Exogenetic Movements Class 9 Geography Notes Maharashtra State Board
Exogenetic Processes Part – I
Many landforms are formed due to internal movements. Many processes occurring on the earth’s surface also lead to the formation or degradation of landforms continuously. In this lesson, we will be studying the exogenetic (external) processes and the landforms formed by them. External processes occur because of the forces working on the earth’s surface. They are mainly solar energy, gravitational force, and kinetic energy associated with the moving objects on the earth’s surface.
Landforms formed on the earth’s surface as a result of internal movements are called primary and secondary landforms. For example continents, mountains, plateaus, plains, etc. Because of the external processes like weathering, erosion, transportation and deposition, the primary and secondary landforms give way to the tertiary landforms. For example, valleys, sand dunes, delta, U-shaped valleys, etc.
The breaking or weakening of rocks is a natural phenomenon. It is called weathering. Weathering can be of three types: mechanical (physical), chemical and biological. In arid climates, mechanical weathering is dominant while in humid climates, chemical weathering is more effective. Biological weathering occurs because of living organisms.
Take an onion. Cut it in the middle. Observe the cut parts. Try to remove each skin layer of these parts.
You will notice that just as we can remove every outer layer of the onion, similarly, in nature rocks undergo such a process. The exposed part of the rock heats more while the inner part is comparatively cooler. As a result, the outer layers of the rocks fall apart from the main rock. This is called exfoliation of the rock. Mechanical weathering mainly occurs because of the following reasons:
- Crystal Growth
- Release of Pressure
The minerals in the rocks expand because of heat and contract when temperature decreases. Due to such continuous contracting and expansion, tension develops in the rock particles. Each mineral reacts differently to the temperature. Some minerals expand more while others do not expand as much. Consequently, the tension formed in the rocks also increases and decreases. As a result, cracks develop in the rocks and they break. In areas, where the diurnal range of temperature is higher, weathering of this type is common in the hot deserts.
You know that the volume of the water increases when it freezes. In areas where the temperatures drop below 0°C for quite some time, the water accumulates in the cracks and crevices in the rocks freezes. Its volume increases. This leads to tension in the rocks and they shatter.
On rocky coasts, waves hit the sea cliff. The water is alkaline. Some water droplets hit the cracks in the rocks. In this alkaline water, the soluble materials in the rock get dissolved. This leads to the formation of small holes in the rocks. This is the effect of the solution. Alkaline water gets stored in these holes. Because of heat, this water turns into water vapor and only crystals of alkaline materials remain in the rocks. Crystals occupy more space. This causes tension in the rock. Holes are formed in the rocks. It looks like a honeycomb.
Release of Pressure:
It is not that tension is created in the rocks only because of temperature, freezing of water, or crystallization. The outer layers of the rocks exert pressure on the inner or lower layers. When this pressure ceases to exist, the lower or inner layers get freed from the pressure. This also leads to weathering.
Some areas experience more rainfall than others. In such areas, soaking of rock water also causes weathering of some rocks like sandstone, and conglomerate. These rocks are formed because of pressure on the agglomeration of sand particles. Mud also makes sand particles come together. When water penetrates such rocks, the particles get loose and separate from the main rock. This is called granular weathering.
Sometimes both temperature and water are responsible for weathering. Differences in temperature cause contraction and expansion widening the joints or the cracks in the rocks. Water accumulates in such wide joints and big blocks of rocks separate from each other. This is called block disintegration.
Take a glass of water and put a piece of chalk in it. On the next day, observe what has happened to it.
Wrap a few iron nails in a wet cloth. Open the cloth after 2 days. Observe what happens.
Water plays an important role in chemical weathering. Rock is a mixture of many minerals. Water is a universal solvent. Many things get dissolved easily in water. The solubility increases because the matter has dissolved in water. And those materials that do not dissolve easily in water get dissolved in such solutions. In areas that receive heavy rainfall, chemical weathering occurs in the following way.
The rainwater travels through the atmosphere before reaching the ground. Carbon dioxide in the air gets mixed in the water in this process. Dilute carbonic acid gets formed. Materials like limestone get easily dissolved in such acids.
e.g. Water + Carbon Dioxide = Carbonic Acid (H2O + CO2 = H2CO3)
Some minerals in the rock get dissolved in water. Limestone is formed due to chemical precipitation between water and alkalis. At Wadgaon Darya in Ahmednagar district, limestone gets precipitated chemically i.e. undergoes chemical weathering again. Similarly, because of the solution, alkalis in the rock dissolve and make them brittle.
