Lecture Sedimentary Rocks (1).pdf - EASC1402(2019/2020 PRINCIPLE OF GEOLOGY 1 SEDIMENTA RY ROCKS Dr MC Cheung Department of Earth Sciences The

Lecture Sedimentary Rocks (1).pdf - EASC1402(2019/2020...

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24/9/2019 1 EASC1402 (2019/2020) PRINCIPLE OF GEOLOGY SEDIMENTA RY ROCKS Dr. MC Cheung Department of Earth Sciences, The University Of Hong Kong Email: [email protected] 1 2 3 Main Objectives: Weathering Processes Classification of Different Sedimentary Rocks Why Can We Read from Sedimentary Rocks? 4 Sedimentary Rock 5 Rocks that are composed of the weathered remains of pre-existing rock, or plant and animal remains. Sedimentary rocks commonly originate from sediments laid down in horizontal strata by water or wind. Sedimentary rocks represent 7% of the Earth’s crust, but they cover 70% of the Earth’s surface. Some sedimentary rocks are fossil-carrying rocks. Formation of Sedimentary Rocks 6 Formation of a sedimentary rock involves four main processes: Weathering and/or erosion Transportation Deposition Consolidation. First, sediment is formed by weathering and/or erosion of rock Then transported , from one location to another; The final location of the sediment is where it is deposited . After deposition, the sediment lithifies to form sedimentary rock. This forms a repeating cycle, each lasting thousands of years
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24/9/2019 2 Formation of Sedimentary Rocks Process 1: Weathering and/ or Erosion 7 Weathering is the breakdown of a rock Erosion is the removal of the broken down rock by transport by wind, water and/ or ice from its original place The broken down material is called sediment Weathering Examples Mechanical wind, stress Chemical water, acid rain Biological plant roots which release acids Erosion Examples Abrasion in a river when small fragments hurl against larger rocks Wave action Plucking a glacier picks up rocks as it moves Weathering and Erosion Mechanical Weathering : Breaking down of rock by physical processes without changes in chemical composition Frost wedging 1) Alternate heating and cooling: Frost wedging (water expands 9% of volume when frozen) Thermal expansion. 8 Mechanical Weathering : 2) Unloading / Decompression Weathering and Erosion causes jointing, rock surface breakdown 9 Grass Island Mechanical Weathering : 3) Wave action Very common in coastal region, e.g. Hong Kong Weathering and Erosion 10 Chemical weathering : Decomposition of rock as the result of chemical attack. Chemical composition changes. Minerals in rocks undergo chemical reactions to form new minerals. Over time produce dramatic results. Water, acids, and oxygen Weathering and Erosion Philippe Bourseiller/Getty Images 11 1. Chemical Weathering From Oxygen : Oxidation Reaction with oxygen (rock rusts), when oxygen reacts with iron to form iron oxide. Rust changes the color of the rocks. Iron oxide is very easy to breakdown, so the weathered region becomes more susceptible to breakage.
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