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Exam Block #3 • Chapter 6 – Weathering ONLY • Chapter 7 – Sedimentary Rocks • Chapter 8 – Metamorphic Rocks How to study for this class: 1. Read the chapter and answer the Review Questions. As you read, follow along with the Chapter Outline (download from our class site below) and these PowerPoint Notes. 2. Review each chapter using the GEODe: Earth CD-ROM in your book. 3. Go online and take the Concept Quizzes and Chapter Test for each chapter at: www.prenhall.com/tarbuck 4. Check your grades online at: http://pages.sbcglobal.net/solanogeo/index.htm BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 1 of 20 Chapter 6 - Weathering 1 2 Rates of Weathering Q: Why do some gravestones weather at a greater rate? Slate Arches National Park, Utah Weathering 3 Limestone The Earth’s surface is dynamic – volcanic forces and mountain building elevate portions of the surface – while opposing processes move material from higher Æ lower elevations. By: Weathering (This Chapter) – the physical breakdown and chemical alteration of rocks. Chemical Weathering – involves a chemical transformation of rock. Q: How do I chemically weather a piece of paper? A: Mass Wasting (Chapter 15) – the transfer of rock down-slope by forces of gravity. Erosion (Chapter 16) – physical removal of material by water, wind, or ice. 1. Mechanical Weathering 5 1. Frost Wedging – repeated cycles of freezing and thawing; water expands about 9% when frozen; this exerts great pressure that cracks the rock. Talus slopes are piles of loose rocks at base of cliffs. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 4 Weathering Mechanical Weathering – physical processes that break rock into smaller pieces. Q: How do I mechanically weather a piece of paper? A: 2. Mechanical Weathering 6 2. Unloading – the great reduction in pressure when the overlying rock is eroded away; generate onionlike layers called sheeting; erosion creates exfoliation domes. Ex: Half Dome, Yosemite. Page 2 of 20 7 2. Mechanical Weathering 2. Mechanical Weathering 8 Jointing – tectonic mountain building events can fracture rocks and create patterns called joints. These allow water to penetrate and start the process of weathering. Ex: Moab, Utah. 9 3. Mechanical Weathering 3. Thermal Expansion – repeated heating and cooling of rocks. This process is not very effective but more readily seen when rock has already weakened by chemical weathering. 1. Chemical Weathering 4. Mechanical Weathering 10 4. Biological Activity – plants (roots); burrowing animals; and humans (road cuts). Tree roots promote further weathering of the rock. 11 Complex processes that break down rock to new minerals and products. Water is the most important agent in these three processes: 1. Dissolution – dissolving of minerals in water, just as sugar does in water; water molecules are polar – that is – they have a small amount of charge and attract ions from minerals. 12 1. Chemical Weathering Weak acids form as groundwater travels through soil; these acids dissolve large quantities of limestones (mineral calcite) to form caves. Acid rain is formed from sulfur and nitrogen oxides released into the atmosphere by the burning of fossil fuels. The granite headstone (left) has not weathered as much as the marble one about the same age. Dissolving of salt (halite) in water. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 3 of 20 2. Chemical Weathering 13 3. Hydrolysis – reaction with water to form new products (like free iron). 2. Oxidation – the process of rusting: 4Fe + 3O2 Æ 2Fe2O3 Iron Oxygen 14 3. Chemical Weathering Iron Oxide (Hematite) Oxidation is important in decomposing of ferromagnesian minerals: olivine, pyroxene, and hornblende, but first iron must be freed from the silicate structure by hydrolysis. Chemical Weathering of Pyroxene Notice: Quartz – is very resistant to chemical weathering and can be transported great distances to the sea to become beach sand. 15 16 Spheroidal Weathering Corners of rocks have more surface area and are weathered to a greater extent, resulting in a spherical shape (Joshua Tree, CA). Hydrolysis Oxidation Spheroidal Weathering 17 Rates of Weathering 18 Several factors influence the rate of rock weathering. 1. Rock Characteristics – weathering rates follow Bowen’s Reaction Series: the first formed ferromagnesian minerals are the least resistant to chemical weathering (iron oxidizes) and the last formed minerals are silica rich and are the most resistant the chemical weathering. Basalt Granite BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 4 of 20 Rates of Weathering 19 2. Climate – warm temperatures and abundant moisture increase chemical weathering. 