Weathering and Soil

1. Weathering and Soil 6.1 Weathering

2. Evidence of Weathering • Weathering – the process that breaks down rocks into smaller and smaller fragments. Rocks break into small pieces called sediment. • Sediments can form soil. • Mechanical Weathering • Mechanical weathering – breaks rocks without changing the chemical composition. Breaks by physical process. • Mechanical weathering can be caused by growing plants, expanding ice, mineral crystal growth, lightning, and expansion and contraction when an area heats and cools.

3. Plants • Roots - wedge rocks apart. • Lichen plants – expand and shrink with the amount of available water. • Ice Wedging • Ice Wedging – ice freezes and expands cracking rocks. • Noticeable in mountains • Can cause potholes. • Chemical Weathering • Chemical weathering – chemical composition of the rock changes. • Water • Can cause new substances to be formed.

4. Acids • Naturally forming Carbonic acid reacts with minerals. • Limestone and Carbonic acid – Figure 6-5 • Carbonic acid and granite – produces clay • Plant roots have acids as well • Oxygen • Oxidation – occurs when iron is exposed to oxygen and water. Causes rust. • Climate and Weathering • Climate – the pattern of weather that occurs in a particular area over many years. • Cold Climates – mechanical weathering happens rapidly. • Warm and Wet Climate – chemical weathering happens rapidly. • Warm Dry – slower chemical weathering.

5. Mechanical and Chemical weathering work together. • Mechanical weathering increases surface area • Chemical weathering increases as surface area increases. Pg. 161 1-3

6. Weathering and Soil 6.2 Soil

7. Formation of Soil • Soil – mixture of weathered rock, organic matter, mineral fragments, water, and air • Soil supports vegetation. • Climate, types of rock, slope, amount of moisture, and length of time rock has been weathering affect the formation. • Composition of Soil • As soil develops, organic materials decay • Humus – dark colored matter • Provides nitrogen, phosphorous, potassium, and sulfur for plants. • Promotes good soil structure, and helps soil hold water • Good quality surface soil is half broken down rock and half hums

8. Soil can take thousands of years to form • Fertile soil often found near rivers • Solids have small spaces in them. • These spaces fill up with air and water. • Soil Profile • Soil profile – different layers of soil • Horizon – each layer in the soil profile • Horizon A • Top Layer • Litter – leaves, twigs, and other organic materials that changes to humus when it is exposed to decomposing organisms • Also known as topsoil • Has more humus and smaller rock and mineral particles than the other layers

9. Horizon B • Lighter in color than Horizon A and contains less humus • Leaching – removal of minerals that have been dissolved in water. • Elements get leached into Horizon B from Horizon A. • Horizon C • Consists mostly of partially weathered parent rock • Leaching from Horizon B • At the bottom is solid rock • Many places land is covered by material that was deposited by glaciers. • Glacial deposits made fertile soil • Midwest farming soils were made this way.

10. Soil Horizons http://www.enchantedlearning.com/geology/soil/

11. Types of Soil • Texture depends on the amounts of sand, silt, and clay • Texture affects how water run through soil • Climate affects soil profiles • Deserts are dry • Prairies are demi dry • Temperate zones – moderate amount of rain, moderate temperature • Soil Types Reflect Climate • Chemical weathering is much slower in areas of little rainfall. • Deserts have little organic material

12. Soil horizons in drier areas are thinner than soils in wetter climates • Precipitation affects how much leaching occurs • Soils that have been leached are light in color • Time changes soil characteristics • Short time means parent rock determines soil characteristics • As weathering continues soil resembles the parent rock less and less • Slope affects as well • Steep slopes, soil horizons are poorly developed • Bottomlands (lots of water) solids are often thick, dark, and full of organic material. • South facing slopes get more sunlight, so different soil profile. • Humus affects soil profiles.

13. Soil Profiles http://soils.usda.gov/gallery/photos/profiles/

15. Soil – An Important Resource • Needed for food, livestock, paper production, and others. • Without plants • soil is more likely to be eroded. • Soil development slows and sometimes stops. • Plowing and Soil Loss • Increasing populations means more food is needed. • Farmers plow increasing amounts of soil. • Plowing soil by machines is effective for crops, but removes plant cover that holds soil in place. • Causes erosion due to wind to speed up • Under these conditions, soil is a nonrenewable resource because it cannot be replaced quickly.

16. Soil Erosion in the Tropics • Tropical rains running down steep slopes erodes soil • Thousands of square kilometers of rain forest are cleared every year. • Only the first few cm of tropical soil have good nutrients • Farming a particular spot can only last a few years. • This is leading to loss of the world’s rain forests. • Farmers work to Minimize Soil Loss • Shelter belts for wind coverage • Cover bare soil with decaying plants • In dry areas, famers graze rather than plow • Crop rotation