EXOGENOUS PROCESSES ➢Also called Gradational Processes, this refers to the processes or forces which originate out side the earth crust that modifies the earth surface and produce different land forms. ➢Comprises degradation and aggradations. Degradation/Denudation & Aggradations/Deposition 1. Denudation or Degradation is derived from a Latin word which means “to lay bare”, and it is the name given to those external agents which wear away the land surface it includes a. Weathering b.Mass wasting C. Erosion and Transportation 2. Aggradations Is the action of laying down of materials transported mechanically by agent of running water, wind, and ice, It include deposition by ➢ Water Features produced include flood plain, levees, alluvial fans, deltas, beaches, lake plains, marine alluvial plains. ➢ Aeolian Features produced include loess plain, sand dunes, seif dune. ➢ Glacial Features produced includes boulder clay, outwash plain, moraines drumlins, eskers. ➢ Living organism Features produced includes coral reef ➢ Evaporation and precipitation Features produced include salt deposit ➢ Organic matter Features produced include coal, oil Relationship: Weathering, Mass wasting(slope failure), Erosion and transportation. Together, these processes are responsible for Denudation of Earth’s surface WEATHERING WEATHERING Is the distengration and decomposition of rock that form the surface of the ground and that lie exposed to weather( wind, precipitation , temperature, pressure and sun shine). ➢ Involves weakening and breaking up of rocks. ➢ No transportation involved, if a rock is changed or broken but stays where it is, it is called weathering. TYPES OF WEATHERING 1.Mechanical(physical) weathering 2. Chemical weathering and Biological weathering ➢ MECHANICAL(PHYSICAL) WEATHERING ➢ Is the physical disintegration and reduction in the size of the rocks without changing their chemical composition. ➢ The size of the rock is changed but the kind of rock is not changed ➢ Ways/processes in which physical weathering occur include the following a). by temperature change e.g exfoliation b). by frost action c). by salt crystalization d). by alternate wetting and drying and pressure release. MECHANICAL WEATHERING A.TEMPERATURE CHANGE ➢Daily (diurnal) and seasonal temperature changes affect certain minerals and facilitates the mechanical weathering of bedrock. ➢ Warmer temperatures may cause some minerals to expand, and cooler temperatures cause them to contract. ➢This gradual expansion and contraction of mineral grains weakens the rock causing it to break apart into smaller fragments or to fracture or causing it to peel off like an onion . ➢ This process is more common in desert climates because they experience extreme fluctuations in daily temperature changes. ➢ Temperature changes are often not the dominant form of weathering, but instead temperature changes tend to accelerate other forms of weathering already occurring. MECHANICAL WEATHERING B. Frost action (freeze thaw action) ➢ Frost action is the breakup of rock caused by the freezing and thawing (contracting and expansion) of water. ➢ Water can seep into the cracks of a rock and as the climate cools the water freezes and expands breaking the rock apart. ➢ Frost action occurs in both arctic and cool temperature because freezing and thaw action is more common in these regions. MECHANICAL WEATHERING B. FROST ACTION MECHANICAL WEATHERING C. EXFOLIATION ➢ ➢ ➢ ➢ ➢ ➢ Exfoliation is the peeling away of large sheets of loosened materials at the surface of a rock caused by temperature change (thermal expansion and contraction). Rock is repeatedly subjected to heat and cold Outer layer expands in heat and contracts in cold. Outer layer of rock peels off (loose rock known as scree) Exfoliation domes (Large rocks characterized by exfoliation) and talus are features formed as a result of exfoliation. Occur in desert, semi-desert and even monsoon region e.g. in Egypt, Sinai and Kalahari desert MECHANICAL WEATHERING C. EXFOLIATION MECHANICAL WEATHERING C.EXFOLIATION MECHANICAL WEATHERING D. PRESSURE RELEASE ➢ Removal of overlying rock load because of continued erosion causes vertical pressure release with the result that the upper layers of the rock expand producing disintegration of rock masses. ➢ Thus this process is controlled by denudation process especially erosion. MECHANICAL WEATHERING D.PRESSURE RELEASE MECHANICAL WEATHERING E. SALT CRYSTALLIZATION ➢ occurs when water enters the holes and cracks in the surface of rocks, these water often carries salt with it. As the water later evaporates, the salt is left behind. Over time, these salt deposits build up, creating pressure that can cause rocks to split and weaken. ➢ Salt crystallization is most common in drier climates, such as hot deserts where capillary action draws to the surface saline water. MECHANICAL WEATHERING E. SALT CRYSTALLIZATION These salt crystals were found growing between rock fractures in California’s Death Valley. MECHANICAL WEATHERING F. WETTING AND DRYING (SLACKING) ➢ ➢ ➢ ➢ It is mechanical weathering process by which a rock is distengrated by its alternative wetting and drying. Occurs when: there is flash floods, seasonal rains, coastal fog, dew which produce moisture that encourages clay minerals in rocks to swell. Repeated expansion on wetting and contraction on drying causes the rock to disintegrate. This process is very common in Tropical and subtropical regions where rains are not through out the year. MECHANICAL WEATHERING ➢ ➢ ➢ G. ABRASION Abrasion occurs when rocks collide against each other while they are transported by water, glacial ice, wind, or gravitational force. The constant collision or gravitational falling of the rocks causes them to slowly break apart into progressively smaller particles. Flowing water is the primary medium of abrasion and it produces the „rounded‟ shape of fluvial sediments. MECHANICAL WEATHERING G. ABRASION CHEMICAL WEATHERING CHEMICAL WEATHERING The processes by which rocks are broken down by chemical reactions. ➢ Involve the change of chemical composition of the rock to f ➢ ➢ ➢ ➢ ➢ ➢ AGENTS OF CHEMICAL WEATHERING Water, (H2O) dissolves some minerals and rock. Acid, (H2SO4, HNO3,H2CO3) contains acids that break down rock or dissolve it. Oxygen,(O2) combines with other elements in the rock in an oxidation reaction. Carbon dioxide, (CO2) dissolves in water to make carbonic acid (H2CO3). Living organisms produce weak acids that break down or dissolve rock. CHEMICAL WEATHERING PROCESSES/ TYPES OF CHEMICAL WEATHERING a. b. c. d. e. Hydrolysis Hydration Carbonation Solution Oxidation CHEMICAL WEATHERING A. HYDROLYSIS ➢ Is the chemical breakdown of a substance (mineral in the rock) when combined with hydrogen in water to give new chemical compound. ➢ Hydrolysis occurs when water combines with the substances in rocks to form new types of substances, which are softer than the original rock types. This allows other forces, such as mechanical weathering, to more easily break them apart. ➢ The most common example of hydrolysis is feldspar, which can be found in granite changing to clay. When it rains, water seeps down into the ground and comes in contact with granite rocks. The feldspar crystals within the granite react with the water and are chemically altered to form clay minerals, which weaken the rock. CHEMICAL WEATHERING B.HYDRATION ➢ It is a chemical weathering process in which, certain minerals of rock absorb water and expand causing internal stress and fracturing of rocks. ➢ Expansion of minerals weaken the rock and cause it to break apart ➢ It is potentially to rocks with minerals that absorb water. ➢ For example hematite, an iron oxide, combines with water to give limonite another iron compound. ➢ there is no chemical change the rock only absorbs water and this can be easily removed e.g. the water in limonite can be removed by heating it. CHEMICAL WEATHERING ➢ ➢ ➢ ➢ C. CARBONATION Carbonation takes place when carbon dioxide reacts with certain types of rocks forming a solution that can easily be carried away by water. Carbon dioxide and water combine to form carbonic acid. Carbonic acid attacks the insoluble calcium carbonate in limestone and turns it into soluble calcium bicarbonate which is eliminated in solution by water. This is the way that karstic landscapes are generated This type of weathering is important in the formation of caves e.g. Ambon cave in Tanga are the best example. CHEMICAL WEATHERING C. CARBONATION CHEMICAL WEATHERING CARBONATION RESULTS INTO THE FORMATION OF LIMESTONE FETURES. CHEMICAL WEATHERING D. OXIDATION ➢ Oxidation takes place when oxygen combines with other elements in rocks to form new types of rock. ➢ These new substances are usually much softer, and thus easier for other forces to break apart. ➢ Oxidation takes place actively in rocks which contain iron, when the oxygen combines wit the iron to form iron oxide which are often easily attacked by other weathering processes. ➢ The process results in the formation of rusting (iron oxide). CHEMICAL WEATHERING OXIDATION RESULTS INTO THE FORMATION OF RUSTING. CHEMICAL WEATHERING RUSTING CHEMICAL WEATHERING SOLUTION It is the process in which minerals present in the rocks directly dissolve in water. The soluble substances are removed by the continuous action of water and the rock no longer remains solid and form holes, rills or rough surface and ultimately falls into pieces or decomposes. The action is considerably increased when the water is acidified by the dissolution of organic and inorganic acids. (e.g) halites, NaCl. NaCl + H2O -> Na+, Cl- , H2O (dissolved ions with water) E. ➢ ➢ ➢ ➢ CHEMICAL WEATHERING NOTE; ➢ Usually two or more chemical weathering processes take place at the same time. ➢ Chemical weathering is most marked in hot regions with high humidity. SIMILARITIES BETWEEN PHYSICAL AND CHEMICAL WEATHERING. ➢ All of them involve the break down of material rocks, however mechanical/physical weathering involves the break down and disintegration of rock into smaller and smaller fragments