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
Mechanical weathering – rock is physically broken down into smaller pieces
Chemical weathering – composition of the rock is changed

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
The volume of ice expands by about 10% when it freezes

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
Mineral ore deposits may occur as a result of leaching

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
Rust is iron oxide

12. Plant acids – acids produced naturally by living things
Acid Precipitation
Rainwater
Oxides that contain nitrogen and sulfur
Faster weathering process
Plant acids – acids produced naturally by living things
Lichens and mosses
Rainwater itself is naturally acidic because it contains carbon dioxide
Industrialized areas emit the oxides that lead to more acidic rain…. Smoke stacks from power plants

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
Rock composition – rate of weathering is also dependent on the agent that holds the particles together
Amount of Exposure – the more exposure a rock receives the faster it will weather
Rainfall & freezing and thawing have the greatest effect on the rate of weathering
High elevations – decreased temperature and increased amount of ice wedging
Steep slopes are weathered more quickly because gravity pulls fragments towards earth at a faster rate with exposes new material more quickly than a gentle slope

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 to 2 mm diameter
Silt 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
Silt particles are too small to be seen easily

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