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Test # 2 Study Guide Weathering What is Weathering? - in-situ process - breaks down rocks and minerals at or near Surface of the Earth - 2 types - Mechanical “Disintegration” - Chemical “Decomposition” Mechanical - Causes - Frost Wedging - Crystal Growth - Salt (NaCl) growth in cracks near the ocean - Salt growth in cracks in paint on cars - Exfoliation - Root Growth - Thermal Expansion/Contraction - Abrasion Chemical - Causes - Dissolution H2O + CO2 → H2CO3 Water + Carbon dioxide → Carbonic Acid CaCO3 + H2CO3 → Ca2+ + 2HCO3Calcite + Carbonic Acid → Calcium Ion + Bicarbonate Ion - produces Karst Terrains - Oxidation FeS2 + 9.5O2 + 2H2O → 0.5Fe2O3 + 2H2SO4 Pyrite + Oxygen + Water → Hematite + Sulfuric Acid - Hydrolysis 2KAlSi3O8 + 2H+ + 9H2O → Al2Si2O5(OH)4 + 4H4SiO4 + 2K2+ Potassium Feldspar + Hydrogen (from water) → Kaolinite Clay Potassium Ions What Factors Control the Rate of Chemical Weathering? Climate – Rainfall, Temperature Living Organism Time Mineral Composition Sedimentary Rocks Studied by Sedimentologists + Silicic Acid + Formed at or near the surface of the Earth from organic or rock fragments or by precipitation from solution. Lithified by compaction or cementation of sediments, deposited in beds or layers - Part of Rock Cycle - Relative Abundance Sedimentation Processes 1) Detrital material is deposited because of energy loss velocity of stream or wind decreases 2) Chemical Precipitation - dissolved material (ions) are converted to solids Ca2+ + CO3 2- → CaCO3 (dissolved) (solid) 3) Biochemical removal of CaCO3, PO4 or SiO2 to form shells or skeletal material (coral, shellfish, diatoms, forams) Mineral Composition Sedimentary rocks are accumulations of minerals - they are rarely composed of just 1 mineral, but 1 mineral may predominate - the most common minerals found in sedimentary rocks are quartz, feldspar, mica, clay and calcite Underlined minerals have the highest degree of SiO4 polymerization are the most difficult to weather and therefore stay around to form sedimentary rocks Clays - produced by the weathering of silicates (e.g. feldspars) Calcite - main constituent of limestone Calcite is produced when calcium and carbonate in solution combine to form calcite Ca2+ + CO3 2- → CaCO3 (dissolved) (solid) The carbonate ion (CO3 2-) is produced when atmospheric CO2 reacts with water CO2 + H2O → 2H+ + CO3 2(air) (ion in solution) Other Materials Dolomite (Ca, Mg (CO3)2) if > 50% (Ca, Mg (CO3)2), rock is called dolomite Iron oxides - (hematite, magnetite, goethite) product of ferromagnesian minerals weathering Evaporites - produced when H2O evaporates - gypsum CaSO4-4H2O - anhydrite CaSO4 - halite NaCl Volcanic matter - ash + cinders Organic matter - gives dark color to rocks such as black shales and coal Origins of Sedimentary rocks 1. Detrital deposits - weathering products of existing rocks (gravel, sand, silt, clay) 2. Chemical deposits - inorganic processes cause minerals to precipitate from solution - organic processes cause the precipitation of minerals through the removal of ions from solutions (e.g. seawater) Sedimentation - Source Material 1) Ultimate source of material is igneous rocks 2) Intermediate source is metamorphic and sedimentary rocks Methods of Transport 1) Water (streams, rivers, glaciers, ocean currents) 2) Landslides (mass wasting) 3) Wind (aeolian deposits) All these transport process are driven by gravity, the source of the energy to transport sediments Effects of Transport 1) rounding 2) sorting Sediments are deposited in layers called strata or stratum. Thin sedimentary layers (<1cm thick) are called laminae Lithification - the process by which unconsolidated sediment is formed into rock There are 3 Lithifying Processes Cementation - pore spaces between detrital grains are filled by a binding agent (e.g. calcite, dolomite, quartz, hematite) Compaction - overlying weight of sediments forces H2O out and mineral grains pack together Chemical Diagenesis - recrystallization 2CaCO3 + Mg2+ Calcite → CaMg(CO3)2 + Ca2+ Dolomite - dissolution / precipitation CaCO3 → CaCO3 or SiO2 → SiO2 - reaction rates increase with burial because: 1) Pressure increases with depth 2) Temperature increases with depth Geothermal gradient 1°C/30 m or 30°C/km Pressure gradient 1 bar/3 m or 300 bars/km (1 bar ~ 1 atmosphere) Rock Types Produced Sediment Rock mud (clays) shale