3. Differential Weathering – different rates of weathering due to rock type or jointing; Ex: Arches National Park. ✓REVIEW QUESTIONS 20 1. Describe the 3 processes of moving rock in the rock cycle. 2. Contrast mechanical and chemical weathering. 3. Where is frost wedging most effective? 4. Describe formation of an exfoliation dome. 5. How does mechanical weathering increase rates of chemical Weathering? SKIP SOIL SECTION ✓REVIEW QUESTIONS 21 6. Granite and basalt are exposed at the surface in a hot wet region. a) Which type of weathering will predominate? b) Which of these rocks will weather most rapidly? Why? 7. Heat speeds up a chemical reaction. Why then does chemical weathering proceed slowly in a hot desert? BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 5 of 20 Chapter 7 – Sedimentary Rocks 1 Detrital & Chemical Sedimentary Rx Detrital – solid pieces from both mechanical and chemical weathering. 2 Chemical – soluble material that precipitates. White Chalk Cliffs of Dover, England % by Volume Minerals in Earth’s Crust 3 1. Detrital (Clastic) – accumulation of material (solid particles) derived from mechanical and chemical weathering. 2. Chemical – precipitation of dissolved substances from solutions produced by chemical weathering. 3. Organic – carbon from plants & animals that form coal. What are the chemical weathering products? Erosion Resulting sedimentary rocks? Turning Sediment Into Sedimentary Rock 4 Sedimentary Rocks Weathering Deposition Diagenesis 5 6 Cementation Diagenesis – refers to changes that take place after the sediments are deposited. Lithification is the process of turning unconsolidated sediments into solid rock by two processes: (1) Compaction – volume may be reduced by 40%. (2) Cementation – minerals precipitate in the pore spaces, such as calcite, silica, and iron oxides. Bedding Planes – The end of one deposit and the beginning of another create flat surfaces separating two beds of sedimentary rock. A very distinguishing characteristic of sed. rocks. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Photomicrograph of sandstone showing quartz grains and calcite cement. Page 6 of 20 Detrital Sedimentary Rocks 7 Clay minerals are the most abundant product of chemical weathering from feldspars. Quartz is also abundant because it is extremely durable and resistant to chemical weathering. Particle Size – is the primary basis for classification of detrital rocks: mud (or clay), sand, and gravel size. Shale (Fine Particle Size) 9 Shale – splits into thin layers along well developed closely spaced planes (fissility). Mudstone – breaks into chunks or blocks. Siltstone – silt-sized particles. Weathering of Shale 8 Mud or Clay (Fine Particle Size) Mud is Lithified into SHALE: About 50% of all sedimentary rocks are shales. The size of the grains provides important information about the environment of deposition. The stronger the current (of water or air), the larger the particle size carried. Therefore, the tiny grains indicate that settling must have occurred in a quiet deposition setting, such as: lakes, river floodplains, lagoons, and deep-ocean basins. 10 Shales Create Oil Traps The clay minerals have sheet-like structures like micas. They settle nearly parallel and become tightly packed. This results in water not being able to penetrate; thus shale often forms a barrier to fluid flow. Ex: Oil and gas reservoirs are often capped by shales. 11 Sand (Medium Particle Size) 12 Sand is Lithified into SANDSTONE: 20% of sedimentary rocks are made of this size. 1. Sorting on Particle Size: (a) Well-sorted grains are all about the same size: Ex: wind blown sand; beach sand. (b) Poorly sorted – large & small particles together. Ex: mountain streams. Grand Canyon, Arizona. Weaker shales crumble and produce debris in which some vegetation grows. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 7 of 20 Sandstone (Medium Particle Size) 13 2. Particle Shape: (a) Rounded grains indicate the distance or time involved in the transportation of the sediment; likely involves air & water. (b) Angular grains indicate material was transported a short distance; likely involves other transport medium, like glaciers. Sandstone (Medium Particle Size) 14 3. Particle composition – long transport leads to the destruction of weaker and less stable minerals (feldspars, ferromagnesians). Quartz is usually the only mineral to survive long trips. Q: Feldspars grains never become well rounded. Why? A: X Types of Sandstone 15 Quartz Sandstone – mostly well-rounded quartz grains; indicates long transport & substantial weathering. Forms at beaches, sand dunes. Arkose – mostly feldspar grains with quartz and micas; source was a granite with short transport distance; usually poorly sorted and angular grains. Forms at alluvial fans. 4 TYPES OF SANDSTONE 17 16 Types of Sandstone Graywacke – quartz, feldspars with silt and clay matrix; also short distance of transport and rapid burial. Forms at offshore turbidity currents (deep sea fans). Lithic Sandstone – many other fragments besides quartz. Forms at deltas. 18 Quartz Sandstone Burial & Lithification Great Sand Dunes National Park, Colorado BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Navajo Sandstone, Zion National Park, Utah. Page 8 of 20 Gravel (Coarse Particle Size) 19 Gravel sized particles are Lithified into: CONGLOMERATE: Rounded gravel sized particles. Ex: mountain streams, glacial deposits. ✓REVIEW QUESTIONS 20 2. List the three basic sedimentary rock categories. BRECCIA: Angular gravel sized particles; rock formed near the source of weathered material. Ex: fault zone. 3. What minerals are most common in detrital sedimentary rocks? Why are these minerals so abundant? 4. What is the primary basis for classifying detrital sedimentary rocks? 6. How are the degree of sorting and the amount of rounding related to the transportation of sand grains? Chemical Sedimentary Rocks 21 22 Limestone 1. Limestone – about 10% of sedimentary rocks; make of calcite (CaCO3). (a) Coral Reefs – secrete a calcareous skeleton. (b) Coquina (kō-kē-na) – a course rock made of shells and shell fragments. Mineral Composition – is the primary basis for classification of chemical sedimentary rocks. They form from the precipitation of chemicals from solution by two methods: 1. Inorganic 2. Organic (biochemical) Cave deposits (travertine) are created by the precipitation of calcium carbonate from solution (inorganic). (a) Coral Reef Formation 23 Modern coral reef (left). Exposed Permian coral reef Texas (right). Limestone (b) 24 (c) Chalk – very fine microscopic marine organisms. (d) Inorganic limestones – direct precipitation from sea water. (d) (c) The White Cliff of Dover, England. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 9 of 20 Chemical Sedimentary Rocks 25 2. Dolostone – magnesium replaces calcium in an already formed limestone. 3. Chert – microcrystalline silica (SiO2); it can be both organic and inorganic in origin. (a) Flint – dark (organic matter present); (b) Jasper – red (iron oxides present); (c) Agate – banded form. 26 Chemical Sedimentary Rocks 4. Evaporites – Halite or Rock Salt (NaCl); Gypsum (hydrous calcium sulfate, CaSO4 • 2H2O) – Plaster of Paris used for wallboard and interior plaster. (a) Halite (b) (c) Chemical Sedimentary Rocks Evaporites form from evaporation of bodies of water. 27 Coal – The accumulation of large quantities of plant remains in a stagnant swamp with little oxygen. Decomposition and burial of organic matter changes form with depth and temperature of burial. Bonneville salt flats, Utah. Modern Swamp Coal – Organic Sedimentary Rock 29 BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 28 Peat Lignite Bituminous Anthracite Review – Classification System 30 Page 10 of 20 ✓REVIEW QUESTIONS 31 32 Sedimentary rocks are important in the interpretation of Earth history. By understanding the conditions under which sedimentary rocks form, geologists can often deduce the history of a rock, including information about the origin of its component particles, the method and length of sediment transport, and the nature of the place where the grains eventually came to rest. That is the geographic setting called the sedimentary environment. ‘The present is the key to the past’. Present-day sedimentary environments are similar to those of the past. 7. Distinguish conglomerate and breccia? 8. Distinguish between the 2 categories of chemical sedimentary rocks. 9. What are evaporite deposits? Name some sedimentary rocks that are evaporites. 12. Chemical sedimentary rocks are classified on…? Sedimentary Environments Sedimentary Environments 33 Sedimentary Environments 34 Where do these sedimentary rocks form? • arkose Where do these sedimentary rocks form? • shale • evaporates • quartz sandstone • conglomerate • graywacke • coal • limestone • lithic sandstone 35 Facies Facies is a term used to describe a unit with different types of sediments next to one another in a single sedimentary layer. When a stream reaches a delta, sand, the coarsest size, deposits at the beach, but the finer clay stays in suspension and is carried further offshore and eventually deposits a shale layer. In warm water regions, the skeletal remains of a coral reef may deposit limestone. 