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Transcript
Weathering Weathering breaks down materials on or near Earth’s surface. Review Vocabulary acid: solution that contains hydrogen ions I. Mechanical Weathering •Mechanical weathering is a type of weathering in which rocks and minerals break down into smaller pieces. •It does not change a rock’s composition. A. Effect of temperature 1.Frost wedging a. Freeze-thaw cycle of water b. Water expands 9% 2. Breaks rock into smaller pieces B. Effect of pressure 1. Small scale a. Plant or tree roots wedged in the cracks of rocks b. Grow and expand, which often causes the rocks to split. 2. larger scale a. overlying rock layers removed by erosion or even mining b. pressure on the bedrock is reduced c. bedrock surface that was buried expands d. long, curved cracks form (AKA joints) 3. Exfoliation a. outer rock layers are stripped away b. resulting in dome-shaped formations II. Chemical Weathering A. rocks and minerals undergo changesin their composition 1. Agents a. water b. oxygen c. carbon dioxide d. acid precipitation 2. Results a. Dissolving Rock b. Formation of new minerals ex: 4Fe + 3O2 2Fe2O3 Iron Oxide (hematite) The composition of a rock determines the effects that chemical weathering will have on it. Temperature is a significant factor in chemical weathering because it influences the rate at which chemical reactions occur. 3. Composition a. Calcite will decompose completely in acid ex: Limestone & Marble 4. Temperature a. As temp. Chem. Reaction b. Doubles every 10 oC increase B. Effect of water 1. Universal solvent 2. Serves as a medium 3. React directly with minerals in a chemical reaction a. Breakdown large molecules ex: Potassium feldspar kaolinite (fine-grained clay) C. Effect of oxygen (Oxidation) 1. Iron in rocks and minerals combines with atmospheric oxygen to form minerals with the oxidized form of iron. Hematite is a common example. Hawaii D. Effect of carbon dioxide 1. Combines with water in atmosphere/ground 2. Forms carbonic acid a. Acid precipitation b. Dissolve certain rocks, such as limestone 3. Decaying organic matter adds CO2 to soil a. Limestone caverns E. Effect of acid precipitation 1. Burning fossil fuels a. Sulfur dioxide b. Carbon dioxide c. Nitrogen oxides 2. Dissolves rocks 3. Harmful to plants and animals III. Rate of Weathering The natural weathering of Earth materials occurs slowly. However, certain conditions and interactions can accelerate or slow the weathering process. Valley of the Moon: San Juan, Argentina A. Effects of climate on weathering 1. Chemical weathering a. Rapid in high temperatures b. Abundant rainfall c. Lush vegetation 2. Physical weathering a. Rapid in cool climates Cleopatra’s Needle (NY, NY) Pylon of Ramses (Egypt) B. Rock type and composition Limestone vs. granite C. Surface area Mechanical weathering breaks rocks into smaller pieces. As the pieces get smaller, their surface area increases, resulting in more total surface area available for chemical weathering. D. Topography 1. Steep slopes promote erosion 2. Continually expose less-weathered material 3. Gravity What effects do mechanical and chemical weathering have on the rock at Earth’s surface? Answer: Mechanical weathering breaks rock apart but does not change its composition. Chemical weathering dissolves some rock and also causes some new minerals, such as iron oxides and clays, to form.