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Chapter Five Weathering, Soil, and Mass Movements.

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1 Chapter Five Weathering, Soil, and Mass Movements

2 Weathering Weathering is a basic part of the rock cycle and a key process in the Earth system. –Weathering: The breaking down and changing of rocks at or near Earth’s surface. There are two types of weathering: 1.Mechanical Weathering 2.Chemical Weathering Even though they are different, they at are work at the same time.

3 Weathering Mechanical Weathering Occurs when physical forces break rock into smaller and smaller pieces without changing the rock’s mineral composition. –Each piece has the same composition. –Breaking a rock into smaller pieces increases the surface area of the rock. In nature, three physical processes are especially important causes of mechanical weathering: 1.Frost wedging 2.Unloading 3.Biological activity

4 Weathering 1.Frost Wedging: When liquid freezes, it expands by about 9%, exerting a tremendous outward force. When water freezes in the cracks of rocks, it enlarges the cracks. This process is known as frost wedging. It is most common in mountainous regions in the middle latitudes. Sections of rock that are wedged loose may tumble into large piles called talus, which typically form at the base of steep, rocky cliffs.

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6 2. Unloading: Large masses of igneous rocks may be exposed through uplift and erosion of overlying rock. When this happens, the pressure exerted on the igneous rock is reduced. This is known as unloading. Unloading causes the outer layers of the rock to expand more than the rock below. Slabs of outer rock separate like layers of an onion and break loose in a process called exfoliation. –Especially common in granite. It often produces large, domed shaped rock formations. –Ex: Stone Mountain in Georgia and Liberty Cap in Yosemite National Park.

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8 3. Biological Activity: The activities of organisms, including plants, burrowing animals, and humans can also cause mechanical weathering. Examples: –Tree roots. –Burrowing animals move rocks to the surface. –Decaying organisms produce acids which cause chemical weathering. –Humans accelerate weathering through deforestation and blasting.

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10 A.Differential Weathering B.Chemical Weathering C.Mechanical Weathering D.Erosion The process that occurs when physical forces break rock into smaller pieces without changing the rock’s chemical composition is called

11 A.Unloading B.Frost Wedging C.Exfoliation D.Spheroidal Weathering Which of the following weathering processes involves the constant freezing and thawing of water?

12 Which of the following is not associated with mechanical weathering? A.Frost Wedging B.Unloading C.Biological Activity D.Reactions with Oxygen

13 What is responsible for the formation of exfoliation domes? A.Frost Wedging B.Biological Activity C.Reactions with Oxygen D.Unloading

14 What type of mechanical weathering is most common in mountainous regions in the middle latitudes? A.Frost Wedging B.Biological Activity C.Oxidation D.Unloading

15 When water freezes, its volume A.Decreases slightly. B.Increases. C.Stays the same. D.Decreases greatly.

16 Weathering Chemical Weathering Chemical weathering is the transformation of rock into one or more new compounds. The new compounds remain mostly unchanged as long as the environment in which they formed does not change. Water is the most important agent of chemical weathering. Water promotes chemical weathering by absorbing gases from the atmosphere and the ground. These dissolved substances then chemically react with various minerals. Oxygen dissolved in water reacts easily with certain minerals, forming oxides. –Ex: Iron-rich minerals get a yellow to reddish-brown coating of iron oxide (rust) when they react with oxygen.

17 Weathering Water also absorbs carbon dioxide when rain falls through the atmosphere. Water that seeps into the ground also picks up carbon dioxide. The dissolved carbon dioxide forms carbonic acid, which reacts with many common minerals. –Carbonic acid is a weak acid found in carbonated drinks.

18 Weathering Water can also absorb the sulfur oxides in the atmosphere produced by the burning of fossil fuels (coal and petroleum). Through a series of chemical reactions, these pollutants are changed into acids that cause acid precipitation. Acid precipitation accelerates the chemical weathering of stone monuments and structures.

