Geology 111 Exam 2 Study Guide
Geology 111 Exam 2 Study Guide 111
University of Louisiana at Lafayette
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This 16 page Study Guide was uploaded by Maria Luisa Cepeda on Thursday March 3, 2016. The Study Guide belongs to 111 at University of Louisiana at Lafayette taught by Dr. Duex in Spring 2016. Since its upload, it has received 36 views. For similar materials see Geology in Geology at University of Louisiana at Lafayette.
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Date Created: 03/03/16
Geology 111 Exam 2 Study Guide Ø Volcanoes o Volcano-‐ landform created by eruption of lava o Most volcanoes are found along convergent margins Ø Sediment: loose material that accumulate on the Earth’s surface including rock fragments, mineral grains weathered from rocks, animal shells, crystals that have precipitated from water. o Sedimentsàphysical + chemical products of weathering o Types of sediment: § Detrital Sediment: fragments of preexisting material of sedimentary, metamorphic and igneous rocks. § Chemical Sediment (inorganic): precipitants from dissolved minerals àex: carbonates, halite § Biochemical Sediment (organic): plants and animal shells or skeletons Ø Sedimentary Rocks: form when loose particles of sediment are compacted, cemented or lithified (hardened to a solid mass) o Thin layer, barely 5% of the Earth’s crust o 75% of exposed rocks on Earth o Economically important o Layers of sed rocks tell us about Earth’s history and past environments and ecosystems (environment of formation) o Most are created by weathering of the continental crust. o Some are remains of organisms’ mineral shells How do Sedimentary Rocks Form? o By surface processes of the rock cycle § These processes move materials from source area (where sed particles are created) to a sink area (where they are deposited in layers) 1. Weathering -‐ breaks down rocks physically and chemically 2. Erosion-‐ carries away particles of rock produced by weathering and moves them away from source area. Occurs mostly when rainwater runs downhill. 3. Transportation-‐ sediment particles are moved to sink areas. Occurs when water, wind, or the moving of ice glaciers transport particles to new places. § Most sediments are transported by currents of air or water § The faster the current, the larger the particles it can transport § Air currentsàmove sediments in smaller quantities than rivers or ocean currents. § Particles become more rounded and slightly smaller as they are transported. -‐ Strong currentsàcarry gravel which includes boulders, cobbles, and pebbles -‐ Moderate Strong currentsàlay down sand beds -‐ Weak Currentsàcarry muds composed of finest clastic particles (silt and clay) 4. Deposition (or sedimentation):occurs when transportation stops, sediment accumulates, particles settle out or dissolved minerals precipitate. 5. Burial: occurs as layers of sediment accumulate and compact previous layers 6. Diagenesis or Lithification –refers to physical and chemical changes caused by pressure, heat and chem reactions. Sediments that are buried within sedimentary basins are lithifiedàor converted into sed. Rock. Groups of Sedimentary Rocks v How are Sedimentary Rocks classified? àSedimentary rocks are classified based on their composition (abundance and kinds of grains) and texture (weathering, transportation, deposition contributed t forming texture) o Detrital (clastic) Sedimentary Rocks: àRock composed of loose sediment of older rocks, fragments and mineral grains (siliclastic: quartz, feldspar, clay) that were eroded (removed) and transported over great distances away from the source location. These materials accumulate in layers. àcan have sizes from >fist size to barely visible. àDetrital sediment does NOT form in situ, because it is transported away from its source. àCreated by physical weathering. àClassified according to particle size and shape. o 3 major types: each one shows the strength of the current that transported sediments. **key: (sedimentàsedimentary rock) • Coarse-‐grained