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9/19/2011 Introduction Rocks and minerals are disintegrated and decomposed by the processes of mechanical and chemical weathering. Chapter 6 Weathering, Soil This breakdown occurs because the parent material reacts with its new physical and chemical environment transforming it into a new equilibrium state. and Sedimentary Rocks Geo-inSight 4., p. 136 Introduction How Are Earth Materials Altered? The products of weathering include soluble salts, ions in solution, and solid particles How does weathering differ from erosion? Weathering is the mechanical and chemical alteration of Earth materials at or near the surface E i involves Erosion i l removing i weathered th d materials t i l from their place of origin-by running water or wind, for example. These products of weathering can be eroded and become sedimentary rock or modified in place to become soils. soils Fig. 6.2, p. 135 How Are Earth Materials Altered? Fig. 6.1, p. 134 How Are Earth Materials Altered? Weathering and erosion take place at different rates Mechanical Weathering Frost action Pressure release Thermal expansion and contraction Crystal growth Activities of organisms. This can occur even on the same body of rock beca se rocks are not because compositionally and structurally homogenous throughout, thereby producing uneven surfaces. The products of mechanical weathering are chemically the same as their parent materials. Geo-inSight 9., p. 137 Fig. 6.9d, p. 142 1 9/19/2011 How Are Earth Materials Altered? How Are Earth Materials Altered? Mechanical Weathering Mechanical Weathering Pressure Release and Sheet Joints Frost Action When water freezes in cracks in rocks it expands and then it contracts when it thaws, thus exerting pressure and opening the cracks wider. Repeated freezing and thawing disaggregates rocks into angular pieces that may tumble downslope and accumulate as talus. Sheet joints are fractures that more or less parallel exposed rock surfaces, especially rocks now at the surface that formed under great pressure at depth. These Th jjoints i t fform iin response tto pressure release; l th thatt iis, when h th the rocks formed, they contained energy that is released by outward expansion. Fig. 6.3a, p. 138 How Are Earth Materials Altered? Fig. 6.4 a-b, p. 138 How Are Earth Materials Altered? Mechanical Weathering Mechanical Weathering Thermal Expansion and Contraction Salt Crystal Growth Volume changes in rocks and minerals with temperature changes Outside expands faster than inside (poor thermal conductivity), and/or dark minerals expand faster than lighter-colored minerals. Over time the stresses produce fracturing and eventual mechanical decomposition. http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/spalling.jpg How Are Earth Materials Altered? Salt crystals form in fractures. As they grow, they exert pressure on the rock causing the fractures to grow. Coastal areas and regions where salt is used on roads are susceptible to weathering through salt action. http://home.tiscali.nl/~wr2777/Salt-weathering.html How Are Earth Materials Altered? Mechanical Weathering How do organisms contribute to mechanical and chemical weathering? Any organic activity such as tree roots growing in cracks contrib tes to mechanical contributes weathering Organic acids and the tendrils of mosses and lichens aid in the chemical alteration of parent material. Fig. 6.5b, p. 139 Chemical weathering Hydration Solution Oxidation Hydrolysis Hot and wet environments accelerate chemical weathering. Chemical weathering occurs in all environments, except, possibly, permanently frozen polar regions. Fig. 6.7, p. 141 2 9/19/2011 How Are Earth Materials Altered? How Are Earth Materials Altered? Chemical Weathering These processes cause a change in the chemical composition. Chemical Weathering Hydration – chemical changes by adding water The parent material is transformed into products including ions in solution, soluble salts and clay minerals. Anhydrite and Gypsum are close “cousins” Anhydrite (CaSO4) has a hardness of 3.5 and density of 3.0 g/cm3. Gypsum (CaSO4·2H2O) has a hardness of only 2.0 and a density of only 2.3 g/cm3. Gypsum is softer, less dense and easier to weather. Fig. 6.6, p. 140 How Are Earth Materials Altered? How Are Earth Materials Altered? Chemical Weathering Oxidation – rocks rust Chemical Weathering Solution – rocks dissolve Carbonate Rocks and Evaporites Rocks such as limestone (CaCO³) are nearly insoluble in neutral or alkaline solutions, but they rapidly dissolve in acidic solutions. Other minerals, such as halite and gypsum, also readily go into solution. Rocks such as sandstone may contain iron minerals that will breakdown when exposed to the atmosphere Rocks containing mafic minerals will also alter to oxide and hydroxide minerals The atoms making up the minerals dissociate, that is, they separate as the rock rusts away. Geo-inSight 4., p. 136 How Are Earth Materials Altered? Chemical Weathering Hydrolysis – breakdown to clays Potassium Feldspar During hydrolysis hydrogen ions react with and replace positive ions in potassium feldspar The result is clay minerals and substances in solution such as potassium and silica. Kaolinite See p.140. How Are Earth Materials Altered? Chemical Weathering Factors That Control the Rate of Chemical Weathering Mechanical weathering enhances chemical weathering by breaking material into smaller pieces, thereby increasing the surface area for chemical reactions reactions. Because chemical weathering is a surface process, the more surface exposed, the faster the weathering. K-spar http://www.mindat.org/photo-46933.html Fig. 6.8 a-c, p. 141 3 9/19/2011 How Are Earth Materials Altered? How Are Earth Materials Altered? Chemical Weathering Chemical Weathering Factors That Control the Rate of Chemical Weathering All chemical weathering processes are enhanced by the presence of water. Climates that have ha e more rainfall are more likel likely to produce faster weathering rates. Factors That Control the Rate of Chemical Weathering The type of material is very important, since certain minerals weather faster at the Earth’s surface than others. others Silicate minerals that form at lower temperatures, such as quartz, are more stable than higher temperature minerals such as olivine. Also, the products of weathering – clay minerals and oxides – are more stable. Highly soluble minerals – such as halite and gypsum – are highly unstable. Therefore, the mineral content of the rock helps determine the rate of weathering. http://www.beringia.com/climate/content/coastmountains.shtml Soils How Does Soil Form and Deteriorate? Soil Composition Soils – Definitions According to soil scientists, a soil is a mixture of weathered materials, air, water and organic matter capable of supporting plant growth. According to an engineer, a soil is any loose material at the Earth’s surface removable without blasting. Regolith is a term geologists use for any unconsolidated material. Soils consist of weathered materials, air, water, humus and also the plants which they support. Fig. 6.10a, p. 143 How Does Soil Form and Deteriorate? Soil Production Soil is produced at a rate of 2.5 cm per century The Soil Profile Soil formation produces horizons that are known in descending order as O O, A A, B, and C. These horizons differ from one another in texture, structure, composition and color. Therefore, soil is a non-renewable resource. We can improve the soil with fertilizer, but the upper portion, the topsoil, is critically important to the future. Fig. 6.10b, p. 143 4 9/19/2011 How Does Soil Form and Deteriorate? How Does Soil Form and Deteriorate? Factors That Control Soil Formation Factors that Control Soil Formation Climate - Certainly climate is the most important factor because chemical processes operate faster where it is warm and wet. Laterite is a deep red soil typical of the tropics where chemical weathering is intense. Laterites are made up of clays and the most p insoluble compounds that were present in the parent material. Soils known as pedalfers develop in humid climates such as that of the eastern United States and much of Canada. Soils of arid and semiarid regions are known as pedocals, and may contain hard, irregular masses of caliche (calcium carbonate) in horizon B. Fig. 6.11, 6.12, p. 144-145 How Does Soil Form and Deteriorate? Fig. 6.12, p. 145 Types of Soil Soils can be divided into Residual and Transported soils. Other Factors That Control Soil Formation Parent material Organic activity Relief and slope Time Residual Soils form in place directly on bedrock; the resulting soil is greatly influenced by the parent bedrock. Transported Soils