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20 When granite (feldspar/quartz) is exposed to water containing carbonic acid, the feldspar is converted mostly into clay minerals. As the feldspar is converted into clay, the quartz grains are released from the granite. Sometimes it is then transported by rivers to the ocean where the tiny particles are carried far from shore and the quartz crystals are deposited near the shore where they become the main component of beaches and sand dunes.

21 Weathering When silicate minerals undergo chemical weathering, the sodium, calcium, potassium, and magnesium they contain dissolve and are carried away by groundwater. The three remaining elements are aluminum, silicon, and oxygen which usually combine with water and produce clay minerals.

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23 Chemical weathering can change the physical shape of rock as well as its chemical composition. –Ex: When water enters along the joints in a rock, it weather the corners and edges most rapidly. As a result, the corners and edges become more rounded. This process is called spheroidal weathering. –The rock takes on a spherical shape. Spheriodal weathering sometimes causes the outer layers of a rock to separate from the rock’s main body. Similar to exfoliation except that they are chemically weathered.

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25 Which of the following is not related to chemical weathering? A.Frost wedging. B.Dissolution C.Reactions with oxygen D.Reactions with water

26 The gradual rounding of the corners and edges of angular blocks of rock is called A.Exfoliation B.Unloading C.Spheroidal Weathering D.Mechanical Weathering

27 Which of the following is the result of chemical weathering? A.A rock that has been changed into one or more new compounds. B.A rock that has been broken into tiny pieces. C.A rock that has been split in two. D.A rock that has lost its outer layers.

28 The chemical weathering of feldspar produces A.Quartz. B.Iron oxide C.Clay minerals D.Calcium carbonate

29 Whenever the characteristics and chemical composition of weathered materials have been altered, they have undergone A.Chemical weathering. B.Mechanical weathering. C.Mass movement. D.Frost wedging.

30 The atmospheric gas that forms a mild acid when dissolved in water is A.Carbon Dioxide B.Oxygen C.Aluminum D.Sulfur

31 Weathering Mechanical weathering affects the rate of chemical weathering. By breaking rocks into smaller pieces, chemical weathering is increased due to the increased amount of surface area of the rock. Two other factors that affect the rate of weathering are: 1.Rock characteristics 2.Climate

32 Weathering 1.Rock Characteristics: Physical characteristics of rock (cracks) are important in weathering because they influence the ability of water to penetrate rock. Mineral composition also affects the rate of weathering. –Ex: Granite vs. Marble; granite is relatively resistant to chemical weathering while marble is not very resistant to chemical weathering (reacts with weak acids).

33 Weathering 2. Climate: Climatic factors, especially temperature and moisture, have a strong effect on the rate of weathering (mechanical and chemical). The climate most favorable for chemical weathering has high temperature and abundant moisture. –Slow in arid and polar regions.

34 Weathering Different parts of a rock mass often weather at different rates. This process, called differential weathering, has several causes. Differences in mineral composition in one cause. –More resistant rock protrudes as pinnacles, or high peaks. Variations in the number and spacing of cracks in different parts of a rock mass is another cause of differential weathering.

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36 What would cause the inscription on a marble gravestone to become harder and harder to read over time? A.Frost wedging B.Mechanical weathering C.Exfoliation D.Chemical weathering

37 Which of the following is not a factor that affects the rate of weathering in rocks? A.Biological evolution B.Climate C.Rock characteristics D.Amount of exposed surface area

38 Which of these factors affects the rate of weathering? A.Climate B.Chemical composition of the exposed rock. C.Surface area of the exposed rock. D.All of the above.

39 Rock features such as the sculpted pinnacles seen in Bryce Canyon National Park are the result of A.Exfoliation. B.Differential Weathering. C.Unloading. D.Frost Wedging.

40 Chemical weathering would be A.Most effective in a warm, dry climate. B.Most effective in a cold, dry climate. C.Most effective in a warm, humid climate. D.Equally effective in any climate.

41 If granite and marble were exposed in an area with a hot and humid climate, A.The granite would weather most rapidly. B.The marble would weather most rapidly. C.Both rocks would weather at the same rate. D.Neither rock would become weathered.