36 Facies Stratigraphic Section ss Regression: fall in sea level. sh ls shoreline sh ls sh ss BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 ss Transgression: rise in sea level. Page 11 of 20 Grand Canyon Stratigraphic Section 37 38 Sedimentary Structures Cross-Bedding (wind or water). Cross-Bedding Formation 39 Ripple Marks (wind or water). Modern sand dune (left). Exposed ancient cross-bedded sandstone (right). Sedimentary Structures 40 Sedimentary Structures 41 Graded Bedding (water). BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 42 Sedimentary Structures Mud Cracks (wet & dry conditions). Page 12 of 20 Sedimentary Structures 43 Fossils (Geology 3 & 4 at Solano). ✓REVIEW QUESTIONS 44 11. Name the sedimentary rock described below: a) An evaporate used to make plaster = b) A fine-grained detrital rock that exhibits fissility = c) Dark-colored sandstone containing angular rock particles as well as clay, quartz, and feldspar = d) The most abundant chemical sedimentary rock = e) A dark-colored hard rock made of microcrystalline quartz = 20. What is probably the single most characteristic feature of sedimentary rocks? Gastropods. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 13 of 20 Chapter 8 – Metamorphic Rocks 1 2 Metamorphic Rocks Metamorphism – “changed form”. Metamorphism takes place where rocks are subjected to conditions unlike those in which it formed. Rocks form new textures or new minerals. • Every metamorphic rock has a parent rock – the rock from which it was formed. • Metamorphism progresses in increments from lowgrade to high-grade, but the rock remains solid! Metamorphic Rocks, Canadian Shield Settings of Metamorphic Rocks 3 Metamorphism occurs in three settings: 4 1. Heat - The most important agent of metamorphism is heat. It causes the ions in a mineral to vibrate more rapidly and migrate to form new minerals. There are 2 sources of heat: (a) contact metamorphism with intruded magma produces a “baked” zone – high temp./low pressure. (b) geothermal gradient increases with depth; minerals become unstable and form new minerals that are stable at that temperature. This may occur at convergent plate boundaries or thick sequences of sediment. 1. Contact or Thermal Metamorphism – rocks near a magma chamber “bake”. 2. Hydrothermal Metamorphism – involves chemical alterations as hot water circulates through fractures in rocks. 3. Regional Metamorphism – extensive areas subject to high temperature and pressure due to tectonic forces. ** All three may occur in mountain belts. ** Geothermal Gradient Agents of Metamorphism 5 Agents of Metamorphism 6 2. Pressure & Stress Buried rocks are subjected to confining pressure and forces are applied equally in all directions. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 14 of 20 7 Agents of Metamorphism 2. Pressure & Stress Rocks subjected to directed pressure (such at convergent boundaries) deform unequally in different directions referred to as differential stress. 8 Agents of Metamorphism 3. Chemically Active Fluids As temperature and pressure break up crystal structures, atoms and ions move into solution and migrate through the rock to form new minerals or ore deposits. Quartz vein deposit with gold & silver. ✓REVIEW QUESTIONS 9 1. What is metamorphism? What are the agents that change rocks? 2. Why is heat considered the most important agent of metamorphism? Metamorphic Textures (Foliated) 10 1. Foliated Texture – flat arrangement of mineral grains within a rock. Pressure is important in the realignment of particles to change the texture of a rock. 3. How is confining pressure different than differential stress? 4. What role do chemically active fluids play in metamorphism? Metamorphic Textures (Foliated) 11 Metamorphic Textures (Foliated) 12 There are three types of foliated textures: 1. Rock or Slaty Cleavage – platy minerals, such as micas, align along planes. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 15 of 20 13 Slaty Cleavage Metamorphic Textures (Foliated) 14 2. Schistosity – “fish scales” – larger pieces of micas have formed; may also contain porphyroblasts (large mineral grains). 