1. EARTH SCIENCE CHAPTER SEVEN WEATHERING, SOIL, AND EROSION

2. 7.1 WEATHERING Weathering is the breakup of a rock due to exposure to processes that occur at Earth’s surface Weathering is the breakup of a rock due to exposure to processes that occur at Earth’s surface Two main types: Two main types: Mechanical weathering (disintegration) occurs when rocks are broken down into smaller pieces but do not change in composition. Mechanical weathering (disintegration) occurs when rocks are broken down into smaller pieces but do not change in composition. Chemical weathering (decomposition) occurs when the rock’s minerals are changed into different substances Chemical weathering (decomposition) occurs when the rock’s minerals are changed into different substances

3. MECHANICAL WEATHERING Several processes: Several processes: Frost wedging Frost wedging Wetting and drying Wetting and drying Abrasion by rock materials Abrasion by rock materials Plant and animal activity Plant and animal activity Exfoliation due to upward expansion Exfoliation due to upward expansion

4. FROST WEDGING Water occupies 10 % more space when it freezes than it does as a liquid. Water occupies 10 % more space when it freezes than it does as a liquid. When water that is trapped inside the cracks and pores rocks freezes, it can break the rock as it expands. When water that is trapped inside the cracks and pores rocks freezes, it can break the rock as it expands. This is called frost wedging or ice wedging. This is called frost wedging or ice wedging.

5. WETTING AND DRYING Liquid water also plays a role in mechanical weathering. Liquid water also plays a role in mechanical weathering. Some rocks that are made heavily of clay and silt (like shale), swell when they are wet and shrink as they dry. Some rocks that are made heavily of clay and silt (like shale), swell when they are wet and shrink as they dry. This repeated action can break the rock over time. This repeated action can break the rock over time.

6. ABRASION Occurs when moving sand, rocks, or pebbles grind and scrape against one another. Occurs when moving sand, rocks, or pebbles grind and scrape against one another. The sand we walk on at the beach was once rock that has been repeatedly broken down and carried by the waves. The sand we walk on at the beach was once rock that has been repeatedly broken down and carried by the waves.

7. PLANT AND ANIMAL ACTIVITY Lichens, plants, and mosses grown on rocks and wedge into their pores—roots can eventually split the rocks. Lichens, plants, and mosses grown on rocks and wedge into their pores—roots can eventually split the rocks. Even trees have been known to grow through boulders.—pic pg 259 Even trees have been known to grow through boulders.—pic pg 259 Many animals (ants, earthworms, rabbits, etc) make holes in the ground that expose rocks to agents like wind, ice, etc. Many animals (ants, earthworms, rabbits, etc) make holes in the ground that expose rocks to agents like wind, ice, etc.

8. UPWARD EXPANSION Upward expansion occurs when granite beneath the earth’s surface is pushed upward by forces below. Upward expansion occurs when granite beneath the earth’s surface is pushed upward by forces below. This extreme force causes rocks on top of the granite sheets to break apart and erode. This extreme force causes rocks on top of the granite sheets to break apart and erode. The granite sheets are then exposed and begin to break and erode also. The granite sheets are then exposed and begin to break and erode also. This is called exfoliation-- This is called exfoliation-- The peeling away of surface layers of exposed bedrock The peeling away of surface layers of exposed bedrock

9. CHEMICAL WEATHERING Can occur anywhere that water or water vapor is present Can occur anywhere that water or water vapor is present Can occur by acids Can occur by acids Can occur by oxygen Can occur by oxygen

10. WATER AND CHEMICAL WEATHERING The reaction of water with other substances is called hydrolysis. The reaction of water with other substances is called hydrolysis. Water readily reacts with some minerals (like feldspar, hornblende, and augite) to dissolve into ions that slowly react with water to form different types of clay minerals. Water readily reacts with some minerals (like feldspar, hornblende, and augite) to dissolve into ions that slowly react with water to form different types of clay minerals.

11. ACIDS AND CHEMICAL WEATHERING Carbonic acid, H 2 CO 3, is an acid commonly found in water. It is formed when CO 2 in the atmosphere dissolves in rainwater. Carbonic acid, H 2 CO 3, is an acid commonly found in water. It is formed when CO 2 in the atmosphere dissolves in rainwater. This acidic mixture of carbonic acid and water breaks down even more minerals to form clay. This acidic mixture of carbonic acid and water breaks down even more minerals to form clay. Sulfur dioxide, nitrogen compounds, and carbon dioxide from pollution forms acid rain. Sulfur dioxide, nitrogen compounds, and carbon dioxide from pollution forms acid rain. This acid rain can very quickly decompose many types of structures. This acid rain can very quickly decompose many types of structures.

12. OXYGEN AND CHEMIMCAL WEATHERING Oxidation is the chemical reaction of oxygen with other substances. Oxidation is the chemical reaction of oxygen with other substances. Oxidation is very effective in weathering rocks that contain iron. Oxidized iron is called rust. Oxidation is very effective in weathering rocks that contain iron. Oxidized iron is called rust.