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43 Soil Soil is an important product of weathering. All life depends on a dozen or so elements that come from Earth’s crust. Weathering produces a layer of rock and mineral fragments called regolith. Soil is the part of the regolith that supports the growth of plants. Three important characteristics of soil are its: 1.Composition 2.Texture 3.Structure

44 Soil 1.Soil Composition: Soil has four major components: –Mineral Matter (Broken-down rock) –Organic Matter (Humus – decayed organisms) –Water –Air The percentages of the four major components varies greatly. In most soils, organic matter (humus) is an essential component for plants nutrients and the soil’s ability to retain water.

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46 2. Soil Texture: Most soils contain particles of different sizes. Soil texture refers to the proportions of different particles sizes. To classify soil texture, the U.S. Department of Agriculture has established categories based on the percentages of clay, silt, and sand in soil. Texture strongly influences a soil’s ability to support plant life.

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48 3. Soil Structure: Soil particles usually form clumps that give soils a particular structure. Soil structure determines how easily a soil can be cultivated and how susceptible it is to erosion. Soil structure also affects the ease with which water can penetrate the soil and thus influences the movement of nutrients to plant roots.

49 Which of the following is not a major component of soil? A.Mineral matter B.Air C.Humus D.Earthworms

50 A soil’s texture is determined by A.Mineral composition B.Type of humus C.Water content D.Particle size

51 The main source of organic matter in soil is A.Water B.Plants C.Fungi D.Bacteria

52 Soil Soil forms through the complex interaction of several factors. The most important factors in soil are: –Parent material –Time –Climate –Organisms –Slope

53 Soil 1.Parent Material: The source of the mineral matter in soil. –May be either bedrock (residual soil) or unconsolidated deposits such as those in a river valley (transported soil). The nature of the parent material influences soils in two ways: –It affects the rate of weathering and the rate of soil formation. –The chemical makeup of the parents material affects the soil’s fertility. Fertility influences the types of plants the soil can support.

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55 2. Time: The longer a soil has been forming, the thicker it becomes. The parent material largely determines the characteristics of young soils. As weathering continues, the influence of the parent material can be overshadowed by the other factors, especially climate.

56 Soil 3. Climate: Climate has the greatest effect on soil formation. Variations in temperature and precipitation influence the rate, depth, and type of weathering. The influence of climate is so great that soil scientists have found that similar soils can be produced from different parent materials in the same climate. Dissimilar soils can be produced from the same parent material in different climates.

57 Soil 4. Organisms: The types of organisms and how many there are in a soil have a major impact on its physical and chemical properties. Scientists name some soils – such as prairie soil, forest soil, and tundra soil – based on the soil’s natural vegetation. Plants are the main source of organic matter in soil (animals/microorganisms are other sources). Microorganisms (fungi, bacteria, and single-celled protozoans) play an active role in decomposing dead plants and animals (nitrogen gas into nitrogen compounds). Burrowing animals mix the mineral and organic matter in soil. –Example: Earthworms – can mix thousands of kilograms of soil each year in a single hectare (10,000 square meters).

58 Soil 5. Slope: The slope of the land can vary greatly over short distances, which results in different soil types. Many of the differences are related to the amount of erosion and the water content of the soil. On steep slopes, erosion is accelerated. –Little water soaks in, little to no plants, which results in thin or nonexistent soils. In flat areas, there is little erosion and poor drainage. –Resulting in waterlogged soils that are typically thick and dark (large amounts of organic matter).

59 Soil The direction the slope faces also affects soil formation. In the temperate zone of the Northern Hemisphere, south-facing slopes receive much more sunlight than do north-facing slopes. Soils on south-facing slopes are usually warmer and drier, which influences the types of plants that grow in the soil.

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61 The factor that has the greatest effect on soil formation is A.Climate B.Parent material C.Time D.Slope orientation

62 In which of the following areas will soil formation be greatest? A.A steep slope in a warm, wet climate. B.A flat area in a cold, wet climate. C.A flat area in a warm, wet climate. D.A north-facing area on a steep slope.

