8UNIT External dynamics of the Earth What do you remember? Can you name the solids in the photograph? nd the liquid? Is the liquid moving? How does it move? What is the name of this formation of water? How do you think it was formed? How do you think this formation will change in the future? Content objectives In this unit you will Identify different types of weathering Learn how erosion, transportation and deposition change the landscape Understand the ways the landscape is shaped by the action of different agents Learn how sedimentary rocks are formed Make an experimental model of a cliff recession Key language Defining Glaciers are masses of moving ice. Expressing possibility The pressure of the roots may cause the rocks to break. Wind may erode, transport and deposit rock materials. Expressing cause and result Weathering is mainly due to the action of atmospheric agents. Mechanical weathering is mainly caused by changes in temperature. 86
1. What is weathering? Weathering is the process which causes rocks and minerals to break down and disintegrate into smaller pieces. It is mainly due to the action of atmospheric agents, such as water, wind, ice and ocean movements. There are two climatic factors which also affect the processes of weathering. These are: Temperature. High temperatures may facilitate chemical reactions which help to disintegrate some types of rock. Humidity. Humidity may also facilitate chemical reactions, for example, in forest areas. In dry desert areas these types of reaction cannot take place. There are three types of weathering: Chemical weathering is the decomposition of rocks through chemical reactions. This process changes the mineral composition of rocks. It is mainly caused by water. Biological weathering is the breakdown of rocks through the activity of living things, for example, plants. Mechanical weathering is the breakdown of rocks and minerals into small-sized particles through physical forces. Mechanical weathering is mainly caused by changes in temperature, such as freezing and thawing, and the effects of wind, rain and waves. ccumulation of rock fragments produced by weathering. Biological weathering. Plants extend their roots into cracks in rocks. The pressure of the roots may cause the rocks to break. The roots may also produce organic acids which help to break down rocks chemically. 87
Example of mechanical weathering Freeze-thaw weathering. This is an example of mechanical weathering, mainly found in mountainous regions. The main cause is a temperature fluctuation from below freezing point to above it. Example of chemical weathering Rainwater is slightly acidic. When it comes into contact with rocks, such as sandstone or limestone, a chemical reaction takes place, causing the rock to break down. Water and warm temperatures can speed up this process, so it mainly occurs in warm, wet places. water Water flows into cracks in a rock during the day. ice Chemical weathering has shaped this landscape. 1. Copy and complete. t night, when the water freezes, it expands. The ice acts as a wedge. Weathering Chemical Process involves chemical reactions Consequences changes in mineral composition of rocks Biological Mechanical The freeze-thaw cycle is repeated, and eventually the rock cracks. 2. How do temperature and water affect each type of weathering? Explain. 3. Where would chemical weathering be more active, in the rainforest or in the desert? Explain. 4. Describe the process of freeze-thaw weathering. First, water flows into cracks in a rock. Then, at night 88
2. Which agents shape landscape? The agents which shape landscape are water, wind, ice and ocean movement. Gravity is the main driving force behind all these agents. These agents are responsible for the following processes: Erosion. Erosion moves small pieces of weathered rocks to another place. For millions of years, erosion has shaped the landscape. Transportation. This process moves rock material which has eroded. This process is usually carried out by the same agent which causes the erosion. Deposition. Moving rock materials are deposited. This process occurs when there is a decrease in the energy of the transporting agent. 5. Copy and complete the definitions. The agents which shape the landscape are: 1 2 3 Erosion: Transportation: 6. How are weathering and erosion different? 7. Explain how beaches may have sand originating from weathered rocks from a far away place. The amount of energy of the agent, in this case, water, affects the intensity of erosion. Different landscapes show how rocks can be more resistant or less resistant to erosion. Water in rivers can break off fragments of rock. Then, the current transports them to another place. Particles can be transported along surfaces or in suspension. Formation of dunes. Sand is deposited by the wind. The wind collects the sand from a beach where it has been deposited by waves. 89
