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

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

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

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

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

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

Organic Activity Plants Animals

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

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.

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

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

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

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

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

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 Silt particles are too small to be seen easily

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

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