form on materials that have been transported to their destination via various agents of transportation, such as gravity, wind or water. Fig. 6.7, p. 141 Types of Soil Residual Soils depend on rock type and climate: Granite – formed by mechanical and chemical processes with a mixture of sand and clays; deep in humid regions and thin in arid regions. Other igneous and metamorphic rocks – composition of soil depends on parent material – could contain more clays or oxides than quartz. Sandstone – thin, sandy soils. Shale – thicker, clay-rich soils. Some clays are expansive, and this can lead to major problems with building foundations. Limestone – leftover materials after calcite dissolution (chert, sand, clay) gives thicker soils (humid) and thinner soils (arid) Types of Soil Transported Soil types depend on material, which depends on agent of transportation: Colluvial soils are formed on the remnants of material moved downslope by gravity. These would be closely associated with their residual counterparts. counterparts Alluvial soils are formed on all sediment deposited by streams (flowing water). These would tend to be a good mixture of sand, silt, clay and organic matter. Glacial soils are formed on sediment deposited by ice. Soil quality would depend on deposited material. 5 9/19/2011 Types of Soil Types of Soils Soils can also be classified by grain size using percent sand, silt and clay. Transported Soils (cont.): Lacustrine and marine soils are those formed on sediment deposited in lakes or the oceans. Deep water deposits tend to be very clay-rich clay rich, whereas nearshore deposits are sandier. Eolian soils are formed on sediment deposited by wind. Fine silt and clay transported by wind (loess) makes some of the world’s best and most fertile soils. http://www.organicrosecare.org/articles/soils_primer.php?continued=yes How Does Soil Form and Deteriorate? How Does Soil Form and Deteriorate? Soil Degradation - Any soil losses, physical Soil Degradation changes, or chemical alteration is called soil degradation, and all lead to reduced soil productivity. Causes include erosion, compaction, and any kind of chemical pollution that inhibits plant growth. Soil erosion is caused mostly by sheet and rill erosion. It is a problem in some areas, especially where accelerated by human activities such as construction,, agriculture, g , ranching, g, and deforestation. Fig. 6.13, p. 146 Fig. 6.14, p. 147 How Does Soil Form and Deteriorate? The Dust Bowl – An American Tragedy Soil Degradation Nutrient depletion Loss of nutrients is most prevalent in areas of land overuse. Improper disposal of chemicals and concentrations of insecticides can destroy soil soil. Geo-Focus Fig. 1 a-c, p. 149 Fig. 6.14, p. 147 6 9/19/2011 Weathering and Resources Sediment and Sedimentary Rock The two primary types of sediment are detrital and chemical. Sedimentary rock is simply rock made up of consolidated sediments. Intense chemical weathering causes the concentration of valuable mineral resources Residual concentrations – bauxite and other valuable minerals are concentrated by selective removal of soluble substances during chemical weathering Bauxite, which forms in lateritic soils in the tropics, occurs in areas where chemical weathering is so intense that only the most insoluble compounds accumulate in the soil. Aluminum is just such an insoluble compound. Laterites are the primary source of aluminum oxide, called bauxite. It is the main source of aluminum ore. Gossans - hydrated iron oxides formed on the earth’s surface by oxidation of iron. Sulfide minerals leach out and concentrate as deposits of iron ore, copper ore, lead and zinc ore beneath the gossan. Detrital sediment consists of solid particles, products of mechanical weathering. weathering Chemical sediments consist of minerals precipitated from solution by inorganic processes and by the activities of organisms thru chemical weathering. Fig. 6.15, p. 150 Sediment and Sedimentary Rocks Sediment and Sedimentary Rocks Sediment Transport and Deposition Sediment Transport and Deposition Sedimentary material weathers, undergoes erosion and transport to a new location. Transportation of sediment results in rounding and sorting. Why are rounding and sorting important in sediments and sedimentary rocks? Both are important in determining how fluids move through