Express EARTH SCIENCE and SP CE The breakdown of rocks by physical means. 1
Mechanical Weathering Upon successful completion of this unit, you will be able to do the following: Explain how weathering and mechanical weathering are related. List and explain the 3 types of mechanical weathering. Describe how plants and animals affect mechanical weathering. Describe how gravity affects mechanical weathering. Explain the process of ice wedging. Be able to use the following terms in their correct context: abrasion animal plant ice wedging weathering cells mechanical weathering soil rocks 2
Mechanical Weathering Background The rock layers buried deep within the Earth s crust were formed by the combined actions of extreme heat and pressure. When geologic forces push these rock strata to the Earth s surface, they become exposed to natural weathering conditions. These natural weathering conditions break down the rocks to form soil. DO THIS: Examine the illustration below that shows volcanic mountain formation. Note that the volcanoes bring new rock to the surface. Deep-sea trench 3
Weathering Weathering is the process where rocks are broken down into smaller pieces. There are two types of weathering: 1. mechanical 2. chemical There are 2 types of weathering: mechanical and chemical. Question 1. What is weathering? 4
Mechanical Weathering Mechanical weathering occurs when rocks are broken into smaller pieces by physical means. There are 3 types of mechanical weathering: 1. plant and animal activity 2. abrasion 3. ice wedging 1. Plant and animal activity breaks rocks into smaller pieces in a variety of ways. Plants develop strong roots that grow into existing rock cracks. As the roots grow, they expand and add pressure to the rock until small pieces of the rock begin to flake away or break apart. DO THIS: Examine the picture of a tree growing on a rock. Note that the roots are creating large cracks in the rock. 5
Burrowing animals, like moles and rabbits dig holes that expose new rocks to the effects of weathering. The holes allow water and other weathering agents to reach the rock layer that had been covered by the soil. DO THIS: Examine this picture showing prairie dogs burrowing into the soil. Note the small rocks that are now exposed. Both plants and animals are able to break rock into smaller pieces. Sometimes the smaller pieces are merely flakes. Mechanical weathering is the process where large rocks are made into smaller rock. 6
Plants and animals can mechanically weather rock by breaking off small flakes and by exposing covered rock to other types of weathering. Question 2. Explain how plants can perform mechanical weathering. Earthworms are a major factor in the weathering of rock. Earthworms tunnel through the soil looking for food. As they tunnel, they bring rock flakes to the surface and expose covered rock to the weathering effects of air and water. 7
Question 3. How do animals contribute to mechanical weathering? 2. Abrasion is the grinding of rocks. Abrasion may occur because of the action of wind, running water, waves, and gravity. Wind, running water, and waves push smaller particles of rock into other rock eventually causing pieces to chip away. Rocks fall and tumble as gravity pulls them down hill. When this happens, the rocks chip or break apart. DO THIS: Examine the picture below. Note how the wind has weathered the rock. 8
Rocks are weathered when small pieces of rock are pushed across other rock or when a rock falls. Question 4. What makes abrasion a form of mechanical weathering? The Grand Canyon continues to be weathered by the action of the small grains of rock that are carried along by the Colorado River. 9
Question 5. What are some examples of abrasion? 3. Ice wedging occurs when water seeps into rock cracks and freezes. Interestingly, water EXPANDS when it freezes. As the water freezes in the rock crack, sections of rock are broken off. Ice wedging is common in areas where the climate includes a cold season. DO THIS: Examine the picture below that shows ice wedging. Note how the water can freeze and expand in rock cracks. 10
Ice wedging breaks rocks because ice gets larger when it freezes in the rock crack. Question 5. Explain how ice wedging can break rocks. 11
Summary Mechanical Weathering Soil is formed by the breakdown of rocks. Mechanical weathering is the breaking down of rocks into smaller pieces by physical means. There are 3 types of mechanical weathering: 1. Plant and animal activity 2. Abrasion 3. Ice wedging Plants break rocks when their roots grow into rock cracks and grow larger over time. Animals help to break rocks by burrowing the soil and bringing small rocks to the surface and exposing covered rocks. Abrasion is the grinding of rocks and other debris against other rocks. Moving water and wind push small pieces of rock into or across other rock causing abrasion. Gravity causes rocks to fall and tumble breaking the rock. Ice wedging is the process where water freezes in the rock cracks and causes the rock to break apart. Ice expands when it freezes. 12
Test Yourself True or False 1. Ice wedging occurs because of animals. 2. Weathering is the process where rock is broken down. 3. Mechanical weathering creates rocks with new chemicals. 4. Gravity pulls rocks causing them to ice wedge. 5. Running water pushes small particles which cause abrasion. 6. Animals that dig tunnels do not help to break down rocks. 7. Rocks weather to become soil. 8. Ice wedging works because water expands when it freezes. 9. Plant roots are not able to break rocks because they are too deep. 10. Abrasion is the grinding of rocks against other rocks. 13
Fill in the Blank Note: Some of the words may be used more than once or not at all. abrasion animal plant ice wedging weathering cells mechanical weathering soil rocks 1. Rocks are broken down by weathering to form. 2. The roots of and the digging of are types of mechanical weathering. 3. are broken down by a process known as. 4. is a special type of weathering that works by physical means. 5. When water freeze in the rock cracks and causes a rock to break apart, this process is known as. 14
Matching Note: Some letters may be used more than once or not at all. 1. ice wedging 2. weathering 3. animal activity 4. abrasion 5. mechanical weathering 6. soil 7. plant activity 8. gravity a. rocks rubbing against rocks b. exposes covered rocks c. works because water expands when it freezes d. plate tectonics e. falling and breaking f. continental drift g. breakdown of rocks h. physical breakdown of rocks i. final product of mechanical weathering j. roots grow into rock cracks and break the rock 15
Constructed Response 1. Explain how weathering and mechanical weathering are related. 2. List and explain the 3 types of mechanical weathering. 3. Explain why soil is the final result of mechanical weathering. 16