Chapter 6

1. Chapter 6 Sedimentary and Metamorphic Rocks

2. Section 6.1Formation of Sedimentary Rocks • Objectives: • Sequence the formation of sedimentary rocks • Explain the process of lithification • Describe the features of sedimentary rocks • Define: • Sediment • Lithification • Cementation • Bedding • Graded bedding • Cross-bedding

3. I. Weathering and Erosion • Rock exposed at Earths surface is continuously broken down by weathering • Weathering – set of physical and chemical processes that break rock into smaller pieces • Sediments – small pieces of rock that are moved and deposited by water, wind, glaciers, and gravity • Sedimentary rocks form when sediments become glued together • Formation begins when weathering and erosion produce sediments

4. A. Weathering • Weathering produces rock and mineral fragments = sediments • Sediments range in size (boulders to microscopic particles) • Chemical weathering – minerals in a rock are dissolved or otherwise chemically changed • More-resistant grains are broken off of the rock as smaller grains • Physical weathering – minerals remain chemically unchanged • Rock fragments break off the solid rock along fractures or grain boundaries

5. B. Erosion • Erosion – removal and transport of sediment • 4 main agents: wind, moving water, gravity, glaciers • After rock fragments have been weathered out of the rock  they are transported to new locations by erosion • Almost always carried DOWNHILL (gravity) • Wind can carry fine sand and dust to higher elevations (eventually pulled downhill by gravity) • Water almost always moved downhill

6. C. Deposition • Deposition – when transported sediments are deposited on the ground or sink to the bottom of a body of water • Sediments deposited when movement stops • When wind stops blowing, river enters quiet lake or ocean • Particles being carried will settle out • Forms layers • Largest grains at bottom • Smallest grains on top

7. D. Energy of Transporting Agents • Fast-moving water can transport larger particles better than slow-moving water • As water slows down, largest particles settle out first, then next largest, etc. • Deposits are characteristic of sediment transported by water and wind • Wind can only move small grains  sand dunes commonly made of fine, well-sorted sand • Glaciers move all materials with equal ease • Large boulders, sand, and mud all carried along by the ice and dumped in an unsorted pile as the glacier melts • Landslides = similar deposits when sediment moves downhill in a jumbled mass

8. II. Lithification • Most sediments deposited in low areas (valleys & ocean basins) • As more sediment is deposited in an area  bottom layers subjected to increasing pressure and temperature • Lithification – physical and chemical processes that transform sediments into sedimentary rocks

10. A. Compaction • 1st step of lithification • Weight of overlying sediments forces sediment grains closer together  causes physical changes • Layers of mud (60% water)  water is squeezed out  layers shrink • Sand does not compact as much as mud because grains made of quartz & do not deform under normal burial conditions • Grain-to-grain contacts in sand form a supporting framework that helps maintain open spaces b/w the grains • Groundwater, oil, natural gas found in spaces in sedimentary rock

11. B. Cementation • Cementation – when mineral growth glues sediment grains together into solid rock • Occurs when a new mineral grows between sediment grains • Also occurs when dissolved minerals precipitate out of groundwater

12. III. Sedimentary Features • Rocks contain features to help geologists interpret how they formed and the history of the area in which they formed

13. A. Bedding • Bedding – horizontal layering • Results from the way sediment settles out of water or wind. • Beds can range in thickness (few mm to several m) • 2 types – depend on method of transport • Size of grains and material within bedding depend on many other factors

14. 1. Graded Bedding • Graded bedding – particle sizes become progressively heavier and coarser toward the bottom layers • Marine sedimentary rocks deposited by underwater landslides • As sliding material slowly came to rest  largest and heaviest material settled out first  followed by progressively finer material

15. 2. Cross-bedding • Cross-bedding – forms as inclined layers of sediment are deposited across a horizontal surface • When deposits become lithified cross beds preserved in rock • Small scale = sandy beaches and sandbars in streams and rivers • Large scale = migrating sand dunes • Zion National Park

16. B. Ripple Marks • Ripple marks form when sediment is moved into small ridges by wind or wave action or by a river current • Back and forth movement of waves forms ripples that are symmetrical • Current flowing in 1 direction (river/stream) produces asymmetrical ripples • If a rippled surface is buried gently by more sediment without being disturbed  it might later be preserved in solid rock

17. C. Sorting and Rounding • Close examination of individual sediment grains reveals that some have jagged edges and some are rounded • When rock breaks apart  pieces are angular in shape • as sediment is transported  individual pieces knock into eachother = edges are broken off and become rounded (over time) • Amount of rounding influenced by how far sediment traveled • Harder the mineral = better chance it has of becoming rounded before it breaks apart • Quartz sand on beaches = nearly round • Carbonate sand = made up of softer seashells and calcite = angular b/c deposited closer to source of sediment