63 Soil that forms on unconsolidated deposits is called A.Transported soil B.Humus C.Residual soil D.Bedrock

64 Soil The processes that form soil operate from the surface downward. Soil varies in composition, texture, structure, and color at different depths. These variations divide the soil into zones known as soil horizons. A vertical section through all of the soil horizons is called a soil profile. In some soil profiles, the soil horizons blend gradually from one to another, in others, they are quite distinct. Mature soils usually have three distinct soil horizons (A, B, and C Horizons).

65 Soil 1.A Horizon: Commonly known as topsoil. Upper part consists mostly of organic matter. Full of insects, fungi, and microorganisms. Lower part is a mixture of mineral matter and organic matter. 2. B Horizon: Commonly known as subsoil. Contains fine clay particles washed out of the A Horizon. In some soils, the clay that accumulates in the B Horizon forms a compact, impenetrable layer called hardpan. Is the lower limit of most plant roots and burrowing animals. 3.C Horizon: Between the B Horizon and the unaltered parent material. Contains partially weathered parent material. Resembles parent material.

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67 In a well-developed soil profile, which horizon is the uppermost layer? A.The C Horizon. B.The B Horizon. C.The A Horizon. D.The Parent Horizon.

68 What kind of material is found in the C horizon of a soil profile? A.Partially weathered parent material. B.Clay Particles. C.Hardpan. D.Mineral and organic matter.

69 How are soil horizons ordered from the top of the profile to the bottom? A.A, C, B B.A, B, C C.C, B, A D.B, A, C

70 The B horizon is also called the A.Topsoil B.Unaltered parent material C.Partially altered parent material D.Subsoil

71 Soil Climate has a major effect on the type of soil that forms. Three common types of soil are: –Pedalfer –Pedocal –Laterite 1.Pedalfer: Usually forms in temperate areas that receive more than 63-cm (25 inches) of rain each year. Present in much of the eastern half of the U.S., mostly in forested regions. The B Horizons in pedalfers contain large amounts of iron oxide and aluminum-rich clays, giving it a brown to red- brown color.

72 Soil 2. Pedocal: Found in the drier western U.S. in areas that have grasses and brush vegetation. Generally contains less clay than pedalfers. Contain abundant calcite, or calcium carbonates, and are typically a light gray-brown. 3. Laterite: Form in hot, wet tropical areas. Are usually deeper than soils that develop over a similar period in temperate areas. The large quantity of water that filters through these soils removes most of the calcite and silica. Iron oxide and aluminum oxide are left behind. The iron oxide gives laterite a distinctive orange to red color. When dried, laterite becomes very hard and practically waterproof (bricks).

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74 Laterite contains almost no organic material. With the lack of organic material, the soil cannot support agriculture for more than a few years. The nutrients that the soil does have are quickly washed out by the plentiful rainwater that filters through the soil.

75 A soil that is characteristic of the humid eastern U.S. is A.Laterite. B.Pedalfer. C.Pedocal. D.Humus.

76 The soil associated with the hot and wet tropics is A.Laterite B.Pedocal C.Pedalfer D.Bedrock soil

77 Laterite soils contain high amounts of A.Organic material B.Iron oxide C.Calcite D.Calcium carbonate

78 Pedalfer soils would most likely be found A.On an island close to the equator. B.In a tropical rainforest. C.In the dry areas of the western U.S. D.In the eastern half of the U.S.

79 Which of the following is not true of laterite soils? A.They form in the wet tropics. B.They are red in color. C.They are enriched in iron oxide. D.They are very productive agriculturally.

80 Soil Soils are among our most abused resources. The loss of fertile topsoil is a growing problem as human activities disturb more of Earth’s surface. Water erodes soil every time it rains (tiny bombs). When water flows across the surface it then carries away dislodged particles, which is called sheet erosion.