3. How does wind shape landscape? Wind may erode, transport and deposit rock materials to change the landscape. ll the processes involving the wind are known as eolian processes. Wind is only an effective agent in places where it is strong, usually in dry regions with not much vegetation. B sand columns barchan erg mushroom - shaped rocks D C. The wind picks up small particles and leaves the larger ones. This eolian process is called deflation. B. The accumulation of sand produces dunes. Ergs are areas of sandy desert. Barchans are crescent-shaped dunes. C. Rock particles (sand) carried by the wind can erode solid rock. D. Large grains of sand carried by the wind are blown at low heights. These erode the base of a rock. The result is mushroom-shaped rocks.this eolian process is called abrasion. ctivity 8. Describe how a mushroom-shaped rock is formed. First, large grains of sand are carried by the wind... 90
4. How do glaciers shape landscape? Glaciers are masses of moving ice. They are only found in alpine and polar regions. Ice moves very slowly from high areas to low areas by the force of gravity. s a glacier moves, it drags rock materials and erodes the surfaces in its path. Glaciers cut U-shaped valleys which are called glacial valleys. B steep relief glacial cirque glacial tongue U-shaped valley moraine D C. Ice causes intense mechanical weathering of rocks, forming very steep relief. B. Glacial cirques are rounded depressions where snow accumulates and changes to ice. C. Ice forms glacial tongues which carry and deposit rock material known as moraine. D. Glacial tongues cut U-shaped valleys. 9. Write the meaning of the following words. Then make a drawing to illustrate each one. barchan erg glacial tongue U-shaped valley moraine 10. How is moraine formed? In which part of the world can you find moraine? Did you know that...? The word glacier comes from French. In the Savoy dialect, glacière means "moving mass of ice." In Old French glace means "ice, from Classical Latin, glacies. 91
5. How do rivers and rain shape landscape? Rivers shape landscape Rivers also carry out erosion, transportation and deposition to change the landscape. The dominant process depends on the velocity of the water: source upper course In the upper course, rivers flow fast. Erosion and transportation are the dominant processes. s a result, valleys are narrow and deep. In the middle course, the velocity of the water decreases. Transportation is dominant. s a result, valleys are wide. The river has less capacity of erosion, so the water flows to form deep bends called meanders. In the lower course, rivers are slow. Deposition is dominant. s a result, the valley flattens out. middle course meanders wide river valley lower course Rainwater shapes landscape Rainwater is a very effective agent for eroding and shaping landscape, especially in areas with dry climates and torrential rains. gullies alluvial cone B Erosion by water forms gullies in the land. Gradually, gullies get deeper and become ravines and gorges. ravine C alluvial fan fairy chimneys 11. Match each statement to the upper, middle or lower course of a river. a. Transportation is dominant here. b. The valley flattens out. c. Erosion and transportation are dominant. d. Meanders are frequent here. 12. Gullies are found in the upper course of a stream, where the landscape is very steep. Can you find gullies in the middle course or lower course of a river?. In the alluvial cone, erosion is very intense and gullies form. B. Fairy chimneys are tall, conical rock formations, caused by erosion. C. lluvial fans are areas which are at risk from floods. alluvial fan fairy chimneys 92
6. How does groundwater shape landscape? Landscapes produced by the dissolving action of water on rock, on or under the surface of the Earth, are known as karst landscapes. Water containing dissolved carbon dioxide is acidic. cidic water can dissolve calcium carbonate (calcite) found in limestone. Most caves are formed by this process. When water drips inside caves, the dissolved calcium carbonate forms deposits. B B sinkhole (doline) cave Stalactites are deposits which grow downward from the cave ceiling. Stalagmites are deposits which grow upward from the cave floor.. Sinkholes or dolines are depressions which are produced by the collapse of underground cavities. B. Caves are large underground cavities where stalactites and stalagmites form. 7. How do oceans shape landscape? beach cliff There are three types of ocean water motions which affect coastal landscapes: Waves are effective agents of erosion, transportation and deposition of rock particles and sand. Wave action shapes sea cliffs which form different coastal landforms. spit sand bar arch Tides are effective agents of transportation. During high tide, waves pick up and carry sand and other materials from beaches. The waves deposit this material when the tide is lower. Currents are effective agents of transportation. Waves undercut the base of sea cliffs. This causes blocks of rock to fall, resulting in cliff recession. The accumulation of sand from erosion forms beaches. 13. What is the main agent which forms karst landscapes? 14. What are the three main water motions which affect coastal landscapes? 93