sediments and sedimentary rocks The amount of rounding and sorting depends on particle size, distance of transportation, and depositional processes. Sediment and Sedimentary Rocks Eventually the sediment comes to rest in a depositional environment. Depositional environments are areas of sediment deposition that can be defined by their physical characteristics (topography, climate, wave and current strength, salinity, etc.). They provide geologist with clues as to how the rock formed and what the geologic past was like. Sediment and Sedimentary Rock How Does Sediment Become Sedimentary Rock? Sediment Transport and Deposition Thru the process of lithification of sediment is converted into sedimentary rock. Major depositional settings are continental, transitional, and marine. Lithification involves two processes 1. Compaction - The volume of a deposit of sediment decreases as the weight of overlying sediment causes a reduction in pore space (open space) as particles pack more closely together. Compaction alone is sufficient for lithification of mud into shale. Each of these depositional settings includes several specific subenvironments. Fig. 6.17, p. 151 Fig. 6.19c, p. 153 7 9/19/2011 Types of Sedimentary Rock Sediment and Sedimentary Rock How Does Sediment Become Sedimentary Rock? Detrital Sedimentary Rocks are made of solid particles of pre-existing rocks. Lithification involves two processes Detrital sedimentary particles are classified according to grain (particle) sizes, in decreasing diameter: 2. Cementation is a process that glues the sediments together. The most common cements are calcium carbonate and silica, but iron oxide and iron hydroxide are important in some rocks. Compaction alone will not form rocks from sand and gravel. Cementation is necessary to glue the particles together into rocks. Gravel (including boulders, cobbles and pebbles) Sand Silt Clay (or mud). Fig. 6.18, p. 152 Types of Sedimentary Rocks Types of Sedimentary Rocks Detrital sedimentary rocks are classified on the basis of particle size. Examples include conglomerate, breccia, sandstone, siltstone, mudstone, and shale. How do conglomerate and sedimentary breccia differ? Both B th begin b i as detrital d t it l gravel. l Conglomerate C l t consists i t off rounded d d gravel, breccia consists of gravel with sharp edges. Chemical and Biochemical Sedimentary Rocks Chemical and biochemical sedimentary rocks are substances derived from solution by inorganic or biochemical processes. Some have a crystalline texture, meaning they are composed of a mosaic of interlocking crystals Others have a clastic texture, meaning that they are made of fragments, like shells that are glued together. Fig. 6.19 a and b , p. 153 Types of Sedimentary Rocks Types of Sedimentary Rocks Chemical Sedimentary Rocks Chemical sedimentary rocks are classified on the basis of composition. Chemical Sedimentary Rocks Evaporites Bedded rock salt (halite) and rock gypsum are chemical evaporite sediments formed by precipitation of minerals during the evaporation of water. Carbonate rocks consist primarily of minerals containing the carbonate ion, such as limestone and dolostone. Dolostone forms when magnesium replaces calcium in limestone. Fig. 6.20b-d, p. 154 Fig. 6.21a-b, p. 155 8 9/19/2011 Types of Sedimentary Rocks Types of Sedimentary Rocks Chemical Sedimentary Rocks Biochemical Sedimentary Rocks Coal is a biochemical sedimentary rock composed largely of altered land plant remains Bedded Chert Marin County, California The origin of chert is highly debated. Fig. 6.21c, p. 155 Sedimentary Facies Fig. 6.21d, p.155 Sedimentary Facies Geologists realize that if they trace a sedimentary layer far enough, it will undergo changes in composition and/or texture. Bodies of sediment or sedimentary rocks which are recognizably different from adjacent sediment or sedimentary rocks and are deposited in a different d depositional iti l ((sub) b) environment i t are kknown as sedimentary facies. Today we recognize modern facies changes when we go from an inland area with rivers to the beach. Marine Transgression and Regression A marine transgression occurs when sea level rises with respect to the land, resulting in offshore ff facies f overlying nearshore facies. A marine regression, caused when the land rises relative to sea level, results in nearshore facies