1 / 39

Chapter 17

Chapter 17. Plate Tectonics. Section 17.1 Drifting Continents. Objectives: Identify the lines of evidence that led Wegener to suggest that Earth’s continents have moved Discuss how evidence of ancient climates supported continental drift

helki
Download Presentation

Chapter 17

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 17 Plate Tectonics

  2. Section 17.1Drifting Continents • Objectives: • Identify the lines of evidence that led Wegener to suggest that Earth’s continents have moved • Discuss how evidence of ancient climates supported continental drift • Explain why continental drift was not accepted when it was first proposed • Define: • Continental drift • pangaea

  3. I. Early Observations • Earth surface = relatively unchanged during course of human lifetime • Except earthquakes, volcanic eruptions, landslides • Changed dramatically on Earth’s timescale • Cartographer = create maps • Abraham Ortelius (Dutch) (1500s) – noticed continents fit on either side of Atlantic Ocean • Alfred Wegener (German) (1912) – proposed hypothesis about continental movement

  4. II. Continental Drift • Continental Drift – proposed that Earth’s continents had once been joined as a single landmass that broke apart and sent continents adrift • Pangea – supercontinent (all the earth) • Began to break apart 200 mya • Continents have continued to move slowly • Wegener – based on more than puzzlelike fit; • Collected: rock, climatic, and fossil data

  5. A. Evidence from Rock Formations • As began to break  mountain ranges fractured as continents separated • Should be areas of similar rock types on opposite sides of the ocean • Layers of Appalachian Mts. Identical to layers in Greenland and Europe • Older than 200 my

  6. B. Evidence of Fossils • Similar fossils of several different animals and plants that once lived on or near land had be found on widely separated continents. • Reasoned: land-dwelling animals could not have swum great distances • Argued: aquatic reptile found only in freshwater  could not have crossed ocean • Ages of fossils predated time frame for breakup of Pangea

  7. C. Climatic Evidence • Clues about ancient climates from fossils • Seed fern resembled low shrubs – found on many parts of earth • Reasoned: area separating fossils too large to have had a single climate • Argued: grew in temperate climates so places where fossils were found was closer to equator • Concluded: rocks containing fossils had once been joined

  8. 1. Coal Deposits • Sedimentary rocks = clues to past environments & climates • Coal forms from compaction and decomposition of accumulations of ancient swamp plants • Existence of coal beds in Antarctica indicated that frozen land once had a tropical climate • Concluded: Antarctica must have been much closer to equator sometime in geologic past

  9. 2. Glacial Deposits • Glacial deposits in africa, india, australia, south america • 290my old • Suggests: areas once covered in ice cap (similar to antarctica today) • Too warm for ice to develop there  proposed: once located near south pole • Suggested 2 possibilities: • South pole shifted position • These land masses had once been closer to south pole • Argued: more likely that landmasses drifted apart than Earth changing its axis

  10. III. A Rejected Notion • Early 1900s, scientific community considered continents and ocean basins = fixed features on Earth’s surface • Continental drift theory was never accepted by scientific community • 2 major flaws in theory: • Did not explain what force could be strong enough to push such large masses over such great distances - Wegener  rotation of Earth (not supported by physicists) • How could continents move through solid mantle? - Wegener  continents plowing through stationary ocean floor • 1960s – reconsidered theory b/c advances in seafloor mapping and understanding Earth’s magnetic field

  11. Section 17.2Seafloor Spreading • Objectives: • Summarize evidence that led to discovery of seafloor spreading • Explain significance of magnetic patterns on ocean floor • Explain process of seafloor spreading • Define: • Magnetometer • Magnetic reversal • Paleomagnetism • Isochron • Seafloor spreading

  12. I. Mapping the Ocean Floor • Until 1950s  ocean floors thought to be essentially flat, unchanging, older than continental crust = INCORRECT • Magnetometer – device that can detect small changes in magnetic fields (ocean floor rocks) • Echo-sounding methods – (sonar) – uses sound waves to measure distance by measuring time it takes for sound waves sent from the ship to bounce off seafloor and return to ship • Can now measure water depth and map topography of ocean floor

  13. II. Ocean Floor Topography • Discovered vast, underwater mountain chains (ocean ridges) run around earth like seams on a baseball • Form longest continuous mountain range on earth • Generated much discussion b/c >80,000 km long & 3 km above ocean floor • Earthquakes & volcanism = common along ridges

  14. Mountain ridges have counter parts = deep sea trenches – narrow, elongated depression in sea floor • Can be 1000s of km long & many km deep • Deepest = Marianas Trench – Pacific Ocean = >11km deep • Tallest mountain (Mt. Everest) = 9km above sea level • What could have formed underwater mountain range that extended around Earth? • What is the source of volcanism associated w/ mountains? • What forces could depress Earth’s curst enough to create trenches nearly 6 times as deep as Grand Canyon?