while chemical weathering involve breaking down material into a substance with a new composition and different physical characteristics ➢ All of them depends on the climate of the given area, warmer temperatures favor chemical weathering in comparison to cooler temperatures, which promote physical weathering ➢ All of them produces material which final transported by transporting agents ,Once materials have been broken down by weathering, they are transported by wind, ice and water, subsequently contributing towards further erosion. ➢ All of them have processes, processes of physical weathering are temperature change(exfoliation and block distengration), wetting and drying, salt crystallization, pressure release and freeze thaw action while processes of chemical weathering are carbonation, hydration, oxidation, carbonation and solution. chemical and physical weathering often go hand in hand. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action. Furthermore, the chemical action at minerals in cracks can aid the disintegration process. Both types can be found in each climate type to a certain degree. ➢ ➢ DIFFERENCES BETWEEN PHYSICAL AND CHEMICAL WEATHERING ➢ Chemical weathering happens when there is change in the composition of rocks through chemical processes and form residual materials. Processes include oxidation, dissolution, and hydrolysis. ➢ Mechanical weathering occurs when there is only physical change in rock structure such as size and shape through physical forces of nature. Processes include exfoliation, abrasion and freeze and thaw weathering. ➢ Climate is an important factor for weathering to take place. Cold temperatures favor mechanical weathering while warm temperatures support chemical weathering. BIOLOGICAL WEATHERING BIOLOGICAL WEATHERING ➢ Biological weathering it is the break up of rocks due to the action of organisms of animal, plant and people. ➢ Mostly by biological weathering the rocks are broken up by physical and chemical means . ➢ Action of plants Wedging action of the plant roots. Some of the plants grow into the joints and cracks of rocks, the roots widen the joints and cause cracks and with time they cause rock blocks separated. ➢ Other plant roots produce organic acid, this acids reacts with some minerals with in the rocks causing decomposition. ➢ Moses and lichens (combination of fungi and algae) grow on rocks and cause occupied to have moisture and thus the environmental is made to undergo chemical decomposition. BIOLOGICAL WEATHERING Action of animals ➢ Burrowing of animals make mechanical break up of rocks through digging example of such animals are rabbits, earthworms and moles. ➢ Large herds of animals such as cattle and zebra pound the rocks with their hooves as they move over them. ➢ When the living organisms in the soils respire give out carbondioxi de gas, that gas dissolve in water to form carbonic acid which cause decomposition of rocks. ➢ When animals die, decay and produce organic acids reacting with minerals to cause decomposition. BIOLOGICAL WEATHERING Action of people ➢ Construction activities by man cause greatly mechanical damage of rock. ➢ Mining activities with the use of explosives and machines cause mechanical break of rock. ➢ The disposal of chemical wastes may contaminate with rock material and easily cause decomposition. ➢ The burning of fuel by which gases of carbon dioxide and sulphur dioxide released out, polluting the atmosphere, the gases dissolve in he rain water and form acidic rains which are corrosive in nature and increase the rate of chemical weathering. FACTORS AFFECTING WEATHERING a) Climate The main climatic control is temperature and rain fall b) Equatorial region Characterized by high temperature and high rainfall hence chemical weathering is dominant than physical weathering. c) d) Tropical region Characterized by seasonal variation of rainfall and temperature hence chemical weathering is dominant during wet season and physical weathering is dominant during dry season. Tundra and polar region These areas are very cold and thus mechanical weathering easily takes place by frost action. . s e t a m i l c t s ➢Fro Found in areas of high mountain ranges as well as close to the North and South poles. The process here is freeze-thaw action. How much weathering happens depend on the number of freeze-thaw cycles, is not so important how severe the frost was. This means in very cold areas the amount of weathering is smaller. Chemical weathering is slow in frost climates, the cold temperature makes the chemical reaction go slower. Himalaya. North Pole. Greenland. . s a e r a e t a r e p m ➢Te Temperate areas have moderate temperatures. Here freeze-thaw action is not important. Exfoliation does not occur. Here what mostly happens is the chemical weathering. Here the rate of weathering is lower than in the Tropics because the temperatures are lower. Valdivia, Chile. Columbia river, USA. Eastern Australia, temperate forest. Cleopatra’s needle in London-Egy pt. s t r e s ➢De Exfoliation is the