Chapter 12: Weathering & Erosion

1. Chapter 12: Weathering & Erosion Modern Earth Science p. 219 through 239

2. Section 12.1: Weathering Processes • Weathering: change in the chemical composition or physical form of a rock • Two types of weathering: • Mechanical Weathering • Chemical Weathering • Temperature and pressure changes • Exposure to gases and water

3. Mechanical Weathering: • occurs when processes physically break rock into smaller pieces, but do not change the chemical composition of the rock

4. Chemical Weathering: • occurs when processes break down a rock by changing its chemical composition

5. Mechanical Weathering: • Strictly physical • Common agents: ice, plants, animals, gravity, running water, wind • Joint – long, curved crack parallel to the surface that forms when a rock formed deep within earth (granite) is uplifted towards the surface and overlying materials are removed, which allows the rock to expand and crack. • Exfoliation – process by which curved sheets, created by fractures at joints, are peeled away from the underlying rock

6. Ice Wedging • Process that occurs when ice freezes, thaws and refreezes • High latitudes where temperature fluctuates above and below freezing point of water • Northern United States

7. Organic Activity • Plants • Animals

8. Abrasion • Collision of rocks with one another, resulting the breaking and wearing away of rocks • Agents of abrasion are gravity, running water and wind • gravity – causes loose soil and rocks to move down the slope of a hill or mountain; rocks break into smaller pieces as they fall and collide • running water or wind – also carry particles of sand and rock that scrape against each other and against other stationary rocks

9. Chemical Weathering • Chemical weathering or decomposition takes place when chemical reactions take place between the minerals in the rock, water, carbon dioxide, oxygen and acids, which alters the structure of the original mineral and leads to the formation of new minerals. • Physical appearance and chemical composition are changed.

10. Hydrolysis • Hydrolysis – change in the composition of minerals when they react chemically with water • Leaching – minerals affected by hydrolysis may dissolve in water and are carried by water to lower layers of rock

11. Carbonation – when carbon dioxide from the air dissolves in the water, a weak carbonic acid solution is produced: • H2O + CO2  H2CO3 • Some minerals come into contact with carbonic acid and form new products • Oxidation – when metallic elements combine with oxygen, often attacks rocks with iron bearing minerals • 4 Fe + 3 O2 2 Fe2O3

12. Acid Precipitation • Rainwater • Oxides that contain nitrogen and sulfur • Faster weathering process • Plant acids – acids produced naturally by living things • Lichens and mosses

13. Section 12.2: Rates of Weathering • The processes of weathering generally work very slowly and rocks do not weather at the same rate; different rates of weathering produces different formations • Rock composition • Quartz - resistant to chemical and mechanical weathering • Limestone (calcite) – resistant to mechanical, but quickly erodes through carbonation • Amounts of exposure • Time • Surface area • Joints and fractures increase surface area • Climates • Rainfall • Freezing and thawing • Topography • High elevations = more rapidly weathered • Steep slopes

14. Section 12.3: Weathering and Soil • Weathering process break and alter all rocks exposed at the surface of the earth • Regolith – layer of weathered rock fragments covering much of the earth’s surfaced • Bedrock – solid, unweathered rock that lies beneath the regolith

15. Soil Composition • Rock material in soil consists of three types of particles: • Clay - < .002 mm diameter • Sand - .06 to 2 mm diameter • Silt - .002 to .06 mm diameter • Proportion of these types of particles is largely dependent on the parent material • High clay = parent material rich in feldspar • High sand = granite or other rock with high quartz • High silt = usually soils found near riverbeds • Transported soil: weathered mineral grains that are carried away and form a soil in a new location • May have a different composition than the bedrock on which it lies

16. Soil Profile • Cross section in which layers of soil and the bedrock beneath the soil can be seen • Residual soil: soil that rests on top of parent material • Horizons: layers of soil in the soil profile, soil profile generally consist of three principal horizons • A horizon – top soil, mixture of organic matter and rock material, almost all living things that live in the soil inhabit the A horizon, contains the humus and other organic material • B horizon – immediately below the A horizon, subsoil, contains the minerals leached from the A horizon, may be considered a zone of accumulation in dry areas • C horizon – bottom layer of the soil, consists of bedrock that has been partially weathered, in the first stages of mechanical and chemical weathering

18. Soil and Climate • Climate is one of the most important factors influencing soil formation, climate determines the weathering processes that occur in a region • Humid tropical climate allows chemical weathering to produce thick soils quickly • Laterites – contains iron and aluminum and doesn’t dissolve easily in water • Constant addition of organic matter sometimes tends to have a layer of humus on the B horizon • Two main types of soil are found in temperate climates • Pedalfer – receives more than 65 cm of rain yearly • Pedocal – receives less than 65 cm of rain yearly • Very acidic and fertile soil, southeast US