overlying offshore facies Note the difference in the vertical rock sequence that occurs in a transgression versus a regression. Reading the Story in Sedimentary Rocks Fig. 6.22, p. 156 Reading the Story in Sedimentary Rocks Sedimentary Structures Some sedimentary structures, such as ripple marks, bedding, cross-bedding, and mud cracks form shortly after deposition. Sedimentary structures are useful in determining the types of environments i t in i which hi h the sediments were deposited. Sediments are most commonly deposited flat in water. One of the most common is strata or Sedimentary Structures Depositional environments are also inferred by comparison of these structures with present-day depositional environments. Cross-bedding preserves layers deposited at an angle. They are common in depositional environments like sand dunes, shallow marine deposits and stream-channel deposits How is cross-bedding used to determine ancient current directions? Understanding how physical features like cross-beds form today can reveal important ancient climate information such as current directions. bedding. Fig. 6.23 a, p. 158 Fig. 6.23b-c, p. 158 9 9/19/2011 Reading the Story in Sedimentary Rocks Reading the Story in Sedimentary Rocks Sedimentary Structures Sedimentary Structures Ripple Marks Mud cracks Depositional environment: streams or shallow marine? Depositional environment: Lagoons and mudflats Streams have a current and leave behind asymmetric dunes. Shallow marine crossbeds exhibit a symmetrical shape from the rocking motion of the waves. Fig. 6.25 a-d, p. 159 Reading the Story in Sedimentary Rocks Fig. 6.26 a-b, p. 159 Reading the Story in Sedimentary Rocks Sedimentary Structures Fossils-Remains and Traces of Ancient Life Graded Beds Depositional environment: Submarine fans – tell us the location of the ancient shelf margin Fossils are the remains of past life and are usually found only in sediments and sedimentary rocks. They provide the only record of prehistoric life, and are used by geologists to correlate strata strata, and to interpret depositional environments. Fig. 6.24a-b, p. 158 Fig. 6.27 a-b, p. 160 Reading the Story in Sedimentary Rocks Reading the Story in Sedimentary Rocks Determining the Environment of Deposition How do we know that the Navajo Sandstone formed as a desert dune deposit? Determining the Environment of Deposition Sedimentary Rocks in the Grand Canyon Under what sedimentary conditions were these rocks deposited? Fig. 6.28 a, p. 161 Fig. 6.28 b, p. 161 10 9/19/2011 Important Resources in Sedimentary Rocks Important Resources in Sedimentary Rocks Many important natural resources are sedimentary rock deposits. These include: Petroleum and Natural Gas Most oil and gas reserves are found within sedimentary rocks. What are stratigraphic and structural traps? Both are areas where petroleum, natural gas, or both accumulate in economic quantities. Stratigraphic traps form because of facies changes in the rock layers ( t t ) (strata). Sand and gravel Coal Clay Evaporites (like salt) Banded-iron formations. Oil and gas Fig. 6.29a p. 162 Important Resources in Sedimentary Rocks Important Resources in Sedimentary Rocks Petroleum and Natural Gas Petroleum and Natural Gas Structural traps form as the result of folding or fracturing (faulting) of rocks. Oil shale is a fine-grained sedimentary rock that contains kerogen from which liquid oil and combustible gases can be derived. None is mined at present in the United States because oil and gas from conventional sources are cheaper. Oil shale and tar sands are increasingly important petroleum reserves. Fig. 6.29b, p. 162 Important Resources in Sedimentary Rocks Fig. 6.29c p. 162 Important Resources in Sedimentary Rocks Uranium Banded Iron Formation Most uranium is used in nuclear reactors. The uranium comes from the minerals carnotite and uraninite. The richest ores are found in Wyoming, Utah, Arizona and New Mexico in ancient stream deposits. Large reserves of low grade ore is found in the Chattanooga Shale, which covers portions of several states. Whyy is banded iron formation such an important sedimentary rock? Banded iron formation consists of alternating thin layers of chert and iron minerals, mostly iron oxides. Nearly all of Earth’s iron ore is mined from ancient banded iron formations. Fig. 6.30 a-b, p. 163 Fig. 6.30b, p. 163 11