18. D. Evidence of Past Life • Fossils – preserved remains, impressions, or any other evidence of once-living organisms • Organism dies and is buried before it decomposes  can be preserved as a fossil if left undisturbed • During lithification  parts of organism can be replaced by minerals and turned into rock • Fossils of great interest to scientists b/c provide evidence of they types of organisms that lived in the distant past, environments that existed in the past, and how organisms have changed over time

19. Section 6.2Types of Sedimentary Rocks • Objectives: • Describe the types of clastic sedimentary rocks • Explain how chemical sedimentary rocks form • Describe biochemical sedimentary rocks • Define: • Clastic sedimentary rock • Clastic • Porosity • evaporite

20. I. Clastic Sedimentary Rocks • Most common sedimentary rock • Clastic sedimentary rocks – formed from abundant deposits of loose sediments that accumulate on Earth’s surface • Clastic – “broken” • Further classified by size of particles, mode of formation and mineral content

21. A. Coarse-grained Rocks • Consists of gravel-sized rock and mineral fragments • Conglomerates – rounded, gravel-sized particles • Gravel transported by high-energy flows of water (mountain streams, flooding rivers, ocean waves, glacial meltwater) b/c of relatively large mass • During transport  gravel becomes abraded and rounded as particles scrape against one another • Lithification turns these sediments into conglomerate

22. Brecchias – angular, gravel-sized particles • Angularity = sediments from which they formed did not have time to become rounded • Suggests particles transported only a short distance and deposited close to their source

23. B. Medium-grained Rocks • Sedimentary rocks that contain sand-sized rock and mineral fragments • Stream/river channels, beaches, deserts • Sandstone – ripple marks and cross-bedding indicate direction of current flow • Used to map ancient stream/river channels • Sandstone – relatively high porosity • Porosity – percentage of open spaces b/w grains in a rock • When pore spaces are connected  fluids can move through • Sandstone layers are valuable - underground reservoirs of oil, natural gas, groundwater

24. C. Fine-grained Rocks • Consist of silt and clay-sized particles • Siltstone and shale • Represent environments such as swamps and ponds w/ slow-moving or still waters • Sediments settle to bottom where they accumulate in thin horizontal layers • Low porosity • Forms barriers that hinder movement of groundwater and oil

25. II. Chemical and Biochemical Sedimentary Rocks • Formation involves processes of evaporation and precipitation of minerals • During weathering, minerals can be dissolved and carried into lakes and oceans • As water evaporates from lakes and oceans, dissolved minerals are left behind • In arid regions, high evaporation rates can increase concentration of dissolved minerals in bodies of water • Example: Great Salt Lake, Salt Lake City, Utah

26. A. Chemical Sedimentary Rocks • Concentration of dissolved minerals in a body of water reaches saturation  crystal grains precipitate out of solution and settle to bottom • Results in layers of chemical sedimentary rocks (most called evaporites) • Most common in arid regions & drainage basins on continents w/ low water flow • Continents w/ low water flow = concentration of dissolved minerals remains high • Even as more dissolved minerals are carried into basins, evaporation continues to remove freshwater and maintain high mineral concentration • Over time, thick layers of evaporite minerals can accumulate on basin floor

27. B. Biochemical Sedimentary Rocks • Formed from remains of once-living organisms • Most abundant = limestone – composed primarily of calcite • Some organisms in ocean use calcium carbonate dissolved in seawater to make their shells • When organisms die  shells settle to bottom of ocean and can form thick layers of carbonate sediment • During burial and lithification, calcium carbonate precipitates out of the water  crystallizes b/w grains of carbonate sediment  forms limestone • Limestone = common in shallow water environments (coral reefs) • Skeletal and shell materials accumulating in coral reefs will eventually become limestone • Limestone = contains evidence of biological origin in form of abundant fossils • Fossils can range from large-shelled organisms to microscopic, unicellular organisms • Some limestone has a crystalline structure (not fossils) - consists of tiny spheres of carbonate sand & some of carbonate mud • Other organisms use silica to make their shells • Shells form siliceous ooze b/c rich in silica • Siliceous ooze is lithified into sedimentary rock called chert

28. Section 6.3Metamorphic Rocks • Objectives: • Compare and contrast the different types and causes of metamorphism • Distinguish among metamorphic textures • Explain how mineral and compositional changes occur during metamorphism • Apply the rock cycle to explain how rocks are classified • Define: • Foliated • Nonfoliated • Regional metamorphism • Contact metamorphism • Hydrothermal metamorphism • Rock cycle

29. I. Recognizing Metamorphic Rock • Metamorphosed – has been changed • Pressure and temperature increase with depth • When temp or pressure is high enough  rocks melt and form magma • When high temp and pressure combine and change texture  mineral composition or chemical composition of a rock without melting it  metamorphic rock forms • Rock changes forms while remaining solid

30. Heat is derived from earth’s internal heat • Through deep burial • From nearby igneous intrusions • Pressure comes from deep burial or from compression during mountain building