This process occurs in rocks that have iron present in them. The iron in the rock comes in contact with water and a chemical reaction takes place between iron and oxygen. Hence, a reddish-colored layer forms on the rocks. This is called rust. When you kept your nails in the wet cloth, you must have noticed a similar thing. A similar process occurs in rocks in areas with high rainfall. There are many more processes involved in chemical weathering besides the ones mentioned above. Daily-life examples include the moistening of table salt in the rainy season, apple slices turning brown, etc.
Besides mechanical and chemical weathering, biological factors are also responsible for weathering of rocks. Have you visited any fort? Have you seen trees growing on their minarets? You also must have seen pieces of rocks getting separated because of the roots of the trees. As the roots grow bigger, they create tension in the rocks and start breaking them.
Ants make large anthills. Rats, mice, rabbits, and other worms and insects make burrows in the ground. These animals are called burrowing animals. Because of their activity, weathering of rock occurs. Besides these, algae, moss, lichen, other flora, etc. grow in the rocks. They also help in weathering.
Mass Movements (Mass Wasting):
The weathered rock materials start moving along the slopes due to gravity and accumulate near the foothills of the gentler slopes. This process has been happening for years. The weathered particles form a conical heap at the foothills. When weathered particles move down due to gravity alone, the process is called mass movement. Mass movements occur in two ways: it is rapid on steeper slopes while it is slower on gentle slopes.
Rapid Mass Movements:
Rockfalls, landslides, and land subsidence occur rapidly. Their effects are very destructive. The probability of these events is higher in the regions having humid climates and steeper slopes. A thick layer of weathered material forms on the slope. When it rains in such areas, the rainwater penetrates the weathered materials, and their weight increases. The weathered materials move very rapidly and come down the slope e.g., the mudslide at Malin Village of Pune district. Sometimes the weathered materials do not move downward but sink ‘in situ’ (where they are). This is called slumping. Such rapid mass movements may also occur because of earthquakes.
Slower Mass Movements:
In areas with dry climates and gentler slopes, mass movements occur slowly. Soil creep is a common phenomenon in such areas. In periglacial regions along the slopes, small layers of soil accumulate because of the movement of soil. This is called solifluction.
Like weathering and mass movements, erosion is also an external process. Erosion occurs through various agents. Wind, running water, glaciers, seawater, and groundwater cause erosion.
Exogenetic Processes Part – II
We have learned in the previous lesson that the earth’s surface wears away because of erosion. The eroded material gets carried away by agents. When the speed of the agent reduces, the materials get deposited. The agents like running water (river), glaciers, wind, sea waves, and groundwater, do the work of erosion, transportation, and deposition. Because of these agents, the earth’s surface keeps on undergoing changes, and new landforms are formed. We will study some of these landforms in this lesson.
Work of Rivers and Landforms:
Running water flows naturally in a direction according to gravity along the slope and makes its way. This is called a flow of water. When many such flows of water come together, a river is formed.
The slope of the land, the type of rock, the volume of water flowing in the river and the length of the flow, the volume of sediments in the river, etc, are the factors on which the erosional, transportation and depositional work of the rivers depend.
Erosional Work of Rivers:
The rivers originate at a much higher altitude than the sea level. Here, the river flows at a great speed and therefore, its power to erode is great. The riverbed and the river banks get eroded because of the fast flow of water, sand particles, and pebbles. etc. and the various tributaries joining the main river. All these lead to the formation of gorges (canyons), V-shaped valleys and waterfalls.
Transportation and Deposition by Rivers:
A river flows down the slope from a hilly region. At the foothills, the change in the slope causes deposition of coarse sediments. As these are deposited in a triangular shape, they form an alluvial fan.
As the steepness of the slope decreases and the transporting capacity of the river reduces, it starts flowing slowly. It bends (meanders) often in its way to cross even small obstacles. By the time the river reaches the sea, its riverbed becomes very wide and its speed becomes very slow. The sediments of the river get deposited in its bed and on its banks. The factors that determine the deposition of sediments are the length of the rivers, volume of water, amount of sediments, and the slope of the river and the earth’s surface. Thus, landforms like flood levees, flood plains, and deltas are formed because of the deposition of sediments.