siliclastic sed rock: gravel (coarsest sediment) and conglomerate (lithified equivalent of gravel) • Medium-‐grained: sand à sandstone • Fine-‐grained: o siltàsiltstone siliclastic material containing water, like mudstone but grains are barely visible o mudà mudstone show poor or no bedding o silt + clayàshale breaks along bedding planes o clays(most abundant fine-‐grained sediment àclaystones **Chemical & Biochemical Sed Rocks** ü Classified according to chem. Composition ü Chemical + biochemical sediment is usually not transported so it is deposited in situ (where it forms) o Chemical Sediments and Sed. Rocks (evaporites): chemical residues and mineral crystals (inorganic) that precipitated from aqueous solutions. o Biochemical Sediment and Rocks (Carbonates): whole and broken parts of organisms (organic), such as shells and plant fragments. Sedimentary Structures: include features formed at time of deposition àSed rocks are characterized by bedding or stratification (diff layers of sediment deposited on top of one another) o Cross-‐bedding: beds (layers of sediment) deposited by wind or water. Well sorted, well grounded grains. § Wind-‐changes direction //// \\\\ § Water-‐ constant direction //// //// o Graded bedding: most abundant in continental slope and deep-‐sea sediments deposited by dense muddy currents. o Ripples:small ridges of sand or silt. Form in environment where wind or water travels in one direction for some time. o Turbidites § Piles of sediment accumulate on continental self § Triggered by earthquakeàsediment becomes unstable so it flows down continental slope as turbidity current § On ocean floor the current slows, forms graded beds. o Mudcracks § Polygonal patterns of cracks that develop in mud as it dries. Ø Economic importance of sed. Rocks o Construction § Sand & gravel § Clay-‐bricks, ceramics § Limestone-‐cement o Evaporites § Salt, chemicals, gypsum, KCl o Fertilizers § PO4 § Iron o Energy Sources § Uranium § Coal § “Oil”àPetroleum & Natural Gas § Oil Shale, Tar Sands Ø Requirements for Oil “pool” o Source Rock: shale (sed rock that forms from consolidated clay, can be split into fragible flat pieces) w/ organic matter o Reservoir Rock: Sandstone or Limestone w/ porosity o A trap so the oil can accumulate, can be stratigraphic or structural-‐ fold/fault. o Proper Timing: organic matter needs to “mature” into oil (via Pressure, Temperature, time) to migrate into reservoir rock and become trapped in the right feature at the right time. àIn short, we need 1. Production: source rock 2. Maturation: makes “oil” 3. Migration: reservoir rock 4. Preservation: trapping 5. Proper Timing Ø Facies: The total rock, mineral, and fossil features that tell something about the environment of formation or deposition. § Sand faciesà nearshore, high energy, coarse material left § Mud faciesàoffshore, lower energy, finer grained sediment (shale) § Ree faciesà skeletal remains (CO3, coral, algaea) in a reef, warm, clear water (limestone) Ø Sandstone, Shale, then Limestoneà TYPICAL TRANSGRESSIVE SEQUENCE § Indicates water gets progressively deeper Ø Metamorphic Rocks: rocks that have undergone changes in texture and/or composition in the solid state. § Every metamorphic rock has a parent rockàthe rock that was metamorphosed. Parent rocks can be any of the 3 rock types (igneous, sedimentary or even metamorphic since it can be metamorphosed again) § Metamorphism: the solid-‐state change ( in mineralogy or physical texture) of a pre-‐existing rock due to changes in its environment caused by intense heat, intense pressure, or fluid activity. § Exposed metamorphic rocks make up large parts of continents Ø How are Metamorphic Rocks Named? § 1 observable feature: foliated vs. non-‐foliated • Foliation: rock cleavage: the tendency for a rock to break along smooth planar surfaces due to alignment of flat minerals § Foliated rocks: parallel layers of platy(flat) minerals such as micas. The foliations form w/ directed pressure causing mineral crystals to slide parallel to one another. àuse grain size first w/ distinctions