  15. III. Ocean Rocks and Sediments 1 Discovery: • Ages of rocks that make up seafloor varies across ocean floor • Variations are predictable • Near ocean ridges = younger than samples taken from areas near deep-sea trenches • Age consistently increases w/ distance from ridge (symmetric trend across ocean) • Oldest parts of seafloor = young = 180 myo • Continental rocks = 3.8 byo • Why is there no trace of older oceanic crust?

  16. 2nd Discovery: • Ocean-floor sediments = few hundred meters thick • Continental layers = 20 km thick • Hypothesis: relatively thin layer of ocean crust = related to age of ocean crust • Thickness of sediments increases w/ distance from ocean ridge • Pattern of thickness across ocean floor = symmetrical across ocean ridges

  17. IV. Magnetism • Earth’s magnetic field generated by flow of molten iron in outer core • Causes compass needle to point to North • Magnetic reversal – happens when flow in outer core changes & earth’s magnetic field changes direction • Cause compass needle to point to South • Reversal occurred many time in earth’s history

  18. A. Magnetic Polarity Time Scale • Paleomagnetism – study of history of Earth’s magnetic field • When lava solidifies  iron-bearing minerals (magnetite) crystallize  behave like tiny compasses and align with earth’s magnetic field • Data studies continental lava flows = construct magnetic polarity time scale

  19. B. Magnetic Symmetry • Oceanic crust = mostly basaltic rock – contains large amounts of iron-bearing minerals of volcanic origin • Hypothesis: rocks on ocean floor would show a record of magnetic reversals • Towed magnetometers & measure magnetic orientation of rocks of ocean floor • Pattern: regions w/ normal and reverse polarity form a series of stripes across floor parallel to ocean ridges • Ages and widths of stripes matched from one side of ridge to the other • Create/interpret a diagram of above pattern.

  20. By matching patterns on seafloor w/ known patterns of reversals on land  scientists determined age of ocean floor from magnetic recordingcreated isochron • Isochron – imaginary line on map that shows points that have same age (formed at same time • Interpret isochron map • Relatively young ocean-floor crust near ocean ridges • Older ocean crust found along deep trenches

  21. V. Seafloor Spreading • Seafloor spreading – theory that explains how new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches • Magma (hotter and less dense than surrounding mantle material) forced toward surface of crust along ocean ridge • 2 sides of ridge spread apart  rising magma fillsgap created • Magma solidifies  small amount of new ocean floor is added to Earth’s surface • Cycle of spreading and intrusion of magma continues formation of ocean floor (slowly moves away from ridge) • Seafloor spreading mostly happens under sea • Iceland – portion of Mid-Atlantic Ridge rises above sea level  shows lava erupting along ridge

  22. Wegener could not explain what caused landmasses to move or how they moved • Seafloor spreading was missing link to complete model of continental drift • Continents = not pushing through ocean crust  more like passengers that ride along while ocean crust slowly moves away from ocean ridges

  23. Section 17.3Plate Boundaries • Objectives: • Describe how Earth’s tectonic plates result in many geologic features • Compare and contrast the three types of plate boundaries and the features associated with each • Generalize the processes associated with subduction zones • Define: • Tectonic plate • Divergent boundary • Rift valley • Convergent boundary • Subduction • Transform boundary

  24. I. Theory of Plate Tectonics • Tectonic plates – huge pieces of crust and rigid upper mantle that fit together at their edges to cover Earth’s surface • 12 major plates + several smaller plates • Move few cm/year (fingernail growth rate) • Theory: • Move in different directions • Move at different rates • Interact w/ one another at their boundaries • Each boundary has certain geologic characteristics and processes associated w/ it • Divergent boundary – where tectonic plates move away from each other • Convergent boundary – where plates move toward each other • Transform boundary – where plates move horizontally past each other

  25. A. Divergent Boundary • Divergent boundary - 2 tectonic plates moving apart • Most = along seafloor in rift valleys • Central rift where process of seafloor spreading begins • Magma rises through rifts faults & forms mid-ocean ridge (continuous mountain chain) • Formation of new ocean crust accounts for high heat flow, volcanism, and earthquakes associated w/ boundaries