Work of Glaciers and Landforms:
In regions where the temperatures are generally below freezing point, precipitation is in the form of snowfall. Layers of snow accumulate on the earth’s surface because of snowfall. The heavy weight of these overlying layers makes the snow move along the slope. At the base of the layer, the snow starts melting because of the friction and the pressure from above. The glacier starts moving slowly along the slope. Like the river, a glacier too carries out the work of erosion, transportation, and deposition.
Erosion Work by Glaciers:
Though its velocity is less, the mass of the ice is greater, and hence the glacier erodes its banks and its bed on a large scale. The erosional work of glaciers produces landforms like cirques, aretes, horns, U-shaped valleys, hanging valleys, and Roche moutonnees (or sheepbacks).
Transportation and Deposition by Glaciers:
The glaciers carry sediments with them. These sediments are called moraines. Depending on the location of the deposits, moraines can be divided into 4 types: ground moraines, lateral moraines, medial moraines, and terminal moraines. Observe figure. The depositional work of glaciers produces landforms like drumlins, eskers, etc.
Glaciers move at different velocities daily ranging from 1cm to 1m. The Jacobshavn Glacier in Greenland is one of the fastest-moving glaciers in the world. It moves at the rate of 46m per day.
Work of the Wind and the Landforms Produced:
You have learned earlier that the movement of air is called wind. Wind is a gaseous agent of erosion. The erosional, transportation and depositional work of wind is more prominent in deserts and semiarid regions. As mechanical weathering occurs on a large scale here, powdered rock and sand spread over a large area. There is hardly any obstacle in the transportation work of the wind. Sand particles also get transported along with the wind and they are carried over longer distances and get deposited where the speed of the wind reduces. In this way, the wind does the work of erosion, transportation, and deposition.
Erosional Work of Wind:
The wind carries small sand particles, small pebbles, etc. along with it. These particles cause erosion along rocks coming in the way due to friction. This leads to the formation of mushroom rocks, deflation hollows, yardangs, etc.
Depositional Work of Winds:
Sand particles that blow with the wind are of different shapes and sizes. Those particles which are very fine are carried over larger distances while the larger ones get transported to shorter distances only. These sand particles get deposited in deserts and semi-arid climates. As a result, specific landforms are formed. Dunes, barchans, seifs, ripple marks, loess plains, etc. are formed by wind deposition.
Work of Sea Waves and the Landforms:
In coastal areas, sea waves carry out erosional, depositional, and transportation work. Winds and tides cause the movements of sea water and as a result, waves come to the coast. Because they hit the rocks at the coasts, erosion of the rocks occurs. In coastal areas having wide beaches, waves carry out depositional work.
Erosional Work of Sea Waves:
When the waves break at the coast, they bring with them water, transported stones, pebbles, sand particles, etc. This leads to the erosion of the coast. Because of the chemical and hydraulic action of the sea waves, erosion occurs. Landforms like wave-cut platforms, sea caves, sea arches, sea cliffs, etc. are formed because of the erosional work of the waves.
Depositional Work of Sea Waves:
The eroded materials accumulate at the sea bed. Because of tides, they keep on moving towards the coast and away from the coast. They become fine because of attrition and hitting each other. Deposition of such materials occurs at the places where the effect of waves is less. Landforms like beaches, sand bars, and lagoons are formed due to the depositional work of the sea waves.
Work of Groundwater and Landforms Produced:
The rainwater seeps below the earth’s surface through porous rocks or cracks in the rocks. This water accumulates in the non-porous layer of the rock. This accumulated water is called groundwater. The soluble minerals in the water get dissolved and flow with the groundwater. This is the erosional work of the groundwater.
When the groundwater evaporates or the volume of soluble minerals is more than the solubility of the groundwater, the deposition of dissolved materials starts. Landforms like sinkholes, limestone caves, stalactites, and stalagmites are formed. Thus, the groundwater carries out the erosion, transportation, and depositional work.
The upper surface of the water accumulated below the ground is called the water table. Factors like seasons, the porosity of rocks, the amount of rainfall, etc. affect the level of the water table. The water table is closer to the ground during rainy seasons while it is deeper in the summers.
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