main weathering process in deserts, this is due to big differences between and minimum and maximum temperatures in a day (Diurnal temperature range). Chemical weathering is extremely slow caused by the lack of moisture. Their weathering rates are the slowest in all the world, we can see it in the archaeological pieces from Egypt from ancient times. They sat that Cleopatra’s Needle in London weathered more in 10 years in the polluted and wet London than in 3500 years in the desert in Egypt. Desert lack soil, so bedrock is often exposed. Caravan desert , China. s a e r a l ropica Amazon River, South America. t d i m ➢Hu They have the fastest weathering rates on Earth. The layer of weathered material lying on top of the bedrock can be up to 40 metres deep, rocks are seen to weather a lot very fast. Thailand, east Asia. b) The nature of the rock Mineral composition The mineral content determine the type and rate of weathering for example rock with calcium carbonate, iron compounds, calcium sulphate and potassium are more subjected to chemical weathering other rocks with no these minerals may distengrate by mechanical means. ➢ Rock structure A rock's composition has a huge effect on its weathering rate. Rock that is softer and less weather-resistant tends to wear away quickly. What is left behind is harder, more weather-resistant rock. This process is called differential weathering. Quartz is one type of rock whose composition, especially its crystalline structure, makes it resistant to mechanical and chemical weathering. This is why quartz remains unchanged on the Earth's surface after surrounding sedimentary rock has been eroded. There are some rocks, like limestone, that weather more rapidly. Limestone has the compound calcite. It is the carbonization of calcite that causes the increased rate of weathering of limestone. ➢ Mineral color Rocks with dark color heat faster than rocks with light minerals, thus the rate of weathering by temperature change is higher to rocks with dark minerals ➢ Quartz rocks LIMESTONE ROCKS c)Topography/ Relief ➢ Topography affects weathering as irregular, steep topography exposes more rock to weathering than horizontal surfaces. ➢ Rocks over gently slopes are more subjectible to chemical weathering as water easily percolate into the ground provided the land scape is permeable and the temperature is reasonably high but it depends on the minerals of the rock. ➢ Steep slopes in highland areas are more liable to mechanical weathering as rocks more exposed to the surface by the running agents. d) Rock age ➢ Old rock weathered easily compared to young rock e) Biotic activities ➢ ➢ ➢ ➢ Presence of action of animals, plants and human activities accelerates the rate of weathering Moses and lichens (combination of fungi and algae) grow on rocks and cause occupied to have moisture and thus the environmental is made to undergo chemical decomposition. Large herds of animals such as cattle and zebra pound the rocks with their hooves as they move over them. Construction activities by man cause greatly mechanical damage of rock. Significance of weathering Positive significance ➢ Facilitate soil formation ➢ Help people to exploit minerals because it weakens the rock. ➢ Weathering creates the building material. ➢ Development of tourism ➢ Occurrence of lakes e.g. polje lakes in karst region ➢ Some forms of chemical weathering help to remove carbon dioxide from the atmosphere ➢ If weathering never occurred, the Earth would have a surface of bare rock, and no plant or animal life could exist. ➢ Geologists can use weathering to determine facts about a region's past. Significance of weathering ➢ ➢ ➢ Negative significance It encourage high rate of erosion. It encourage high rate of mass wasting. It cause the lowering of the land scape. MASS WASTING MASS WASTING ➢ ➢ ➢ ➢ ➢ Mass wasting or mass movement is the movement of a large mass of rock, soil and debris downward due to the pull of gravity. Causes of mass wasting The most notable cause of mass wasting is gravitational force however the ability of gravity to cause mass wasting depends on the following factors. Slope Force of gravity has an easier time of pulling materials down a steep slope as compared to a gentle slope. Water or rainfall Too much water or rainfall will also weaken a mountain or hillside's ability to resist gravity. Vegetation If a mountain slope has decreased vegetation, then it will be more prone to mass wasting because it does not have the protective plant covering above ground or the soil anchoring roots system below ground. Disruptive events/ Earth quakes This can cause sections of mountain to detach due to the violent shaking that occurs. MASS WASTING CAUSES OF MASS WASTING ▪ Human activities Human activities like ploughing, mining , constructions, deforestation, overgrazing, wars and others, weaken a lot the stability of earth’s materials (soil and rocks) and encourage mass wasting. ▪ The nature of the material Earths material (soil and rocks ) can be unstable or stable . The stable