders exposed at the ground surface may be abraded by sand and silt particles. Abrasion rates appear to be highest where strong winds carry hard sand grains from soft and friable rocks upwind. Sand particles are carried within a metre or two of the ground surface, and abrasion is not important above that height.
MASS WASTING
Mass wasting refers to the transfer, sliding, falling, creeping or flowing of rock materials produced by the agents of denudation (weathering and erosion down slope) under the influence of gravity. It is a link between weathering and transport by agents of erosion. The force of gravity acts constantly on all rocks and debris. Mass wasting is the downward movement of soil and rock under the influence of gravity. It is most frequent on slopes above 25 degrees with little vegetation and annual rainfall over 900mm and often occurs after heavy storms when soil becomes waterlogged and heavy.
Processes of mass wasting in the tropics
Mass wasting is common with the humid tropics because of the availability of moisture as a result of frequent downpour. While gravity forces loose materials down slope, there is resistance to movement from friction and cohesion that have to be overcome before any movement can take place. Usually mass movement takes place slowly, but sometimes it takes place suddenly. The rate of movement depends on the steepness of the slope. The rate of movement of loose material moves down faster over steeper slopes. Other factors influencing movement include the nature and weight of materials, and the amount of water in the material. More dense materials tend to move quickly on a steep slope whereas light materials move slowly, sudden movements give rise to landslides.
Creep, flows, and slides, are the most common form of mass wasting in the tropics due to the deep weathered profile of the humid tropics and some cases like in the volcanic island of Sumatra where the materials are very susceptible to slide.
Land Slide A landslide is the sliding down under force of gravity of a mass of land on a mountain or hillside. This takes place when large quantities of loosened surface rocks and soil suddenly slide down a cliff face or valley side. A landslide may either take the form of sliding or slumping. The latter is common on slopes made of clay A number of actions may trigger the occurrence of a landslide. The undercutting of the base of a steep slope by a river or by the sea and the steeping of a slope by human activities such as quarrying or clearing up vegetation from a steep slope. An earthquake or prolonged heavy rains in mountainous areas such as in the Sumatra cause landslides. Buildings and roads can be buried. When landslides occur in a populated area, loss of life and property may take place.
Soil Creep This is the slow downward movement of soil under force of gravity common on all sloping land. Rainwater enables soil particles slide over each other. Boulders and stones in the soil, or resting on it, are carried down the slope by the soil. Other factors, which influence soil creep, include heating and cooling of soil, alternate wetting and drying of the soil, tramping of grazing animals and burrowing of animals in the soil. Soil creep can be recognized by fences and trees that lean down the slope.
Mudflow This is a moving mass of soil made fluid by continued heavy rains on a slope. Mudflows can take place on desert slopes that are unprotected by vegetation cover. Other places where mudflows take place are on the slopes of an erupting volcano when heavy rains fall on the volcanic ash.
Debris flow is a fast-moving body of sediment particles with water or air or both that often has the consistency of wet cement. Debris flows occur as a series of surges lasting from a few seconds to several hours that move at 1 to 20 m/s. They may flow several kilometers beyond their source areas. Some are powerful enough to destroy buildings and snap off trees that lie in their path. Mudflows triggered by water saturating the debris on the sides of volcanoes are called lahars e.g the debris flow on the volcanic mountain Pinatubo of the Philippines.
Conclusion
The exogenic processes (weathering, erosion, and mass wasting) create a new landform and their resultant effect can be of devastating or benefit to man, and also these processes are also part of the cycling of lithospheric materials. The impact of erosion and mass wasting is being felt globally, with the first causing land degradation and the latter causing loss of properties and lives in some cases.
Sources
Allen, P. A. (1997) Earth Surface Processes. Oxford: Blackwell Science.
Selby, M. J. (1993) Hillslope Materials and Processes, 2nd ed. With a contribution by A. P. W. Hodder. Oxford: Oxford University Press.
Thornes, J. B. (ed.) (1990) Vegetation and Erosion: Processes and Environments. Chichester: John Wiley & Sons.
Bunnett, R. B (1999), Physical Geography in Diagrams for Africa, Longman Group; Hongkong.
Bowen, Ann and John Pallister (2001), A2 Geography. Heinemann Educational Publishers; Oxford.
Micheal, A.S (1991). Global Geomorphology, New York: Wiley & Sons
http://www.physicalgeography.net/physgeoglos/d.html#deposition
http://www.physicalgeography.net/physgeoglos/e.html#erosion