8. How are sedimentary rocks formed? Sedimentary rocks are generally formed through the following processes: 1 Sedimentation. Layers of sediment are deposited over millions of years. 2 Compaction. The weight of the sediments on top compresses the sediments at the bottom. The water is squeezed out from between the pieces of rock. s a result, crystals of different salts form. 3 Cementation. The crystals form a sort of glue which sticks or cements the pieces of rock together. 1 water 2 Sedimentation Compaction 3 Cementation sedimentary rock limestone sandstone conglomerate gypsum Types of sedimentary rocks 15. Copy and complete the definitions. 16. Why do sedimentary rocks form in layers? The formation of sedimentary rocks: 1 2 3 Sedimentation: 17. If you look at a large piece of sedimentary rock, which layer was the earliest one formed? Which was the most recent layer formed? 18. Explain the difference between compaction and cementation. Compaction occurs when the weight of the sediments on top... 94
Hands on Experimental models: Cliff recession Materials large, rectangular tray sand water bucket spade 1. Prepare the model. First, pack wet sand in the bucket until it is full. Next, turn the bucket upside down in the tray. The sandcastle represents the cliff. Then, fill the tray half way up with water. Finally, use the spade to make gentle waves. The waves should hit the base of the sandcastle. 2. Observe the process. Continue to make waves for about ten minutes. During this time, you can observe these processes: The waves will undercut the base of the sandcastle. Small blocks or pieces of sand will fall off. The fallen sand will accumulate at the bottom or form little islands. s the fallen sand is moved by the waves, it will form a submerged, sandy surface. 3. Compare the model to the real process. On a real cliff, fragments of rock accumulate as they are broken off by the action of the waves. In the model, this process is represented by blocks or pieces of fallen sand broken off by the waves. part of the sandcastle is left projecting over the water. In the case of a real cliff, a beach could form under this projection. 19. Record the stages of recession in the sandcastle cliff. dd drawings or photos of the experiment. Label the changes which take place in the sandcastle cliff. 20. Do you think cliff recession could be a problem for people living in coastal areas? Explain your answer. 95
+ 21. Copy the following drawings in your notebook. 28. What is the difference between stalactites and stalagmites? Where do you find them? 29. Copy and complete. How rivers shape landscape 1 2 3 dd labels to explain the process, and indicate the type of weathering. 22. Find examples of chemical, mechanical or biological weathering near where you live. Which type of weathering is most dominant? Why is this explained by the climate? Course Velocity Processes Results Upper course Middle course Lower course 30. Copy the following drawing of a glacier in your notebook. 23. On the Iberian Peninsula, many cliffs are eroding because of wave motion. However, not all of them are eroding at the same rate. Why? 24. Can chemical weathering continue during transportation? Explain how. 25. Copy the drawing in your notebook. Label the cirque and the glacial tongue, and indicate where the moraine has been deposited. 31. Copy this diagram of a cliff. Include an explanation of the process it shows. Indicate whether it shows erosion or sedimentation, and what the result will be. 26. What type of weathering would dominate in a region with a cold, dry climate? Explain. 27. The Gran Canyon was formed by the Colorado River over millions of years. Find out how long it is, and how wide it is at its narrowest and widest points. dd labels to explain the erosion process represented. Describe the formations which erosion is causing. 32. Can a beach be formed in a coastal area with strong currents? Explain why or why not. 96
What should you know? 8 Weathering Mechanical weathering breaks rocks by physical forces. Chemical weathering decomposes rocks by chemical reactions. Biological weathering breaks down rocks by the action of living things. EXTERNL DYNMICS OF THE ERTH Erosion, transportation and deposition Shaping of the landscape Erosion moves weathered pieces of rocks to a different place. Transportation moves eroded rock material along surfaces or in suspension. Deposition deposits moving rock materials. This occurs when there is a decrease in energy in the transporting process. Wind may erode, transport and deposit rock materials to change the landscape. It is only an effective agent in places where it is strong, usually in dry regions with scant vegetation. Glaciers are masses of moving ice. They cut U-shaped valleys known as glacial valleys. Rivers also carry out erosion, transportation and deposition which change the landscape. The dominant process depends on the velocity of the water. Rainwater is a very effective agent for eroding and shaping relief features in areas with dry climates and torrential rain. Groundwater causes the dissolution of limestone rock and forms karst landscapes. Ocean waves, tides and currents shape coastal landscapes. Formation of sedimentary rock Sedimentary rock is formed from sediments through the following processes: Sedimentation Compaction Cementation Projects INVESTIGTE: Write a report on ways of preventing damage from rainwater, groundwater and wind. INVESTIGTE: What relationship is there between weathering, acid rain, and the deterioration of monuments? WEB TSK: Describe a trip to a glacier or to a desert. 97