sand sandstone gravel conglomerate lime muds limestone + dolomite SEDIMENTARY ROCK TEXTURES 1) Clastic Texture - composed of mineral and rock fragments - texture varies with depositional process or environment (desert, glacier, delta, shoreline) - chemical sedimentary rocks sometimes show clastic texture (shell material) - grain size is useful in classifying sedimentary rocks 2. Nonclastic or Crystalline Texture - composed of interlocking grains - exhibited by chemical sedimentary rocks that precipitated from a solution - initial grain size of the precipitate is small - “grains” settle out and accumulate as mud - mud is compacted causing the grain size to increase because of Pressure Detrital Rocks 1. Conglomerate - >35% gravel size or larger - breccia - angular fragments - tillite - glacial deposits 2. Sandstone - > 50% sand-sized particles - has four components a) sand-sized grains b) matrix - clays, micas, <1/16 mm c) cement - SiO2, CaCO3 d) pore space - important for movement and storage of fluids such as water, oil and gas. 3. Siltstone + Claystone (shales) - claystone - finest, >50% is < 1/256 mm - siltstone - >50% is between < 1/256 and 1/16 mm - mudstone - mixture of sand, silt and clay none of which makes up >50% - shale - all fine-grained detrital rocks Chemical Sedimentary Rocks - subdivided by composition - carbonates and evaporites are the most abundant 1. Limestone - Mostly CaCO3 ~ 90% of Limestone is formed by living organisms - Limestone is easily dissolved in H2O CaCO3 → Ca2+ + CO3 2(solid) (dissolved) 2. Dolomite - > 50% CaMg(CO3)2 - formed from limestone by partial replacement of Ca by Mg 3. Evaporites - minerals precipitate from lakes and seawater - principal minerals are gypsum, anhydrite, halite - least soluble minerals precipitate first - as H2O evaporates, the remaining H2O becomes more saline - when the salinity >3 times seawater, gypsum precipitates - when the salinity >10 times seawater, halite precipitates 4. Coal - derived from ancient swamp-dwelling plants - rich in carbon - formed in areas of low relief, poor surface drainage, warm temperatures, and abundant rainfall - removal of water and volatiles with compaction results in increasing coal rank Relative Abundance of Sedimentary Rock Types Shale Sandstone Limestone ~ 70 % ~ 20 % ~ 10 % Sedimentary Features Bedding - horizontal bedding - parallel layers - cross-bedding - beds inclined at an angle Graded Bedding - characterized by the progressive decrease in particle size upward - formed by the sudden decrease in velocity of sediment-laden currents Ripple Marks - small ridges formed on the surface of sediments by moving wind or water. Mud Cracks - polygonal pattern formed on sediment surface from drying and shrinking of freshly deposited mud. The dried mud can also preserve rain drops and salt hoppers Nodules, Concretions and Geodes - formed sometime after sediment was deposited. - irregular or disc-shaped body differing in composition from the host sediment. - formed parallel to bedding planes - usually <1/3 m in diameter (e.g. chert nodules in Limestone) Concretions - spherical or disc-shaped sedimentary rock that is more firmly cemented than the surrounding host rocks - the cement is usually calcite, silica or iron oxide - the spheres are usually up to <1 m in diameter (some <3 m) Geodes - hollow in the center and commonly lined with crystals of quartz or calcite - common in Limestone - up to 30 cm in diameter Fossils - preservation or imprints of hard parts of plant and animal remains - abundant in Limestone, Shale and some Sandstone - useful in determining the age of the rock Metamorphic Rocks Metamorphism - solid state change in response to temperature, pressure and chemical environment. - takes place within the Earth’s crust below the zone of weathering and cementation and above the zone of remelting. What are the agents of Metamorphism? 