  26. Seafloor spreading along divergent boundary can cause ocean basin to grow wider • Most divergent boundaries form ridges • Some form on continents • Rift valley – continental crust begins to separate  creates long, narrow depression • Example: currently forming in East Africa – might eventually lead to formation of new ocean basin

  27. B. Convergent Boundaries • Convergent boundaries – 2 tectonic plates moving toward each other • When 2 plates collide  denser plate descends below the less-dense plate = subduction • 3 types of convergent boundaries – classified according to type of crust involved • Ocean crust – high in iron & magnesium = dense, basaltic rocks • Continental crust – feldspar & quartz = less-dense, lighter colored granitic rocks • Difference in density affects how they converge

  28. 1. Oceanic-Oceanic • Subduction zone formed when one plate (denser b/c cooling) descends below another • Creates ocean trench • Subducted plate descends into mantle  recycling oceanic crust formed at ridge • Water carried into Earth by subducting plate lowers melting temp of plate  melts at shallower depth • Magma = less dense  rises back to surface  erupts and forms an arc of volcanic islands that parallel the trench • Example: Marianas Trench & Marianas Islands • Example: Aleutian Trench & Aleutian Islands (volcanic peak)

  29. 2. Oceanic-Continental • Oceanic plate converges w/ continental plate • Denser oceanic plate is subducted • Produces trenches and volcanic arc (chain of volcanoes along edge of continental plate) mountain range w/ many volcanoes • Example: Peru-Chile Trench & Andes mountain range

  30. 3. Continental - Continental • 2 continental plates collide • Form long after oceanic plate has converged w/ continental plate • Continents carried by oceanic plates & oceanic plate can be completely subducted, dragging attached continent behind • Oceanic crust descends beneath continental crust • Continental crust behind cannot descend b/c continental rocks are less dense & will not sink into mantle • Edges of both continents collide  become crumpled, folded, and uplifted = vast mountain range • Example: Himalayas

  31. C. Transform Boundaries • 2 plates slide horizontally past each other • Long faults (100s km long) • Shallow earthquakes • Crust is deformed/fractured somewhat • Most offset sections of ocean ridges • Sometimes occur on continents (San Andreas Fault system b/w SW Cali & rest of Cali) = earthquakes

  32. Section 17.4Causes of Plate Motion • Objectives: • Explain the process of convection • Summarize how convection in mantle is related to movements of tectonic plates • Compare and contrast the processes of ridge push and slap pull • Define: • Ridge push • Slap pull

  33. I. Convection • What force causes plates to move? • Large-scale motion in mantle drives movement of tectonic plates • Mantle – earth’s interior b/w crust and core

  34. A. Convection Currents • Convection – transfer of thermal energy by movement of heated material from one place to another • Cooling of matter causes it to contract slightly and increase in density = sinks • Warmed matter is displaced and forced to rise • Up and down flow = convection current • Convection current – aids in transfer of thermal energy from warmer regions to cooler

  35. Earth’s mantle composed of partially molten material heated unevenly by radioactive decay from both mantle and core beneath • Radioactive decay heats up molten material in mantle • Causes enormous convection currents to move material throughout the mantle

  36. B. Convection in the Mantle • Mantle is solid, but much of it moves like a soft, pliable plastic • Part of mantle that is too cold and stiff to flow – located beneath crust – attached to crust – moves as part of plates • Cooler mantle –denser so sinks into hotter mantle toward center of Earth • Heated mantle material near core is displaced and rises • Convection currents sustained by rise and fall of material  results in transfer of energy b/w hot interior and cooler exterior • Can be 1000s km across • Flow at rates of only few cm per year • Hypothesis: convection currents set in motion by subducting slabs

  37. C. Plate Movement • Rising material in convection current spreads out as it reaches upper mantle • Causes both upward and sideways forces • Forces lift and split lithosphere at divergent plate boundaries • As plates separate, material rising from mantle supplies magma that hardens to form new ocean crust • Downward part of convection current occurs where sinking force pulls tectonic plates downward at convergent boundaries

  38. II. Push and Pull • Hypothesis: there are several processes that determine how mantle convection affects movement of plates • Oceanic crust cools, moves away from divergent boundary, becomes denser and sinks compared to newer, less-dense oceanic crust • As older portion of seafloor sinks, weight of uplifted ridge is thought to push oceanic plate toward trench formed at subduction zone = ridge push • Slab pull = weight of subducting plate pulls trailing slab into subduction zone (tablecloth)

More Related