13. RATES OF WEATHERING Under average conditions, weathering is a slow process. Under average conditions, weathering is a slow process. The process is influenced by three major factors: The process is influenced by three major factors: Surface exposure—all weathering happens at the surface, so more surface area=faster process Surface exposure—all weathering happens at the surface, so more surface area=faster process Composition of the rock—certain types of rock are more susceptible to oxidation and acid reactions, thus they would weather faster than other rocks in the same area Composition of the rock—certain types of rock are more susceptible to oxidation and acid reactions, thus they would weather faster than other rocks in the same area Climate—in warmer, wetter climates—both mechanical and chemical weathering play about the same role. However, in dryer, colder climates, mechanical weathering plays a greater role than chemical weathering Climate—in warmer, wetter climates—both mechanical and chemical weathering play about the same role. However, in dryer, colder climates, mechanical weathering plays a greater role than chemical weathering

14. 7.2 EROSION and DEPOSTION Erosion is the removal and transport of materials by natural agents such as wind and running water. Erosion is the removal and transport of materials by natural agents such as wind and running water. Is affected by climate Is affected by climate Is affected by the composition of the rock Is affected by the composition of the rock The overall effect depends on how the process of erosion compares with the ongoing process of uplift The overall effect depends on how the process of erosion compares with the ongoing process of uplift Deposition is when the materials are dropped in another location. Deposition is when the materials are dropped in another location.

15. AGENTS OF EROSION Gravity Gravity Running Water Running Water Glaciers Glaciers Wind Wind Living things Living things To be discussed more in Chapter 8 To be discussed more in Chapter 8

16. 7.3 SOIL Soil is made of loose, weathered rock and organic material in which plants with roots can grow Soil is made of loose, weathered rock and organic material in which plants with roots can grow Without the weathering process we studies in Section 1, there would be no soil. Without the weathering process we studies in Section 1, there would be no soil. Without soil, what would we eat? Without soil, what would we eat?

17. HOW SOIL FORMS—PARENT MATERIAL The material from which soil forms is called the parent material. The material from which soil forms is called the parent material. If the parent material is the bedrock beneath the soil, the soil is called a residual soil. If the parent material is the bedrock beneath the soil, the soil is called a residual soil. If the parent material is not from the underlying bedrock but from weathered particles that were carried there by wind, waves, etc., the soil is called a transported soil. If the parent material is not from the underlying bedrock but from weathered particles that were carried there by wind, waves, etc., the soil is called a transported soil.

18. HOW SOIL FORMS—THE PROCESS Forms as parent material is weathered away. Forms as parent material is weathered away. Rate varies by location. Rate varies by location. Climate plays a role. Climate plays a role. Organic materials (decaying plant and animal remains) are added. Organic materials (decaying plant and animal remains) are added.

19. SOIL PROFILE A cross-section of the earth exposed by digging A cross-section of the earth exposed by digging Allows us to study the “layers of soil” Allows us to study the “layers of soil” Mature soils have three distinct zones or soil horizons Mature soils have three distinct zones or soil horizons A Horizon A Horizon B Horizon B Horizon C Horizon C Horizon

20. A-HORIZON Called topsoil Called topsoil Generally gray to black in color Generally gray to black in color Contains humus—organic material that forms from decayed plant and animal materials Contains humus—organic material that forms from decayed plant and animal materials Tends to be sandy Tends to be sandy

21. B-HORIZON Called subsoil Called subsoil Usually red or brown from oxides that formed in topsoil and then got washed down into the subsoil Usually red or brown from oxides that formed in topsoil and then got washed down into the subsoil Often contains clay and soluble minerals like calcium and potassium Often contains clay and soluble minerals like calcium and potassium

22. C-HORIZON Made of slightly weathered parent material Made of slightly weathered parent material Often has rock fragments Often has rock fragments Lies just above the bedrock or parent material Lies just above the bedrock or parent material

23. SOIL COMPOSITION Several factors affect soil composition: Several factors affect soil composition: Parent material Parent material Time Time Plants and animals Plants and animals Topography Topography Climate Climate

24. SOIL AS A RESOURCE SOIL AS A RESOURCE The world’s population is increasing at an alarming rate. The world’s population is increasing at an alarming rate. All of these people need to eat!!!!!! All of these people need to eat!!!!!! However, less than 25 % of the Earth’s land can be used to grow crops. However, less than 25 % of the Earth’s land can be used to grow crops. Soil fertility is the ability of soil to grow plants. Soil fertility is the ability of soil to grow plants. Certain types of soil are best suited for certain crops. Certain types of soil are best suited for certain crops. Many things currently threaten soil fertility. Many things currently threaten soil fertility.

25. SOIL DEPLETION Soil depletion occurs when the soil gradually becomes so lacking in nutrients that it can no longer be used to grow a usable crop. Soil depletion occurs when the soil gradually becomes so lacking in nutrients that it can no longer be used to grow a usable crop. Farmers can do some things to prevent this. Farmers can do some things to prevent this. Allow the soil to rest by rotating crops Allow the soil to rest by rotating crops

26. SALINIZATION Irrigation is often used to make soils fertile. Irrigation is often used to make soils fertile. Irrigation water often brings in dissolved minerals. Irrigation water often brings in dissolved minerals. These dissolved minerals are often salts. These dissolved minerals are often salts. Over time, the buildup of salt in the soil is called salinization. Over time, the buildup of salt in the soil is called salinization. Often renders the soil completely useless Often renders the soil completely useless

27. SOIL CONSERVATION Several methods exist: Several methods exist: Windbreaks—also known as shelter belts—lines of trees planted along the edges of fields Windbreaks—also known as shelter belts—lines of trees planted along the edges of fields Contour farming—farmers plant crops parallel to land contours to prevent water from running rapidly down a slope Contour farming—farmers plant crops parallel to land contours to prevent water from running rapidly down a slope Terraces—flattens a slope into separate terraces to slow the speed of runoff Terraces—flattens a slope into separate terraces to slow the speed of runoff Strip cropping—alternate rows of crops to minimize soil erosion Strip cropping—alternate rows of crops to minimize soil erosion No-till—method of farming where everything is done at once (plowing, planting, fertilizing, weed control) and the soil is left completely alone until harvest time No-till—method of farming where everything is done at once (plowing, planting, fertilizing, weed control) and the soil is left completely alone until harvest time