1. Heat - Temperature range from 100° to 900°C - reactions involved in the formation of new minerals proceed faster at higher temperatures 2. Pressure - Pressure range from 1 to 10,000 bars (What’s a bar?) - effects of pressure (1) causes recrystallization and formation of new minerals (closer atomic packing) (2) causes rocks to flow and creates linear features (foliation) 3. Chemically Active Fluids - hydrothermal solutions heated by or released from magma - fluids are important in dissolving, transporting and precipitating minerals - fluids tend to increase reaction rates Does the appearance of the rocks change when metamorphosed? Metamorphic Textures 1. Foliated - planar or parallel orientation of mineral grains - foliation imparts a rock cleavage - types of foliation a) slaty - smooth, regular cleavage planes on a microscopic scale slaty cleavage b) phyllitic - more irregular cleavage planes barely visible to the naked eye c) schistose - rough, clearly visible cleavage surfaces, flakes d) gneissic - coarse foliation or bands of different mineral composition 2. Unfoliated - no preferred orientation of breakage Can a rock containing only one mineral be metamorphosed? Composition 1. Monomineralic - composed dominantly of 1 mineral - marble (calcite) - quartzite (quartz) - unfoliated or weakly foliated 2. Multimineralic - most contain only 3 minerals - includes most metamorphic rocks - common minerals are those of igneous and sedimentary rocks - new minerals are formed as a result of metamorphism: diopside tremolite garnet sillimanite chlorite How do we name Metamorphic rocks? Texture Monomineralic Unfoliated (Contact Mm) Foliated (Regional Mm) Metamorphic Rock Types quartzite marble MultiMineralic hornfels slate phyllite schist gneiss 1. Slate - metamorphosed shale (low grade metamorphism) - increasing P+T convert clays to chlorite and micas 2. Phyllite - metamorphosed slate - brighter, coarser grained than slate - at 250° - 300°C, chlorite and mica develop large flakes that gives the rock a silky sheen on fresh surfaces 3. Schist - contains visible flakes of minerals such as micas, talc, chlorite, or hematite - rock cleaves parallel to platy or fibrous minerals - most abundant of the regionally metamorphosed rocks - generally named for the predominant mineral in rock (e.g. chlorite schist, biotite schist, etc.) - characterized by > 50% platy minerals 4. Amphibolite - medium to high-grade regional metamorphic rock composed largely of ferro-magnesian minerals. - composed chiefly of hornblende and plagioclase - typically dark colored (green, gray, black) - may be meta-igneous or meta-sedimentary 5. Gneiss - coarse grained, banded - high grade regional metamorphism - segregation of minerals into distinct bands (mica; quartz + feldspar) - rocks named for igneous counterpart (granite gneiss, diorite gneiss, etc.) 6. Marble - contact or regional metamorphism of limestone or dolomite - No rock cleavage or foliation - interlocking grains of carbonate 7. Quartzite - metamorphosed quartz-rich sandstone - unfoliated - distinguished from sandstone by: a) no pore spaces and no cement b) quartzite breaks through grains not between them 8. Hornfels - contact metamorphism of Shale, Limestone, Sandstone, Tuff (or basalt) - fine-grained, hard, unfoliated - form at Temperatures > 550°C 9. Anthracite - Coal with high Carbon content because Pressure and Temperature drive off water and other volatile gases - characterized by a conchoidal fracture 10. Migmatite - rock formed by the highest grade of regional metamorphism ~ equal to partial melting of the rocks - First rocks to melt are layers with a granitic composition - melting occurs starting at 700 to 900°C 11. Blueschist - rocks form in high Pressure low Temperature conditions - these conditions occur in subduction zones - blueschist rocks are named for the diagnostic blue amphiboles and layered-silicates that grow under these conditions Is there more than one type of Metamorphism? Types of Metamorphism 1. Cataclastic / Dynamic - mechanical, localized deformation - this type of metamorphism is caused by shearing and/or grinding associated with intense folding or faulting - recrystallization and chemical changes are not common in these rocks - rocks with this type of metamorphism include: friction breccias - angular, fragmented rocks mylonites - finely granulated rocks 2. Contact Metamorphism - rocks in contact with hot, igneous intrusion - occur in High Temperature, Low Pressure environments T = 150 - 800°C P = 1 - 3000 bars - metamorphism restricted to zones called aureoles (halos) around the intrusion - isochemical process in which no ions are added or lost, they are just rearranged. 