Chapter 17

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

26. 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

27. 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

28. 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

29. 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)

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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)