Transcript Weathering
THE ROCK CYCLE Sediment is produced either directly or indirectly by the weathering and erosion of pre-existing rocks. Grains can originate as: A) Broken down particles from older, pre-existing rocks. B) Pyroclastic particles from explosive volcanism. C) Secondary Minerals (iron oxide, clay minerals) that form at the site of weathering. These grains are usually referred to as extrabasinal sediments because they originate outside the depositional basin and are transported into it. Main source of siliciclastic (quartz composition) rocks like shale and sandstone. Grains can also originate in two other ways: A) Chemical and biochemical precipitates These form what we call chemical sedimentary rocks. These grains are usually referred to as intrabasinal sediments because they are forming within the depositional basin. They are the source for limestones, dolostones, cherts, and evaporites B) Organic residues that form from the decomposition of plant and animal remains. Enriched in carbon, these residues are the constituents of coal. The fact that one generation of sedimentary rock can be the source material for another suggests that the processes responsible for sedimentary rock formation occur in cycles – a fact first recognized by James Hutton in 1788. Thus, the part of the Geologic Cycle that we are most interested in is the Sedimentary Cycle: 1) Weathering + Erosion 2) Transportation 3) Deposition 4) Lithification 5) Diagenesis 6) Recycling, again Let’s Begin with WEATHERING: What Happens To Rocks At The Earth’s Surface: DEFINITIONS: 1. Weathering – The destruction and decomposition of rocks at the Earth’s surface. 2. Erosion – The incorporation and transport of weathered material by wind, water, and ice. 3. Mass Movement – The downslope movement of weathered material under the force of gravity WEATHERING 1. Mechanical – The physical breakdown of rocks into smaller and smaller pieces. 2. Chemical – The chemical reactions that alter or destroy minerals. 1. MECHANICAL WEATHERING a) Frost Wedging – Water increases by 9% in volume when it freezes. 1. MECHANICAL WEATHERING b) Exfoliation or Sheeting – A pressure release phenomenon. Downward pressure Granite pluton SHEETING 1. MECHANICAL WEATHERING c) Organic activity – tree roots (also chemical), animal burrowing, lichens (chelation), bacteria. d) Day/Night Temperature Variation Desert Pavement 1. MECHANICAL WEATHERING e) Spheroidal Weathering: CHEMICAL WEATHERING 1) Dissolution H2O + CO2 + CaCO3 2) Oxidation 4Fe + 3 O2 Ca2+ + 2HCO3- 2Fe2O3 (Hematite or Rust) 3) Hydrolysis Carbonic Acid + Silicate Mineral 4) Hydration CaSO4 + 2H2O (Annhydrite) Dissolved ions + Clay Minerals CaSO4* 2H2O (Gypsum) 1) Dissolution – Karst Topography 1) Dissolution – Florida Sinkhole 1) Dissolution – Cave Formation Extreme Karst Topography: The Stone Forests of Yunnan Province, China 2) Oxidation: 2) Oxidation: Honeycomb Weathering at Hocking Hills 2) Oxidation: Artists Drive: Oxidized Volcanics in Death Valley 3) Hydrolysis HYDROLYSIS HYDROLYSIS WHY DO ROCKS WEATHER ? Form Under Higher Pressure/Temperature Unstable at Surface Pressure & Temperature BOWEN’S REACTION SERIES Temperatures at which silicate minerals crystallize out of a cooling melt 1200o 600o WHAT CONTROLS THE TYPE & RATE OF WEATHERING? WHAT CONTROLS THE TYPE & RATE OF WEATHERING? Obelisk was carved in granite 3500 years ago. Chemical Weathering & Rock Type Pedestal was carved in marble (limestone) only 1600 years ago.