materials may not easily move down the slope . N.B CAUSES OF MASS WASTING = FACTORS AFFECTING MASS WASTING TYPES OF MASS WASTING ➢ Mass movements can be divided into four main classes. These are falls, slides, creeps and flows. The classes are based on how quickly the rock and sediment moves and how much water there is. a) Soil creep ➢ Soil creep is a very, very slow form of mass wasting. It's just a slow adjustment of soil and rocks that is so hard to notice unless you can see the effects of the movement. ➢ These effects would be things like fencepost shifted out of alignment, or telephone poles tipping down slope. ➢ Rain water lubricates soil particles and enables them to slide slowly over each other, this action can take several years or even a century. ➢ Other factors which help soil creep are the heating and cooling of the soil, alternate wetting and drying of the soil, over grazing on steep slope and the burrowing of animals in the soil. TYPES OF MASS WASTING b) Mud / Soil flow ➢ A flowing mass of soft, wet, unconsolidated earth and fine-grained de bris, made fluid by rain or melted snow and often building up great speed. The mud travels down a steep unstable slope very quickly. ➢ Mud flow occur in tundra regions during the early summer when the frozen soil thaws and turns into a semi liquid state, thus enabling it to slide over the still frozen sub soil. ➢ Mud flow also can occur on desert slopes which are not protected by vegetation when amore rain falls than the soils can absorb, also can occur on the slopes of an erupting volcano if heavy rains fall on the volcanic ash covering the slopes. TYPES OF MASS WASTING c) Land slide ➢ A large mass of rocks and earth that suddenly and quickly moves down the steep slope such as a cliff face , a valley side or an embankment side of a mountain or hill. ➢ Are caused by the lubricating action of water and the pull of gravity, a land slide may either take the form of sliding or of slumping . ➢ Actions that help to produce a landslide are the undercutting of the base of a steep slope by a river or by the sea or the steepening of a slope by human action also earthquake and prolonged rain can cause landslide TYPES OF MASS WASTING d) Rock fall ➢ Refers to the free falling of mass of rock from a steep cliff, This is the most rapid and fastest of all types of mass wasting, and occur most frequently in mountains or other steep areas during early spring when there is abundant moisture and repeated freezing and thawing. ➢ Rock falls are caused by the loss of support from underneath or detachment from a larger rock mass. Ice wedging, root growth, or ground shaking, as well as a loss of support through erosion or chemical weathering may start the fall. Significance of mass wasting ➢ ➢ ➢ ➢ ➢ ➢ ➢ ➢ ➢ Positive significance Formation of lakes. Bring about soil development. May improve soil fertility. Enable tourist attraction. Negative significance Loss of human life. Bring about land degradations Loss of human properties Changing of the river course. Accelerates the occurrence of soil erosions. The control of mass wasting ➢ ➢ ➢ Planting of trees over the sloped landscape to hold the earths materials more firmly. Making of terraces across of slopes To prohibit the construction of houses over the steeply sloped landscape. EROSION EROSION Erosion is the process by which the surface of the Earth gets worn down often by forces such as water, wind, ice or wave. Types of erosion Erosion categorized according to its nature of occurrence type of eroding agent and the appearance of the affected land. According to the nature of occurrence, erosion broadly divided into geological and accelerated erosions. a)Geological erosion It is the type of erosion which occurs more naturally as not aided by human activities, caused mainly by movement of water, ice and wind. b) Accelerated erosion Agents of erosion Different between weathering and erosion WEATHERING EROSION Weathering means disintegration and decomposition of the rock without any significant transportation of the resultant product. Erosion is the loosening and carrying away of rock debris by the moving agent operating on the earth surface. Weathering aids erosion by weakening and disintegrating rocks. Erosion in turn aids weathering by removing the cover of soil and loose debris and exposing them to the weather for weathering. It is the production of rock waste under virtually static condition. It is the transportation of rock waste from the site of its formation. Weathering includes only breaking of rock. Erosion includes both breaking of rock and transportation of the broken part. It is a static process. It is a dynamic process. Weathering can take place without subsequent erosion. Erosion is possible without previous weathering. There are 3 types of weathering. That are physical, chemical and biological weathering There are 5 types of erosion. That are Water, ice, Wind, Gravity and Thermal After weathering, weathered materials doesn’t move. After erosion, eroded materials move.