3. Gneissic Texture – segregation of light and dark minerals into bands. Slate quarry near Alta, Norway. Metamorphic Textures (Nonfoliated) 15 Foliated Metamorphic Rocks 16 Slate (Left) & Phyllite (Right) 18 There are no alignment of mineral grains. They exhibit interlocking equidimensional crystals. Marble Quartzite Nonfoliated Metamorphic Rocks 17 BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 16 of 20 Mica Schist 19 Gneiss 20 Migmatites 21 Common Foliated Rocks 22 Migmatites – ‘Mixed Rock’ – partial melting has occurred. As a granite approaches 750° C, the lightcolored silicates (quartz & potassium feldspar) will melt, but the mafic silicates will remain a solid. M Granite d te el m Un Migmatite (Mixed Rock) d te el ✓REVIEW QUESTIONS 23 6. What is foliation? Distinguish between slaty cleavage, schistosity, and gneissic textures. 7. Describe the mechanisms by which minerals develop a preferred orientation. 8. List some changes that might occur to a rock in response to metamorphic processes. Metamorphic Environments 24 1. Contact Metamorphism Aureole (or-ē-ōl) is a zone of alteration around emplaced magma. Most are fine-grained, dense, baked clay minerals called hornfels. 9. How can you tell slate and phyllite apart? BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Page 17 of 20 Contact Metamorphism 25 Metamorphic Environments 26 2. Hydrothermal Metamorphism – hot fluids circulate ion-rich solutions to move and deposit heavy minerals. Good example are black smokers at mid-ocean ridges. Metamorphic Environments 27 3. Regional Metamorphism The greatest quantity of metamorphic rock is produced by this method and associated with mountain building. Metamorphism Along Fault Zone 29 Metamorphic Environments 28 4. Other Metamorphic Environments Burial Metamorphism – as basins subside, thick accumulations of sediment cause low-grade metamorphic conditions. Along Fault Zones – near the surface, rocks behave like a brittle solid and fracture creating a fault breccia. At greater depth along the fault, temp. and pressure are higher and rocks are ductile and flow creating a sheared rock called mylonites. Impact Metamorphism – meteorites striking the surface subject rocks to metamorphic conditions. Tektites are beads of silica-rich glass that cool to teardrop shape as they fall. 30 Impact Metamorphism Upper crust - brittle Lower crust - ductile BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 Meteor Crater, Winslow, Arizona Page 18 of 20 Metamorphic Zones 31 There exists systematic variations in the mineralogy and texture of rocks in each metamorphic zone. Index Minerals 33 Higher grade metamorphism is located near the core of ancient mountain ranges. Metamorphic Environments 32 Index Minerals Index Minerals: the grade of metamorphism can be determined by the minerals present. 34 Metamorphic Environments Metamorphic rocks containing the same assemblage of minerals belong to the same metamorphic facies – implying they formed in the same metamorphic environment. 35 Blueschist is a low temp & high-pressure environment found locally in the coast range. Eclogite facies is high temp & high pressure. Blueschist is a low temp & high-pressure environment found locally in the coast range. Eclogite facies is high temp & high pressure. Glaucophane BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 36 Metamorphic Environments Garnets Page 19 of 20 Ancient Meta. Environments 37 ✓REVIEW QUESTIONS 38 10. Name the metamorphic rock described: Most continental shields are composed of meta/igneous rocks indicating periods of ancient mountain building. a) Calcite-rich and often nonfoliated = b) Loosely coherent rock composed of broken fragments that formed along a fault zone = c) Represents a grade of metamorphism between slate and schist = d) Very fine-grained and foliated; excellent rock cleavage = e) Foliated and composed predominately of platy minerals = f) Composed of alternating bands of light and dark silicate minerals = g) Hard, nonfoliated rock resulting form contact metamorphism = ✓REVIEW QUESTIONS 39 11. Contrast contact and regional metamorphism. Which generates the greatest quantity of metamorphic rock? ✓REVIEW QUESTIONS 40 16. How are gneisses and migmatites related? 12. Where does most hydrothermal metamorphism occur? 17. Which type of plate boundary is regional metamorphism associated? 13. Describe burial metamorphism. 18. Why do the cores of major mountain chains contain metamorphic rocks? 14. How to geologists use index minerals? 15. Describe textural changes from slate through gneiss. BLOCK EXAM #3 - CHAPTERS 6, 7 & 8 19. What are shields? How are these relatively flat areas related to mountains? Page 20 of 20