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\
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 2+ + 2HCO 3 -
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