Weathering: Sedimentary Petrology and Stratigraphy PDF

WEATHERING Sedimentary Petrology and Stratigraphy GEM2105 BY JOSEPHINE MAXIMUS UNIVERSITY OF GUYANA 1  OUTLINE Introduction and Definitions What is Weathering? Types of Weathering Physical/Mechanical Chemi...

WEATHERING Sedimentary Petrology and Stratigraphy GEM2105 BY JOSEPHINE MAXIMUS UNIVERSITY OF GUYANA 1  OUTLINE Introduction and Definitions What is Weathering? Types of Weathering Physical/Mechanical Chemical Biological Types of Physical Weathering Types of Chemical Weathering Factors Affecting Rates of Weathering UNIVERSITY OF GUYANA 2  INTRODUCTION AND DEFINITIONS Sedimentary Petrology : The study is concerned with the mineralogical composition, characteristics, and origins of sediments and sedimentary rocks. Sedimentology: A branch of science that deals with sediment, sedimentary deposits, processes and rocks Stratigraphy: Study of sedimentary strata. Sediment: These are loose solid particles (grains) derived from the physical and chemical weathering of pre-existing rocks formed at the surface under low temperature and pressure. UNIVERSITY OF GUYANA 3 Sedimentary Rocks: Sedimentary rock cover roughly three-fourths of Earth’s surface. They have special genetic significance because their textures, structures, composition, and fossil content reveal the nature of past surface environments and life forms on Earth. Thus, they provide our only available clues to the evolution of earth’s landscapes and life forms through time. Sedimentary rocks contain minerals and fossil fuels that have economic significance. Petroleum, natural gas, coal, salt, phosphorus, sulfur, iron and other metallic ores, and uranium are examples of some of the extremely important economic products that occur in sedimentary rocks. UNIVERSITY OF GUYANA 4 TYPES OF ROCKS Sedimentary Rock Metamorphic Igneous rock Processes of formation of Sedimentary Rock Weathering Erosion Transportation Deposition Diagenesis- Burial, compaction, cementation, recrystallization, replacement and authigenesis UNIVERSITY OF GUYANA 5  TYPES OF WEATHERING, EROSION, TRANSPORTATION AND DEPOSITION Weathering: This is the breakdown of rock materials. The disintegration and decomposition of rock at or near the surface of the earth or simply the breaking of rocks. It affects the rocks in place and no transport is involved. (Boggs, 2006) Erosion: Removal/Transportation of sediment that has been broken down through weathering by agents such as wind, running water, glaciers, waves, gravity, and humans. Types of Weathering: Physical/Mechanical, Chemical and biological. Physical Weathering This involves the disintegration of rocks by breaking them into smaller fragments without changing their composition. Types of Physical Weathering 1) Freezing Thaw(Frost scattering) / wedging: UNIVERSITY OF GUYANA 6 Breakage of rocks as a result of water freezing 2) Salt Crystal 3) Thermal Expansion/Isolation In arid climates, dissolved salt in groundwater When the heat of an intense fire or sun bakes a precipitates and grows as crystals in open pore spaces rock, the outer layer of the rock expands. On in rocks. This process pushes apart the surrounding cooling, the layer contracts. This contract and grains and weakens the rock so that when exposed to expansion create physical stress known as thermal wind and rain, the rock disintegrates into separate pressure. E.g. desert rocks & heat from a wildfire grains. The same phenomenon happens along the coast, where salt spray percolates into rock and then dries. Salts can lead to expansion by becoming rehydrated and recreating a weathering pattern that looks like a honeycomb. UNIVERSITY OF GUYANA 7 4) Wetting and Drying: 5) Stress Release/Exfoliation: cracks and fractures caused by cycles of water Fracture of rock due to overburden. When the saturation and drying. When mud soaks water, the overburden pressure is released it causes expansion volume expands and when dry it contracts leading to and results to fracture. This change creates forces in cracking e.g. Mud cracks in deserts or streams where the rock sufficient to make the outer part of the you have low & high tides. E.g. mud cracks. rock break off into sheet-like pieces along joints parallel to the surface. E.g. Exfoliation in granitic rock. UNIVERSITY OF GUYANA 8 6) Abrasion: Surface scraping. Interaction between the rock surface and other materials e.g. materials carried by the wind, and water of glacier. UNIVERSITY OF GUYANA 9 Chemical Weathering The mineral composition is altered and the internal structure of the mineral is changed. Water is the driving force for chemical mineral composition is altered by the addition or removal of elements CO2 +H2O H2C03 H++(HCO3) HO- Carbon +Water = Carbonic acid =Hydrogen ion +Bicarbonate TYPES OF CHEMICAL WEATHERING 1) Hydrolysis: Decomposition reaction involving minerals taking in H+ or HO- ion from acids to alter mineralogy, chemistry and crystal structure. Dominant rock mineral is Aluminum (Al) Silica (Si) and oxygen (O ) During hydrolysis, water chemically reacts with minerals and breaks them down (lysis means loosen in Greek) to form other minerals. For example, hydrolysis reactions in feldspar produce clay. 2KAISi3O8+ 2H+ +H2O AI2Si2O(OH)4 +2K+ +4SiO2 K-Feldspar +Hydrogen ions+Water =Kaolinite +Potassiumions+Silica UNIVERSITY OF GUYANA 10 2) Dissolution: 3) Oxidation: Break down rock and mineral into ions in a solution. Free oxygen reacts with metal-bearing minerals to Some rock dissolves in water while others require the create metal oxides