Chapter 7: Weathering & Soil

1. Chapter 7: Weathering & Soil

2. Aim: How does weathering change Earth’s surface?

3. Weathering • Is surface processes that work to breakdown rocks into smaller pieces • Smaller loose pieces are sediments • Gravel, Sand, silt and clay describe sizes of sediments • Size order: gravel, sand, silt, clay (largest to smallest) • Wears mountains down to hills • Produces strange rock formations • 2 Types of weathering: • Mechanical Weathering • Chemical Weathering

4. Aim: How do mechanical weathering and chemical weathering differ?

5. Mechanical Weathering • Occurs when rocks are broken apart by physical processes • Chemical makeup of the rock stays the same • 2 ways this can happen: • Plants and Animals • Water and nutrients that collect in the cracks or rocks enable plants to grow • As the roots grow they enlarge the cracks • Ex: tripped on crack in the sidewalk near a tree

6. Burrowing animals loosen sediments and push them to the surface • Sediments reach the surface and other weathering processes occur • Ice Wedging • Occurs in temperate and cold climates • Water enters cracks in rocks and freezes • Pressure builds up in the cracks causing them to expand and break apart the rock • The ice melts and this happens all over again • The process of freezing and thawing occurs over and over, especially in mountains

7. Ice wedging wears mountain peaks • It can also break up roads and highways • Mechanical weathering reduces rocks to smaller pieces giving the rock more surface area • As the surface area increases more rock is exposed to water and oxygen which aids in chemical weathering

8. Chemical Weathering • Chemical reactions dissolve the minerals in rocks or change them to different minerals • Changes the chemical composition of the rock • Naturally occurring acids, such as carbonic acid, react with calcite in limestone • The acid weathers away the limestone to form caves • Kaolinite clay is created when acids react with feldspar in granite • Clay is an end product in weathering

9. Plant acids can dissolve minerals in rocks and breaks the rocks into smaller pieces • This weathering also enables nutrients to be available for plants • Oxygen also causes weathering • Oxidation occurs when some materials are exposed to oxygen and water • Ex: rust

10. Effects of Climate on Weathering • Chemical weathering is more rapid in warm, moist climates • Mechanical weathering is important in dry climates and in cold climates

11. Aim: How does soil form?

12. Formation of Soil • Is a mixture of weathered rock, decayed organic matter, mineral fragments, water and air • Can take thousands of years to form • Climate, slope, types of rock, types of vegetation and length of time of weathering affect rock formation • There are different kinds of soil all over the world based on these factors • 4 steps

13. Weathering of rocks • Natural acids in rainwater weather the surface of exposed bedrock • Water can freeze in cracks to break them apart • Plant Growth • Plants begin growing in cracks of rocks • As they grow they continue the process of breaking down rocks • Then a thin layer of soil begins to form

14. Living Organsims • Insects & worms live underground with the plant roots • Their wastes and dead material add organic matter to the soil • Organic Matter • as organic matter increases, the bedrock continues to breakdown • This causes the soil layer to thicken • Rich topsoil supports trees and plants with large root systems

15. Aim: What are the characteristics of soil?

16. Composition of Soil • Most organic matter in soil comes from plants • Animals and microorganisms remains also add organic matter to the soil • Decayed organic matter over time turns into a dark-colored material called humus • Humus is a source of nutrients for plants • Animals burrowing in the ground mix the humus with fragments of rocks • Good quality soil have equal amounts of humus and weathered rock material

17. Layers of Soil • Layers of soil are called horizons • All soil horizons form a soil profile • Most soils have 3 horizons • A Horizon • top layer-AKA topsoil • In a forest it is covered with litter • Litter helps prevent erosion and evaporation of water from the soil • Topsoil has more humus and fewer rock and mineral fragments than the other horizons

18. Is dark and fertile • B Horizon • is below the A horizon • Lighter in color (less organic matter) • Less fertile • Leaching moves minerals that have been dissolved in water from A horizon down into the B horizon

19. C Horizon • Bottom layer • Contains partially weathered rock • Thickest horizon • Not affected by leaching and doesn’t have much organic material • Coarser sediments • Most like the parent material, which is rock

20. Aim: What determines the different types of soil?

21. Types of Soil • Many different types of soil exist • Climate affects soil • Desert soil has a small amount of organic matter and is thin • Prairies have thick, dark A horizon soil due to the organic matter from the large amount of grasses present • Forest soil has a thinner A horizon than prairies because trees and leaves don’t supply as much organic matter as grasses do

22. The type of rocks soil comes from affects the type of soil • Sandstone turns into a sandy soil • Clay soil comes from basalt • Type of vegetation that grows in an affects soil formation • Slope of the land affects soil development • The soil on the slope is poorly developed • Soil in a valley is rich in organic matter

23. Aim: How has human activity affected Earth’s soil?

24. Soil Erosion • Soil erodes when it is moved from the place where it formed • Water and wind move sediments and cause erosion • More severe on steep slopes • Humans cause erosion to form faster by removing vegetation (ground cover) which helps increase soil erosion • Serious problem for agriculture • Soil loses nutrients, which plants need to grow

25. When soil erodes farmers compensate for the nutrient lose by buying fertilizer • Forest harvesting also causes soil erosion • Removing forests creates severe problems for the environment, especially rain forests • Forests are cleared for lumber, farming and grazing • Overgrazing also increases soil erosion • Sheep and cattle are grazed on land until no ground cover remains • Without protection from plants that soil is carried away by wind and water

26. Too much soil erosion, sediments can damage the environment • This can cause streams to be covered, eggs from organisms die, or reduce downstream water quality

27. Aim: How can humans prevent soil erosion?

28. Preventing Soil Erosion • Plant shelter belts of trees-this blocks the strength of the wind • No-till farming-farmers do not plow the fields • This practice provides cover for the soil all year round and reduces water runoff and soil erosion • Contour farming used on slopes-this slows the flow of water down the slope and reduces soil erosion • Terracing-leveled areas are built on slopes (looks like steps)

29. This practice reduces runoff by creating flat areas and shorter sections of slope