2b. Contact Metasomatism - ions are added and lost by exchange between the fluids and the rocks - escaping gases and magmatic solutions cause ionic transfer into the surrounding rocks or heated groundwater around the intrusion can cause the same thing to occur. - process results in recrystallization / alteration of country rocks - develops in late stages of mountain building and occurs at shallow depths - often results in the formation of ore deposits (Cu, Zn, Pb, Fe) 2. Regional Metamorphism - is extensive in nature and can involve areas that are 100’s of km by 100’s km - occurs deep in the Earth at high Pressure and Temperature - associated with roots of folded mountains formed by plate collisions + continental shields - results in the formation of new silicate minerals - zones of regional metamorphism (at same pressure) low grade 300-500°C medium grade 500-600°C high grade >600°C - metamorphic grade is reflected by mineral assemblages (index minerals) Where do Metamorphic Rocks Occur on Earth? Geologic Time and Dating - Length of Geologic Time - Relative Dating - Absolute Dating Absolute Dating - K-Ar (Potassium-Argon) method is the most widely used (half life of 1.25 b.y.) - K (potassium) occurs in many different minerals - works best for Igneous and Metamorphic rocks that have formed or cooled quickly - works by trapping Argon in Crystal structure - Method is no good for rocks / minerals that have cooled slowly or have been reheated. - Method can be precise to +/- 5 m.y. for rocks 2.5 b.y. old Carbon - 14 Dating - half life 5730 years - produced by cosmic radiation in upper atmosphere - incorporated into all organic matter while living - upon death 14 C begins to decay to 14 N by beta decay - method good for material up to 50,000 years old (e.g. 15,000 year old moccasins from Missouri cave) - 1 to 4% variation in production rate of 14 C is a small problem Problems with Absolute Dating 1) most radioactive elements only occur in trace amounts and do not form minerals of their own. 2) typical concentrations of radioactive elements in rocks and minerals is in the parts per million (ppm) range (e.g Uranium and Thorium concentration in granite is ~ 5 ppm). 3) the low concentration of radioactive parent and daughter elements can affect how precise an age we can determine. Instruments can measure the abundance of elements in the parts per billion range 4) the accuracy to which a parent decays to a daughter element (half-life) is know can also affect the precision of and date. 5) the parent → daughter half-life for Uranium 235 → Lead 207 is considered accurate to < 1%. 6) parent and daughter isotopes can can move after the radiometric clock has started. - causes - weathering of minerals - alteration of minerals (hydrothermal) - heating of samples during burial or metamorphism 7) many rocks cannot be dated because: - rocks are too young to date - rocks are too old to date (parent element has all decayed) - they contain insufficient concentrations of radioactive elements (e.g. quartz does not accommodate radioactive elements) Absolute Dating Safeguards - run duplicate analyses - use several different radioactive isotopes to determine an age - if different methods agree you probably have a good age - compare absolute ages to relative ages Other Methods for Measuring Absolute Time - Tree ring growth - oldest tree is a huon pine in Tasmania ~ 10,500 years (covers 2.5 acres and may be 30 - 40,000 years old) - Ice Layers in Glaciers - glaciers in Greenland, Antarctica and highest mountains are up to 3,000 m thick (2 miles) - glaciers are built up by annual deposition of snow and packing of this into a more dense material called firn. - there is a nearly continuous record over 65,000 years - composite cores provide us with a record extending back 165,000 years - Varves - layers of clay deposited in glacial lakes (1 dark and 1 light colored layer = 1 year) - layers in Baltic Sea extend back 20,000 years - Carbon-14 dating of organic matter in dark layers helps confirm age of layers (if not enough organic matter we can only establish total - not absolute time Folds, Faults and Mountains Folding, Faulting + Mountain Building are studied by Structural Geologists Why do rocks fold or break ? It has to do with deformation characteristics What processes cause rocks to be deformed ? - compression - Tension - Shearing Where on Earth do rocks become deformed ? What are Synclines and Anticlines ? How do we measure their orientation? - strike - dip What controls the structures that form? 1) rock composition 2) amount + orientation of pressure - symmetric - asymmetric - overturned - recumbent Plunging Synclines and Anticlines Domes + Basins Faults What is a Fault ? - Hanging Wall - Footwall Types - Normal - Oblique - Reverse / Thrust - Strike-Slip Why do we want to understand Faults? Implications of Faults - Ore Deposits - Landslides - Dam construction - Earthquakes Recognizing Faults - scarps - Valleys and Lakes - Offset Hills / streams - Surface rupture Where do Faults occur on Earth? Folds + Faults in Oil Exploration - Structural Traps Mountain Building What is a Mountain? - part of the crust of the Earth that stands >300 m (1000’) above the surrounding land. How do Mountains form ? - By a process called Orogenesis - Volcanic Activity - Folding + Thrusting - Upwarping - Faulting Mountain Unbuilding Earthquakes What is an Earthquake ? - it is the vibration of the Earth produced by the rapid release of energy - the vibrations radiate in all directions from the focus What causes an Earthquake ? - it is caused by the rapid release of elastic energy stored in rocks on either side of a fault. - discovered by H.F. Reid who studied 1906 San Francisco Earthquake What is elastic energy ? What evidence is there that earthquakes occur along faults ? How do we measure Earthquake activity ? - measured using the Mercalli Intensity Scale - Mercalli Scale ranges from I to XII - it measures the effect that seismic waves has on things - measured using the Richter Scale - Open ended logarithmic scale that starts at 0 - it measures the amplitude of waves measured by a seismograph Richter Scale - frequency of Earthquakes What type of seismic waves are generated by an Earthquake? Body Waves - Primary (P) + Secondary (S) Surface Waves – Love + Rayleigh - What can we learn from the different seismic waves ? - Location of the Epicenter of the Earthquake What factors contribute to damage caused by Earthquakes ? 1) Proximity to the Epicenter 2) The intensity of the Earthquake 3) Duration of the vibrations - most earthquakes last for < 1 minute - Loma Prieta, CA, 1989 - 15 seconds - North Ridge, CA, 1994 - 40 seconds - Alaska, 1964 - 3 to 4 minutes 4) Proximity to Faults 5) Nature of the material on which a structure is built - Liquefaction - Amplification of waves - Landslides / Subsidence 6) The design of the structure - materials used - reinforcement - joints between walls and ceiling - connection between building and foundation - in support elements - height of building 7) Fires after the Earthquake 8) The presence of large bodies of water - Tsunami Can we predict when Earthquakes will occur ? No ! “Prediction provides a happy hunting ground for amateurs, cranks, and outright publicity-seeking fakers” What research is being done to find a way to predict when and where an Earthquake will occur ? 1) Short-Range Predictions 2) Long-Range Predictions Short-Range Predictions - monitor possible precursors to an earthquake 1) swarms of microearthquakes prior to a Earthquake 2) Preseismic Uplift / Subsidence (Geodetic Level Surveys) 3) Tilting of Land 4) Geophysical observations (other than seismic) 5) Radon Gas 6) Changes in water tables 7) Seismic Activity Short-Range Predictions - monitor possible precursors to an earthquake 1) swarms of microearthquakes prior to a Earthquake problem - microearthquakes do not always precede a quake 2) Preseismic Uplift / Subsidence (Geodetic Level Surveys) 4) Geomagnetic observations - resistivity - magnetic - low frequency sounds 5) Radon Gas 6) Changes in water table 7) Seismic Activity (this is really a warning system) Long-Range Predictions - work on premise that Earthquakes are repetitive. 1) Seismic Gaps 2) Paleoseismology (Old Earthquakes) 3) Fault Creep 4) Mapping of existing faults and scarps 1) Seismic Gaps - Parkfield, California 2) Paleoseismology (Old Earthquakes) - Gurvan Bulag thrust fault disrupting alluvial fans in Mongolia 3) Fault Creep (using lasers / strain meters) 4) Mapping of existing faults and scarps The problem with making predictions 1) Disruptive to society 2) Costly if not correct - Iben Browning - predicted a major Earthquake would occur in New Madrid on Dec 2 or 3, 1990 - 100’s of schools were closed for a week - industry all but unproductive for weeks before - adults were terrified which affected their children - public may no longer believe legitimate predictions positive - temporary preparedness for earthquake 3) Makes public immune to actual risk if too many are false