31. A. Metamorphic Minerals • Fractional crystallization bowens reaction series – all minerals are stable at certain temperatures as they crystallize from magma along a range of different temperatures • Principles also apply to minerals in solid rock • During metamorphism  minerals in rock change into new minerals that are stable under the new temperature and pressure conditions • These minerals are said to undergo solid-state alterations

32. B. Metamorphic Textures • 2 textural groups: foliated and nonfoliated • Geologists use textures and mineral composition to identify metamorphic rocks • gneiss • marble

33. 1. Foliated Rocks • Layers and bands of minerals = foliated metamorphic rock • High pressure during metamorphism causes minerals w/ flat or needlelike crystals to form w/ long axes perpendicular to pressure • Parallel alignment of minerals creates layers

34. 2. Nonfoliated Rocks • Nonfoliated = composed mainly of minerals that from w/ blocky crystal shapes • Quartzite – hard, often light-colored formed by metamorphism of quartz-rich sandstone • Marble – formed by metamorphism of limestone • Some smooth texture from interlocked grains of calcite  used in sculptures • Fossils are rarely preserved in metamorphic rocks

35. Under certain conditions, new metamorphic minerals can grow large while the surrounding minerals remain small • Porphyroblasts – large crystals (few mm to few cm in size) • Resemble very large crystals that form in pegmatite granite  but not the same • Instead of forming from magma  form in solid rock through reorganization of atoms during metamorphism • Garnet – commonly forms porphyroblasts

36. II. Grades of Metamorphism • Different combinations of temperature and pressure result in different grades of metamorphism • Low-grade metamorphism – low temps and pressures and a particular suite of minerals and textures • High-grade metamorphism – high temps and pressures and different suite of minerals and textures • Intermediate-grade metamorphism – in between low- and high-grade metamorphism

37. Metamorphosed shale • Change in composition as conditions change from low-grade to high-grade • Geologists can create metamorphic maps by plotting location of metamorphic minerals • Knowing temps that certain areas experienced when rocks were forming helps geologists locate valuable metamorphic minerals (garnet, talc) • Studying distribution of metamorphic minerals helps geologists interpret metamorphic history of an area

38. III. Types of Metamorphism • Effects of metamorphism can be result of contact, regional, or hydrothermal metamorphism • Minerals that form and degree of change in the rocks provide information as to type and grade of metamorphism that occurred

39. A. Regional Metamorphism • High temperature and pressure affect large regions of Earth’s crust  produce large belts of regional metamorphism • Can range in grade from low to high • Results include changes in minerals and rock types • also includes folding and deforming of rock layers that make up the area

40. B. Contact Metamorphism • Molten material (igneous intrusion) comes in contact w/ solid rock  local effect = contact metamorphism • High temp & med to low pressure form mineral assemblages • Temp decr. w/ distance from intrusion  metamorphic effects decrease w/ distance • Metamorphic minerals form at high temps which are closest to intrusion • Heat does not penetrate far into surface rocks  contact metamorphism from extrusive igneous rocks is limited to thin zones

41. C. Hydrothermal Metamorphism • Very hot water reacts w/ rock and alters its chemical and mineral composition  hydrothermal metamorphism • As hot fluids migrate in and out of rock during metamorphism  original mineral composition and texture of rock can change • Chemical changes common during contact metamorphism near igneous intrusions and active volcanoes • Valuable ore deposits of gold, copper, zinc, tungsten, lead are formed this way

42. IV. Economic Importance of Metamorphic Rocks and Minerals • Salt for cooking, gold for trade, metals for construction, fossil fuels for energy, • Many produced by metamorphic processes (gold, silver, copper, lead)

43. A. Metallic Mineral Resources • Occur mostly in form of metal ores • Deposits of pure metals are occasionally discovered • Many deposits are precipitated from hydrothermal solutions • Concentrated in veins • Spread throughout rock mass

44. Examples: • Gold, silver, copper – hydrothermal quartz veins near igneous intrusions or in contact metamorphic zones • Most hydrothermal metal deposits – metal sulfides (galena) (pyrite) • Iron ores (magnetite) (hematite) – oxide minerals formed by precipitation from iron-bearing hydrothermal solutions

45. B. Nonmetallic Mineral Resources • Metamorphism of ultrabasic igneous rock produces talc & asbestos • Talc – hardness = 1  dusting powder, lubricant, texture in paints • Asbestos – noncombustible & low thermal / electricity conductivity  fireproof and insulating materials (construction) • Cancer causing properties • Graphite – main ingredient in lead pencils – formed by metamorphism of coal

46. C. The Rock Cycle • Igneous rocks crystallize from magma • Sedimentary rocks form from cemented or precipitated sediments • Metamorphic rocks form from changes in temperature and pressure • Any rock can be changed into any other type of rock

47. Rock cycle – continuous changing and remaking of rocks • Heat and pressure can change igneous rock into metamorphic rock • Metamorphic rock can be changed into another metamorphic rock or melted to form an igneous rock • Metamorphic rock can be weathered and eroded into sediments that might become cemented into a sedimentary rock

48. Draw a Diagram of the Rock Cycle