Weathering of Rocks

Exam 1

Why we see weathering 1.Most minerals are not stable at the Earth’s surface 2.The Earth wants to be flat – lowest energy state Topography provides energy

Bowen’s Reaction Series and Weathering

IgneousSedimentary Metamorphic Melting Erosion + Deposition Melting Erosion + Deposition Burial + Heating Weathering in the Rock Cycle

Three Dynamic Processes of Breaking and Removing rock 1.Weathering - the disintegration and decomposition of rock at or near the surface 2.Erosion 3.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) 1)Which site is at the greatest risk from physical weathering? Why? 2)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

Mechanical Weathering 1.Frost Wedging 2.Salt Wedging 3.Biological Wedging 4.Unloading

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

Salt Wedging

Biological Wedging

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

Sheeting Unloading

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

Types of Chemical Weathering 1.Hydrolysis - any reaction in which water participates. a)Ion exchange – H+ replaces other cations. b)Dissolution - mineral completely dissolves, leaving only ions in solution. c)Oxidation - reaction in which elements gain or lose electrons (example: rust).

Dissolution Minerals dissolve in water For example: salts

Carbon Dioxide + Rain Becomes Acid Dissolves Minerals (i.e. carbonates such as calcite) Carries away---Ions Acid Hydrolysis - Dissolution H 2 O + CO 2  H 2 CO 3 CaCO 3 + H 2 CO3  Ca HCO 3 -

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

Oxidation 1.Iron silicate dissolves 2.Iron oxidizes 3.Oxidized iron combines with water 4.Leaves iron oxide

Results of Weathering 1.Water moves through joints a.Frost wedging b.Chemical dissolution – get clays 2.Joints enlarge 3.Corners are attacked more aggressively – get rounding of the material 4.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 1.Rock structures – chemical/mineral composition, physical features 2.Topography 3.Climate 4.Vegetation 5.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: 1.More chemical weathering in soil. 2.Soil has structure (layers called soil horizons). 3.Soil retains nutrients and moisture, essential for abundant plant growth.

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: 1.Bedrock 2.Stream sediments 3.Volcanic ash

Controls of Soil Formation 1.Parent Material 2.Time 3.Climate 4.Plants and Animals 5.Slope