Chapter 7 Section 1 & 2 Weathering PDF

24/11/01 Chapter 7 Section 1 & 2 How do mechanical and chemical weathering differ? 1  24/11/01 What variables affec...

24/11/01 Chapter 7 Section 1 & 2 How do mechanical and chemical weathering differ? 1  24/11/01 What variables affect the rate of weathering? Mechanical Weathering Weathering is the process in which materials on or near Earth’s surface break down and change. Mechanical weathering is a type of weathering in which rocks and minerals break down into smaller pieces. It does not change a rock’s composition. Copyright © McGraw-Hill Education Weathering 2  24/11/01 Mechanical Weathering Effect of temperature When water freezes, it increases in volume. If the temperature drops to the freezing point, water that has collected in the cracks of rocks or rock layers freezes, expands, and exerts pressure on the rocks, which can cause the cracks to widen slightly. When the temperature increases, ice melts in the cracks of rocks and rock layers. The freeze-thaw cycles of water in the cracks of rocks is called frost wedging. Copyright © McGraw-Hill Education Weathering Mechanical Weathering Effect of pressure On a small scale, plant or tree roots wedged in the cracks of rocks exert pressure as they grow and expand, which often causes the rocks to split. On a larger scale, when overlying rock layers are removed by processes such as erosion or even mining, the pressure on the bedrock is reduced. The bedrock surface that was buried expands, and long, curved cracks can form. Copyright © McGraw-Hill Education Weathering 3  24/11/01 Mechanical Weathering Effect of pressure Exfoliation is a mechanical weathering process in which outer rock layers are stripped away, often resulting in dome-shaped formations. Copyright © McGraw-Hill Education Weathering Chemical Weathering Chemical weathering is the process by which rocks and minerals undergo changes in their composition. Agents of chemical weathering include water, oxygen, carbon dioxide, and acid precipitation Copyright © McGraw-Hill Education Weathering 4  24/11/01 Chemical Weathering The composition of a rock determines the effects that chemical weathering will have on it. Temperature is a significant factor in chemical weathering because it influences the rate at which chemical reactions occur. Copyright © McGraw-Hill Education Weathering Chemical Weathering Effect of water Water is an important agent in chemical weathering because it can dissolve many kinds of minerals and rocks. Water serves as a medium in which chemical reactions can occur, and it can also react directly with minerals in a chemical reaction. Copyright © McGraw-Hill Education Weathering 5  24/11/01 Chemical Weathering Effect of oxygen The chemical reaction of oxygen with other substances is called oxidation. Iron in rocks and minerals combines with oxygen to form minerals with the oxidized form of iron. Hematite is a common example. Copyright © McGraw-Hill Education Weathering Chemical Weathering Effect of carbon dioxide When carbon dioxide combines with water in the atmosphere, it forms a very weak acid called carbonic acid that falls to Earth’s surface as precipitation. The slight acidity of precipitation can cause it to dissolve certain rocks, such as limestone. When slightly acidic water from precipitation seeps into the ground, it combines with carbon dioxide in the soil, and becomes a stronger acid. Carbonic acid slowly reacts with minerals such as calcite in limestone to dissolve rocks. Copyright © McGraw-Hill Education Weathering 6  24/11/01 Chemical Weathering Effect of acid precipitation Sulfur dioxide, carbon dioxide, and nitrogen oxides are released into the atmosphere by human activities. When these gases combine with water and oxygen in the atmosphere, sulfuric, carbonic, and nitric acids form and cause acid precipitation. Copyright © McGraw-Hill Education Weathering Chemical Weathering Effect of acid precipitation Because strong acids can be harmful to many organisms and destructive to human-made structures, acid precipitation often creates problems, such as making forests more vulnerable to disease. Copyright © McGraw-Hill Education Weathering 7  24/11/01 Rate of Weathering The natural weathering of Earth materials occurs slowly. However, certain conditions and interactions can accelerate or slow the weathering process. Copyright © McGraw-Hill Education Weathering Rate of Weathering Effects of climate on weathering The interaction between temperature and precipitation in a given climate determines the rate of weathering in a region. Copyright © McGraw-Hill Education Weathering 8  24/11/01 Rate of Weathering Rates of chemical weathering Warm, lush areas such as the tropics experience the fastest chemical weathering. Copyright © McGraw-Hill Education Weathering Rate of Weathering Rates of physical weathering Physical weathering can break down rocks more rapidly in cool climates. Conditions in such climates do not favor chemical weathering because cool temperatures slow or inhibit chemical reactions. Copyright © McGraw-Hill Education Weathering 9  24/11/01 Rate of Weathering Rock type and composition Not all the rocks in the same climate weather at the same rate. The effects of climate on the weathering of rock also depend on the rock type and composition. Copyright © McGraw-Hill Education Weathering Rate of Weathering Surface area Mechanical weathering breaks rocks into smaller pieces. As the pieces get smaller, their surface area increases, resulting in more total surface area available for chemical weathering. Copyright © McGraw-Hill Education Weathering 10  24/11/01 Rate of Weathering Topography The slope of a landscape helps determine the rate of weathering. Steep slopes promote erosion and continually expose more rocks to weathering. Copyright © McGraw-Hill Education Weathering Gravity’s Role The removal of weathered rock and soil from its original location is a process called erosion. Erosion can remove material through a number of different agents, including running water, glaciers, wind, ocean currents, and waves. Copyright © McGraw-Hill Education Erosion and Deposition 11  24/11/01 Gravity’s Role After the rock and soil are transported, sometimes thousands of kilometers away from their source, they are dropped in another location in a process known as deposition. Gravity is associated with many erosional agents because the force of gravity tends to pull all materials downslope. Copyright © McGraw-Hill Education Erosion and Deposition Erosion by Water Stream erosion can reshape entire landscapes. Water flowing down steep slopes has additional erosive potential resulting from gravity, causing it to cut downward into the slopes, carving steep valleys and carrying away rock and soil. Copyright © McGraw-Hill Education Erosion and Deposition 12  24/11/01 Erosion by Water Rill erosion develops when running water cuts small channels into the side of a slope. When a channel becomes deep and wide, rill erosion evolves into gully erosion. Copyright © McGraw-Hill Education Erosion and Deposition Erosion by Water Rivers and streams Once a river enters the ocean, the current slows down, which reduces the potential of the stream to carry sediment. As a result, rivers deposit large amounts of sediment near the region where they enter the ocean. Over time, deltas form. Copyright © McGraw-Hill Education Erosion and Deposition 13  24/11/01 Erosion by Water Wave action The work of ocean currents, waves, and tides carves out cliffs, arches, and other features along the continents’ edges. The constant movement of water and the availability of accumulated weathered material result in a continuous erosional process. Deposition of sand particles forms features such as beaches and sandbars; erosion removes sand from these features. Copyright © McGraw-Hill Education Erosion and Deposition 14