Weathering vs. Erosion What is weathering? What is erosion? Breakdown of earth materials What is erosion? Removal of earth material Glaciers Wind Water

Types of Weathering There are two primary types of weathering: Physical – Breaking a material into smaller particles Chemical – Changing the materials composition

Physical Weathering Physical or mechanical weathering Breaking a rock or mineral into smaller pieces No change in composition

Physical Weathering There are six types of physical weathering Abrasion Frost wedging Salt cracking Organic activity Pressure relief Thermal expansion

Physical Weathering 1) Abrasion – A rock is pelted by a stream of small particles Particles carried by wind or water Think of sandblasting

Physical Weathering 2) Frost wedging – Water fills a crack Freezing water expands cracks Destroys rock

Physical Weathering 3) Salt cracking – Similar to frost wedging Most common in coastal regions

Physical Weathering 4) Organic activity – Animals burrowing Plants grow and the roots spread

Physical Weathering 5)Pressure relief cracking – Depth equals higher pressure Pressure makes a rock smaller Pressure is released, and the rock expands and cracks

Physical Weathering Pressure relief (cont’d) Cracks are called joints Joints - rocks separate but don’t move

Physical Weathering Pressure relief (cont’d) Within igneous bodies, the joints tend to form parallel to the surface This is called exfoliation

Physical Weathering 6) Thermal expansion – Heating and cooling = expansion and contraction Different minerals expand and contract differently Weakens a rock

Chemical Weathering Chemical weathering – A rock is broken down and it’s composition changes Exposure to air or water initiate chemical changes

Chemical Weathering Chemical weathering can be broken into a number of categories Oxidation Hydrolysis Weathering by solution

Chemical Weathering 1) Oxidation – Elements reacting with free oxygen Oxygen combines with elements, removing them from the mineral Rusting 4 Fe (+3) + 3 O2 (-4) = 2 Fe2O3 (Hematite)

Chemical Weathering Oxidation (cont’d) – Pyrite (FeS2) is commonly oxidized when exposed to oxygen at the surface

Chemical Weathering 2) Hydrolysis – Hydrogen ions (H+) replaces other positively charged ions Happens within slightly acidic water

Chemical Weathering Hydrolysis (cont’d) – Acidic water travels across a rock, replacing cations with hydrogen Common reaction: Potassium feldspar + hydrogen ions + water = Muscovite mica + potassium ions + dissolved silica 12H2O + 2H+ + 3KAlSi3O8 = KAl3Si3O10(OH)2 + 2K+ + 6H4SiO4

Chemical Weathering 3) Weathering by solution – Also called dissolutioning Ions pulled into solution Halite (salt) and calcite

Chemical Weathering Weathering by solution (cont’d) – Water is dipolar Pure, neutral water can weather by solution

Chemical Weathering Weathering by solution (cont’d) – Water attaches to the ions The salt is dissolved

Chemical Weathering Weathering by solution (cont’d) – Water isn’t normally neutral Natural water is usually slightly acidic H2O + CO2 = H2CO3 = H+ ion + HCO3) Excess H+

Chemical Weathering Weathering by solution (cont’d) – Carbonic acid reacts with calcite (CaCO3) in limestones Calcite dissolves into solution CaCO3 + H2CO3 = Ca + 2HCO3

Chemical Weathering Acid Rain – Defined – Precipitation with a pH < 5.0 Excess acid rain can: Rapidly weathers certain rock types Effects groundwater quality

Chemical Weathering Acid rain (cont’d) – Result of water and industrial wastes mixing Wastes from burning fossil fuels

Chemical Weathering Acid rain (cont’d) - Waste products include: SO2 NO2 These wastes create: Nitric acid - NO2 + O2 + H2O = HNO3 Sulfuric acid - SO2 + O2 + H2O = H2SO4

Chemical Weathering Acid rain (cont’d) – Acid rain dissolves marble or limestone (weathering by solution) Excess acid rain pollutes ponds, lakes or reservoirs Acid rain can leach into the groundwater

Rate of Weathering Variables influencing the rate of weathering include Climate Rock type Particle size Particle shape

Rate of Weathering 1) Climate – Very important consideration re: weathering Moisture

Rate of Weathering 2) Rock type – Different rock weather at different rates Granite weathers slowly Mineral strength Limestone weathers quickly Calcite

Rate of Weathering 3) Particle size – Rock or particle size influences rate of weathering More surface area exposed = quicker weathering

Rate of Weathering Particle size (cont’d) – Small particles have more surface area exposed More surface area = faster weathering

Rate of Weathering 4) Particle shape – A particle with numerous edges or corners will weather quickly A particle with few edges or corners will weather slowly

Rate of Weathering Particle shape (cont’d) A corner weathers quickest An edge weathers slower A face weathers slowest