based on composition ♦ Grain size increases from fine to coarse: slate (very fine grained), phyllite, schist (Coarse grained), gneiss (coarse grained w/muscovite and biotite), with mineral or rock modifiers • Non-‐foliated rocks: have no layering, have following textures: crystalline, sandy, glassy) ànamed mainly on the basis of composition ♦ Calcite=marble ♦ Quartz= quartzite ♦ Clay= hornfels ♦ Green minerals= greenstone Ø Agents of Metamorphism § Confining Pressureàapplied equally in all directions • Increases with depth below Earth’s surface • Rocks become more dense as pore space is squeezed, may remain nonfoliated § Directed Pressure (differential stress)à pressure that isn’t equal in all directions • Causes rock to get foliated § Temperature • Increasing temp with depth • Radioactive decay • Deformation • Migrating magmas § Fluidsà enhance metamorphism by increasing rate of chemical reactions • Fluid Source ♦ Water trapped in pore space of sed. Rocks ♦ Magmatic fluids Ø Types of Metamorphism & Ways of Describing it: § Regional • Results from variable temperature and pressure generated by processes at convergent plate boundaries • There is directed pressure (differential stress) and high temp that causes foliation of the rocks • Index Minerals: form at different Pressure, Temp conditions. It indicates a grade of metamorphism. ♦ Ex: polymorphs of AlSiO5 same composition, 3 different structures • Metamorphic zonesà characterized by distinctive mineral assemblages § Contact • Produced at or near an igneous body; high Temp and Fluids alter the surrounding rock § Burialàmost common type of metamorphism as rocks form and get buried. • Confining pressure causes it. • Changes in rocks due to accumulation of thick piles of sed. Rocks. § Shock (impact) • Associated w/ impact of extraterrestrial bodies (High Pressure & Temp; short time) Ø v Geological Time: Ø The Geologic Timescale *You need to know the 4 major divisions/Eons and their ages and eras in the 3 Eras in the youngest Eon § Divisions (from longest to shortest) Eons, Eras, Periods, Epochs • Eons: 4 major divisions ♦ Hadean 4.6-‐4.0 Ga ♦ Archean 4.0-‐2.5 Ga ♦ Proterozoic 2.5-‐0.543 Ga ♦ Phanerozoic 543 Ma-‐ now Ø Eras: Cenozoic, Mesozoic and Paleozoic Ø Modern Dating of Rocks § Absolute Dating: uses radiometric or other finite methods of dating. Produces ages in numbers • Determine age of sample ♦ 1. Measure parent and daughter isotopes ♦ 2. Calculate number of half-‐lives elapsed ♦ 3. Multiply # half-‐lives by half-‐life value of isotope pair § Relative Dating: uses physical characteristics, positions, and field relations of rocks to determine age. Places events in chronological sequence • Principles of Relative Dating: ♦ Original Horizontality: sediments and extrusive igneous rocks are deposited in horizontal beds ♦ Superposition: younger rocks are on top of older rocks ♦ Faunal Succession: Ø organisms have changed over time and the order is preserved in fossil record. Age of a rock layer can be determined by its fossil content. ♦ Cross-‐cutting Relations: Ø A rock or feature is younger than any rock or feature that it cuts ex: faults or dikes are younger than the rocks that they cut ♦ Inclusions: a rock is younger than any rock it includes or encloses. Ex: a sandstone is younger than the fragments of granite that are mixed in with the sand. ♦ Unconformities Ø Disconformity: gap in the rock record between 2 parallel layers of sed. Rock. Very irregular surfaces Ø Angular unconformity: boundary between tilted sed rocks and horizontal overlying rocks. Ø Nonconformity: boundary between igneous and/or metamorphic rocks and overlying sediments Ø Geologic Ages § Age of the Earthà4.6 billion years § Age of oldest rocks on Earthà3.96 billion years § Age of oldest fossil on Earth à3.77 billion years (in Australia) Ø Paleontology: the study of life in the past based on the fossils of plants and animals § Fossils that are preserved in sed rocks are used to determine relative age + environment of deposition
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