Weathering of Rocks

Exam 1

Why we see weathering Most minerals are not stable at the Earth’s surface The Earth wants to be flat – lowest energy state Topography provides energy

Bowen’s Reaction Series and Weathering

Three Dynamic Processes of Breaking and Removing rock Weathering - the disintegration and decomposition of rock at or near the surface Erosion Mass wasting

Types of Weathering Physical (Mechanical) –breakdown of rocks into smaller pieces. Chemical – decomposition of rocks by chemical reactions.

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Daily Questions Examine the five World Heritage Sites in Figure 9.1 (pg. 244) Which site is at the greatest risk from physical weathering? Why? Which site is at the greatest risk from chemical weathering? Why?

Mechanical Weathering Mechanical weathering breaks down rocks/minerals Smaller particles increases the amount of exposed surface area Does not change the chemical composition of the minerals\

Mechanical Weathering - making smaller pieces

Types of Mechanical Weathering Frost wedging – water penetrates into cracks, expands when it freezes. Must have: Adequate moisture Cracks in rocks Freeze/thaw cycles

Salt Wedging

Biological Wedging

Unloading Removal of overlying material allows rock to expands Top layers expand more than deeper layers Causes sheeting or exfoliation

Unloading Sheeting Sheeting

Chemical Weathering Chemical alteration of minerals. Results in new minerals and ions in solution. Water and acid are essential.

Dissolution Minerals dissolve in water For example: salts

Acid Hydrolysis - Dissolution Carbon Dioxide + Rain H2O + CO2 H2CO3 CaCO3 + H2CO3 Ca2+ + 2HCO3- Becomes Acid Dissolves Minerals (i.e. carbonates such as calcite) Carries away--- Ions

Acid Hydrolysis & Secondary Minerals Carbon Dioxide + Rain Becomes Acid Dissolves Minerals (i.e. silicates) Leaves ----Clay Carries away--- Ions Silica

Oxidation Iron silicate dissolves Iron oxidizes Oxidized iron combines with water Leaves iron oxide

Results of Weathering Water moves through joints Joints enlarge Frost wedging Chemical dissolution – get clays Joints enlarge Corners are attacked more aggressively – get rounding of the material Fines (clays and silts) wash out of rock

Weathering Products Regolith – a loose layer of broken rock and mineral fragments. Sediments Dissolved Ions

Relative susceptibility To weathering

Products of Weathering

Factors influencing Weathering Rates Rock structures – chemical/mineral composition, physical features Topography Climate Vegetation Time

Climate and Weathering

Benefits of Weathering Creates soil Produces clay, sand, and gravel Produces minerals

Soils

Terminology Regolith – blanket of loose, weathered rock debris covering unweathered bedrock. Soil – uppermost part of regolith. Rock particles, new minerals, organics

How soil differs from regolith: More chemical weathering in soil. Soil has structure (layers called soil horizons). Soil retains nutrients and moisture, essential for abundant plant growth.

Composition, color, texture More Terminology Soil profile – sequence of soil horizons. Soil horizon – layer within a soil profile that has distinct characteristics. Composition, color, texture

Soil Horizons

A horizon Zone of leaching (lots of chemical weathering) Organic rich, often dark in color Decaying organic matter releases nutrients

B horizon Zone of accumulation – minerals (clay and iron oxide) are washed down from above. High clay content, reddish color from iron. Able to retain moisture because of clay content

C horizon Parent material – grading from weathered to unweathered. Parent material can be: Bedrock Stream sediments Volcanic ash

Controls of Soil Formation Parent Material Time Climate Plants and Animals Slope