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82 After flowing as a thin sheet for a short distance, the water forms tiny streams called rills. As more water enters the rills, they erode the soil further, creating trenches known as gullies.

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84 Human activities that remove natural vegetation, such as farming, logging, and construction, have greatly accelerated erosion. Without plants, soil is more easily carried away by wind and water. Scientists can estimate the rate of erosion due to water by measuring the amount of sediment in rivers. These estimates indicate that before humans appeared, rivers carries about 9 trillion kilograms of sediment to the oceans each year. The amount of sediment currently transported to the sea by rivers is about 24 trillion kilograms per year.

85 Soil Wind generally erodes soil much more slowly than water does. During a long drought, strong winds can remove large quantities of soil from unprotected fields. –Example: 1930’s Great Plains Dust Bowl. The rate of erosion depends on soil characteristics and on factors such as climate, slope, and type of vegetation. In many regions, including about one-third of the world’s croplands, soil is eroding faster than it is being formed. This results in lower productivity, poorer crop quality, and a threatened world food supply.

86 Soil Another problem caused by erosion is the deposition of sediment. Rivers that accumulate sediment must be dredged to remain open for shipping. As sediment settles in reservoirs, they become less useful in storing water, controlling floods, and generating electricity. Some sediments are contaminated with agricultural pesticides. Sediments also contain soil nutrients, which may come from natural processes and from added fertilizers. –Excessive nutrient levels in lakes stimulate the growth of algae and plants, which accelerates a process that eventually leads to the early death of the lake.

87 Soil We can significantly slow erosion by using soil conservation measures: –Preserve environments –Protect the land. These measures include planting rows of trees (windbreaks), plowing along the contours of hills, and rotating crops. Preserving fertile soil is essential to feeding the world’s rapidly growing population.

88 Compared to the past, rates of soil erosion are A.About the same. B.Faster. C.Slower. D.More unpredictable.

89 Which of the following human activities has caused an increase in soil erosion? A.Clear-cut logging. B.Clearing land for construction. C.Plowing land for farming. D.All of the above.

90 The rate of soil erosion depends on A.Climate. B.Slope steepness. C.The type of vegetation. D.All of the above.

91 Since humans have appeared, the amount of sediments carried by rivers has A.Increased dramatically. B.Increased slightly. C.Stayed about the same. D.Decreased by about half.

92 What is the correct order for water eroding soil? A.Gullies, rills, sheet erosion. B.Sheet erosion, rills, gullies. C.Sheet erosion, gullies, rills. D.Rills, sheet erosion, gullies.

93 Mass Movements The transfer of rock and soil downslope due to gravity is called mass movement. –Ex: Landslides The combination of weathering and mass movement produce most landforms. Once weathering weakens and breaks rock apart, mass movement moves the debris downslope. There a stream usually carries is away. Stream valleys are the most common of Earth’s landforms.

94 Mass Movements Several factors make slopes more susceptible to the pull of gravity. –Saturation of surface materials with water. –Oversteepening of slopes. –Removal of vegetation. –Earthquakes. 1.Water: Heavy rains and rapid melting of snow can trigger mass movement by saturating surface materials with water. When the pores in sediment become filled with water, the particles slides past one another easily.

95 Mass Movements 2. Oversteepened Slopes: Loose soil particles can maintain a relatively stable slope up to a certain angle (25 to 40°), depending on the size and shape of the particles. If the steepness of the slope exceeds the stable angle, mass movement is likely. Such slopes are said to be oversteepened. This can result when: A.Streams undercut a valley wall. B.Waves pound against the base of a cliff. C.People, through excavation during construction of roads/buildings.

96 Mass Movements 3. Removal of Vegetation: Plants make slopes more stable because of their root systems. When plants are removed, mass movements are likely. 4. Earthquakes: Earthquakes are one of the most dramatic triggers of mass movements. They can dislodge rock and unconsolidated material, which can cause more damage than the earthquake itself.