leading to a rusty colouration. presence of an acid. E.g. carbonates and Evaporites Dominant rock mineral is iron(Fe) and Manganese(Mn). create ions in solution(CaSO3). Dominate mineral rock Oxidation reactions in rocks transform iron-bearing is calcium (Ca) magnesium( Mg)and potassium( K). minerals (such as biotite and pyrite) into a rusty brown Dissolution primarily affects salts and carbonate mixture of various iron-oxide and iron-hydroxide minerals but even quartz dissolves slightly. E.g. Acid minerals. rain 4Fe2+ +8(HCO3)- +O2 +4H2O 2Fe2O3 +8H2CO3 H2CO3 +CaCO3 Ca2+ +2(HCO3)- Iron ions +Bicarbonate ions +oxygen +water =Hematite +Carbonic acid Calcite +Carbonic acid = Calciumion +Bicarbonateion Mount Roraima, Guiana Shield UNIVERSITY OF GUYANA 11 4) Hydration: Biological Weathering water is added to the chemical structure of the Breakdown of rocks due to biological processes. It can be both mechanical mineral called hydrous minerals. Dominate mineral and chemical. As its name suggests, biological weathering is a type of is Ferric oxides and evaporates. the absorption of weathering brought about by various activities of living organisms. Along water into the crystal structure of minerals, causes with other types of weathering, biological weathering can contribute to the further degradation of rocks and rock particles by making them more some minerals, such as certain types of clay, to susceptible to other environmental factors, whether be FeO +H O 2 FeO (OH) expand. 2 3 Such expansion 2 weakens rock. it biotic or abiotic factors. Hematite +Water =Goethite Types Of Biological Weathering Plants- This type of weathering occurs when a force or pressure is applied to break rocks apart or degrade the minerals in them. Increasing the exposed surface area of rocks makes it possible for other physical factors to speed up their degradation. g algae, fungi Animals- Burrowing animals like shrews, earthworms, and even ants contribute to biological weathering. In particular, these animals create holes in the ground by excavation and move the rock fragments to the surface. As a result, these fragments become more exposed to other environmental factors that can further enhance their weathering. E.g. birds, piddock shells, ants, termite & trees Activities of man. UNIVERSITY OF GUYANA 12  Examples of Biological Weathering Effect of Termite Effect of Algae & Fungi Piddock Shell UNIVERSITY OF GUYANA Effect on Trees 13 Activity of man  Factors affecting Rates of Weathering Bedrock type/ Mineral Composition Climate Surface area Minerals Composition Pollution Tectonics/Length of exposure UNIVERSITY OF GUYANA 14 1) Bedrock Type/ Mineral Composition Minerals that crystalize first out of a cooling magma are less stable. Quartz is the second most common mineral and has a conchoidal fracture. It has a hardness of 7 and is chemically stable Feldspar is the most common mineral. Rocks high in iron are most susceptible to oxidation. Dissolution affects rocks high in calcite, such as limestone and marble. Some minerals are more susceptible to weathering than other Order of Mineral Stability to Chemical Weathering UNIVERSITY OF GUYANA 15 2) Climate: Effect of Bauxite Weathering in Guyana Climate has an effect on temperature and rainfall. The near-surface rocks of Guyana are largely affected The higher the temperature and rainfall the more by chemical weathering. The products of weathering the weathering process. Chemical weathering is may give rise to mineral deposits formed in situ; known higher in warm and wet climates and least in cold as residual deposits. and dry climates while physical weathering is higher in cooler regions. UNIVERSITY OF GUYANA 16 83% of the World's supply of bauxite is found on stable continental platforms. Distribution in Space: The Guiana Shield Province of South America (this includes Venezuela, Guyana, Suriname, Guiana, and parts of Brazil and Colombia). Distribution in Time: Spore dating (1964) suggests the bauxite development period between the late Eocene and early Oligocene. Following climatic conditions are of primary importance in the formation of bauxite deposits: 1) A humid tropical or subtropical climate 2) Temps above 20°C: favor the passage of SiO2 into solution allowing its removal as the silicates break down. 3) Defined wet and dry seasons: The leaching of silica proceeds during the dry season while during the wet season the Al2O3 and Fe2O3 are formed. The alternation of wet and dry seasons leads to the concentration of these compounds. Guyana’s climatic conditions facilitate the chemical weathering that was needed to form world-class bauxite deposits. https://www.bizlatinhub.com/company-formation-guyana/ UNIVERSITY OF GUYANA 17 3)Surface Area The larger the surface area , the higher the weathering process. UNIVERSITY OF GUYANA 18 4)Pollution: Pollution speeds up weathering. Factories release carbon dioxide and other gases into the air These gases dissolve in the rainwater, causing acid rain to form Acid rain contains nitric and sulphuric acid and causes rocks and minerals to dissolve faster Effects Of Acid Rain Leaching of soil e.g. dissolves Mg, Ca and K Kill aquatic life Dissolves waxy coating protecting leaves from bacteria Corrodes metals, textile Solution Burning of fossil fuel An alternative source of energy Carbon capture UNIVERSITY OF GUYANA 19 6) Tectonics /length of Exposure: Tectonics results in the uplift and exposure of new rocks. The higher the exposure the more the rate of weathering. UNIVERSITY OF GUYANA 20 QUESTION TIME UNIVERSITY OF GUYANA 21