Weathering, Erosion, and Soils

1. Weathering, Erosion, and Soils Mandy Meeks, Baldwin Arts and Academics Magnet, Earth Science

2. Weathering

3. What is weathering? • Weathering refers to surface processes that break down rock. • Weathering breaks rock into small loose pieces called sediment. • Examples: sand, silt, and clay • Over millions of years weathering has changed Earth’s surface.

4. Mechanical Weathering • Mechanical weathering occurs when rocks are broken apart by physical processes. • The overall chemical makeup of the rock stays the same- each fragment of rock has the same characteristics as the original rock.

5. Mechanical Weathering- Plants and Animals • When plants grow in cracks in rocks, the roots enlarge the cracks. • Burrowing Animals loosen sediment and bring it to the surface where other weathering processes act on it.

6. Mechanical Weathering- Ice • Ice Wedging occurs in cold climates where water enters cracks in rocks and freezes. • Water expands as it turns to ice, extending the cracks and breaking rocks apart. • Ice wedging is a problem for roads in highways in the mountains, causing potholes to form.

7. Mechanical Weathering: Surface Area • Mechanical weathering reduces rocks into smaller pieces, which increases their surface area. • This exposes more of the rock to more water and oxygen, which speeds up the process of chemical weathering.

8. Chemical Weathering • Chemical Weathering occurs when chemical reactions dissolve the minerals in rocks or change them into different minerals. • This changes the chemical composition of the rock and often makes the rock weaker.

9. Chemical Weathering- Natural Acids • Carbonic Acid is formed when carbon dioxide and water react in the air. • Carbonic acid causes minerals like calcite to dissolve. • This causes stones like limestone to weather away, forming caves. • Chemical weathering causes the mineral feldspar, which is found in granite and sandstone, to be broken down into kaolonite, forming clay.

10. Chemical Weathering- Plant Acids • Plant roots and decaying plants give off acids that dissolve minerals in rocks. • As the rock breaks apart, nutrients become available to the plants.

11. Chemical Weathering- Oxygen • Oxidation is when some materials chemically react when exposed to oxygen and water. • Example: rusting of iron • Magnatite, a mineral that contains iron, oxidizes to form limonite. • Many oxidized minerals have a reddish color.

12. Weathering and Climate • Climate is the pattern of weather that occurs in a particular area over many years. • In cold climates, freezing and thawing causes many rocks to break down due to ice wedging. • Chemical weathering occurs faster in warm, wet climates.

13. Weathering and rock type • Some rocks weather more quickly than others. • Example: Marble and granite

14. Soils

15. Formation of Soil • Soil is a mixture of weathered rock, decayed organic matter, mineral fragments, water, and air. • Rock fragments do not become high quality soil without organic matter. • Soil can take thousands of years to form.

16. Factors Affecting Soil Formation • Climate • Slope of Land • Types of Rock • Types of vegetation • Amount of time rock has been weathering

17. Composition of Soil • Rock and mineral fragments • Organic matter • Humus- decayed plant and animal material in soil

18. Soil Profiles • Soil is divided up into layers called horizons. • All horizons of soil form the soil profile. • Soils have three horizons- A (top layer, lots of humus), B(middle layer, less humus), and C (bottom layer, consists of partially weathered rock).

19. Erosion

20. Soil Erosion • Soil erodes when it is moved from the place where it formed. • In normal conditions there is a balance between soil erosion and soil production. • Agents of erosion include gravity, water, wind, and glaciers. • Soil erosion is harmful because plants do not grow well when topsoil has been removed.

21. Human Causes of Erosion • Agricultural cultivation • Plowing, leaving fields bare after harvest • Forest Harvesting • Overgrazing

22. Erosion by gravity • The greater an object’s mass is, the greater is gravitational force is. • Mass movement is any type of erosion that happens as gravity moves materials downslope. • Material can move downhill in a large mass (slump) or when sediments slowly move downhill (creep). • Other kinds of mass movement include rockslides and mudslides

23. Glacier Erosion • A large mass of ice and snow moving on land under its own weight is a glacier • Continental glaciers cover 10% of Earth; they once covered as much as 28% of Earth. • Glaciers erode land as they pass over it and deposit eroded material somewhere else

24. Wind Erosion • Wind erodes the Earth via deflation (blowing small loose sediment around) and abrasion (windblown sediment striking rock). • Wind erosion can lead to sandstorms and dust storms. • Dunes form when sediment is deposited by the wind. • Many dunes migrate away from the wind unless they are planted with grass. • Planting vegetation is one of the best ways to slow wind erosion.

25. Water Erosion • Water that does not soak into the ground or evaporate but instead flows across Earth’s surface is called runoff. • The amount of rain and the length of time rain is falling affect runoff. • Slope of the land and vegetation also affects runoff.

26. Types of Water Erosion • Water can travel in channels, carrying away soil and causing rills and gullies. • Water can travel in sheets, picking up and carrying away sediments. • Water can form streams • Sediment carried by a stream is called the load.

27. Groundwater • Water that soaks into the ground is called groundwater. • The water table is the upper surface of groundwater. • Carbon dioxide dissolved in groundwater can form carbonic acid, causing caves and sink holes as it weathers away limestone.

28. Consequences of Erosion • Building on steep slopes • Lots of time and $$ spent trying to slow down erosion • Beachfront developments are affected by shoreline erosion. • Loss of Soil Productivity • Sediment pollution • Runoff from pesticides and fertilizers