97 The process responsible for moving material downslope under the influence of gravity is called A.Erosion B.Weathering C.Mass movement D.Soil formation

98 What is the force behind mass movements? A.The Sun’s energy B.Flowing water C.Gravity D.Moving ice

99 Which of the following is not true about mass movements? A.Some mass movements are too slow to be seen. B.Mass movements always lead to landslides. C.Gravity is the driving force behind all mass movements. D.Mass movements are always downslope.

100 What factor commonly triggers mass movements? A.Saturation of surface materials with water. B.Earthquakes C.Removal of vegetation D.All of the above

101 Why can the removal of vegetation trigger mass movements? A.The soil loses nutrients and begins the crumble. B.The plant roots bind the soil and regolith together. C.The shaking triggers mass movements. D.The plant roots lubricate the loose sediment.

102 Oversteepened slopes often lead to mass movements because A.Plants cannot grow on them. B.The angle of their slope is between 10 and 20 degrees. C.The angle of their slope is less than 20 degrees. D.The angle of their slope is greater than 40 degrees.

103 During what season would you expect mass movements to be a greater threat? A.A dry summer. B.A wet spring before vegetation is growing. C.A wet spring with lots of growing vegetation. D.A dry autumn after the leaves have turned.

104 Mass Movements Geologists classify mass movements based on the kind of material that moves, how is moves, and the speed of the movement. 1.Rockfalls: Occurs when rocks or rock fragments fall freely through the air. Common of slopes that are too steep for loose material to remain on the surface. Result from the mechanical weathering of rock caused by freeze-thaw cycles or plant roots. Sometimes trigger other mass movements.

105 Mass Movements 2. Slides: A block of material moves suddenly along a flat, inclined surface., Slides that include segments of bedrock are called rockslides. Often occur in high mountain areas. –i.e. Andes, Alps, Rockies. Rockslides are among the fastest mass movements (speeds over 200 kmph ≈ 125 mph). Triggered by rain or melting snow.

106 Mass Movements 3. Slumps: Is the downward movement of a block of material along a curved surface. Usually does not travel very fast or very far. Slumps leave a crescent-shaped cliff just above the slump. Common on oversteepened slopes where the soil contains thick accumulations of clay.

107 Mass Movements 4. Flows: Mass movements of material containing a large amount of water, which move downslope as a thick liquid. Flows that move quickly, called mudflows, are common in semiarid mountainous regions. –i.e. Southern California Follows the contours of the canyon, taking trees and boulders with it.

108 Mass Movements Earthflows are flows that move relatively slow – from about a millimeter per day to several meters per day, and may continue for years. Occurs most often on hillsides in wet regions. When water saturates the soil and regolith on a hillside, the material breaks away, forming a tongue- shaped mass. They range in size from a few meters long and less than 1 m deep to over 1 km long and more than 10 m deep.

109 Mass Movements 5. Creep: The slowest type of mass movement. Usually only travels a few millimeters or centimeters per year. –Because it is slow, you cannot directly observe it. Alternating between freezing and thawing contributes to creep. Effects are easy to recognize: –Structures once vertical tilt downhill. –Displacement of fences. –Cracks in walls and underground pipes.

110 A mass movement that involves the sudden movement of a block of material along a flat, inclined surface is called a A.Slide B.Rockfall C.Slump D.Flow

111 When a block of material moves downslope along a curved surface, the type of mass movement is called A.A rockfall B.A rockslide C.A slump D.Creep

112 What is the slowest type of mass movement? A.A slump B.A rockfall C.An earthflow D.Creep

113 A relatively rapid form of mass movement that is most common in dry mountainous regions is A.Creep B.A mudflow C.A slump D.An earthflow

114 Which of the following statements best describes a slump? A.Slippage of a block of material moving along a curved surface. B.Blocks of rock sliding down a slope. C.Rapid flow of water-saturated debris, most common in mountainous regions. D.Slow downhill movement of soil and regolith.

115 Alternate freezing and thawing often leads to A.Creep B.Slumps C.Mudflows D.Earthflows


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