Plate Tectonics

1. Plate Tectonics Chapter 8

2. Section 1 What Is Plate Tectonics? • Theory that describes the formation, movements, and interactions of Earth’s lithospheric plates. • Lithosphere is broken into large “plates” that move (“float”) on top of the asthenosphere. • 3 ways • Away from each other (divergent plate boundaries) • Toward each other (convergent plate boundaries) • Slide past each other (transform plate boundaries)

3. Early Ideas About Plate Movements • Have you ever noticed that the continents look like they would fit together like the pieces of a puzzle??? If you have, you’re not alone…. • 1912 Alfred Wegener • hypothesis  continental drift • Continents have “drifted” over time • Observations to support hypothesis • Shape of continents • Fossils ~Mesosaurus *South America & Africa • Distinctive rock formations • Climate change evidence ~coal deposits Continental Drift Video

4. Evidence for Joined Continents Interaction

5. The Theory of Plate Tectonics • Continents (and ocean basins) are part of lithospheric plates. • Plate movement = continent movement • Evidence/Explanations of many geologic processes • Location of volcanoes & earthquakes • Formation of new crust (ocean floor)

6. Locations of Earthquakes & Volcanoes • Occur in concentrated areas • Mark locations of plate boundaries • Where plates move apart, together, or past each other • Strain builds up  releases causing EQs • Molten rock rises  volcanic activity • Pacific Ocean • Ring of Fire Plates & Volcanoes Video

7. Ring of Fire Locations of Earthquakes & Volcanoes

8. Magnetism & the Age of the Ocean Floor • Magnetic properties & ages of igneous rocks on the ocean floor provide evidence for theory of plate tectonics. • Some igneous rocks contain magnetic minerals. • Provide record of direction of Earth’s magnetic field when rock formed

9. Magnetism & the Age of the Ocean Floor • Records of Earth’s magnetic field • Some rocks recorded reversals in magnetic field • normal polarity • what we know today as N & S magnetic poles • reversed polarity • present N magnetic pole became the S magnetic pole • present S magnetic pole became the N magnetic pole • A # of magnetic reversals have taken place at different times over millions of years

11. Magnetism on the Ocean Floor

12. Magnetism & the Age of the Ocean Floor • Mid-ocean ridge: long chain of volcanic mountains on the ocean floor with a deep central valley • Magnetic reversals are recorded in bands of rock on opposite sides of the ridge • Mirror image • Center of ridge shows current orientation of Earth’s magnetic field (normal or reversed) • Rocks at center of ridge are youngest (most newly formed) • As move further from center rocks get older Magnetic Polarity Recorded in Rocks Animation

13. Magnetism & the Age of the Ocean Floor • Mid-ocean ridges  boundaries between lithospheric plates • New rock along ridge is formed by hot, molten rock which rises up between the plates (because it is less dense) • As new rock forms plates spread • Older rocks pushed away from ridge (both sides) • Rock at center of ridge hottest • Temperature decreases as move out from center

14. HW • Read pgs 172-175 • Answer #s 1, 2, 3

15. Section 2 Types of Plate Boundaries • Earth’s lithosphere is broken in to large plates • move in 3 ways • Divergent plate boundaries • Convergent plate boundaries • Transform plate boundaries

16. Plates move apart Sometimes called spreading centers Most are along ocean floor Creates mid-ocean ridge Divergent Boundaries

17. Divergent Boundaries • Rift valley • Center of mid-ocean ridge • Border between 2 diverging plates • Sea-floor spreading  molten rock forces through cracks (rifts) in valley • Oceanic crust forms as rock cools • older crust moves away from mid-ocean ridge as the new crust is formed

18. Divergent Boundaries • Rift valley (continued) • Broken into sections • offset from each other by breaks (fracture zones) • Perpendicular (right angle) to ridge • Source of earthquakes at mid-ocean ridges Divergent Plate Boundary Animation

19. Mid-Atlantic Ridge East Pacific Rise Mid-Ocean Ridges

20. Convergent Boundaries • 2 plates move towards each other • 2 broad classifications • Subduction boundaries • Collision boundaries

21. Subduction Boundaries • Oceanic plate subducts (plunges) below another plate • Boundary between the 2 plates is called a subduction boundary • Features of subduction boundaries • Deep-sea (or ocean) trenches • Deepest part of the ocean • Volcanic activity • Island arcs • Along the coast of continents • Earthquakes

22. Subduction Boundaries • Ocean-Ocean subduction • 2 Oceanic plates collide • Deep-sea trench formed • accompanied by chain of volcanic islands (volcanic island arc) on the overriding plate • Example: • Pacific Plate subducts under Phillippine Plate • Pacific Plate pulled down • Forms Mariana Trench • Phillippine Plate overrides Pacific Plate • Forms Mariana Islands (volcanic island chain) Subduction and Collision Boundary Animations

23. Ocean-Ocean Subduction Boundary

24. Ocean-Continent Subduction Boundary • Ocean-Continent Subduction • The denser oceanic plate subducts below the less-dense continental plate • Ocean trenches form. • Volcanoes & mountains are formed on the continental plate. • Example: • Nazca Plate (off west coast of S. America) subducts under the South American Plate • Nazca Plate pulled down • Forms Peru-Chile Trench • South American Plate overrides Nazca Plate • Forms Andes Mountains and volcanoes (along western edge of S. America) Subduction and Collision Boundary Animations

25. Ocean-Continent Subduction Boundary

26. Collision Boundaries • Continent-continent collision • Neither plate subducts, so the crust piles up (because the continents join to form a single larger continent) • Mountains are formed. • Example: • Indian subcontinent colliding into Eurasian Plate • Formed Himalaya Mountains Subduction and Collision Boundary Animations

27. Continent-Continent Collision

28. Transform Plate Boundary • 2 plates slide past each other at a fault. • Stress is released as an earthquake. • Example • California—San Andreas Fault • Pacific Plate (Southwestern Cali.) is moving NW • N. American Plate (rest of N. Amer.) moving SE

29. Transform Plate Boundary Transform Boundary--San Andreas Fault Animation N

30. HW • Read pgs 176-179 • Answer #s 1, 2, 3, 4, 6

31. Causes Of Plate Movement Ch. 8.3 Convection cells (caused by density differences) in the asthenosphere (mantle) cause hot material to rise, move outward (pushing the plates/ridge push),& sink as the material cools (pulling plates down/slab pull). Convection in the Mantle Animation

32. The Big Picture

33. HW • Read pgs 180-181 • Answer # 1

34. Plate Movements & Continental GrowthSec 4 • Reconstructing the Past • Many different kinds of evidence give clues as to what Earth looked like in the past • Rocks in Ural & Appalachian Mountain • evidence of past subduction  formed at convergent boundary, but neither mountain range is located near a plate boundary today • Ages of rocks that form in ocean basins • Magnetic record of igneous rocks • Can reveal latitude of formation • Fossils • Organisms that lived in shallow seas  fossils found on high mountaintops • Rocks that show evidence of having been covered by glaciers, but now are in tropical areas

35. Plate Movements & Continental Growth • Plate Tectonics & Pangaea • Evidence suggests 250 million years ago (mya) that all the continents were welded together into one land mass  Pangaea

36. Plate Movements & Continental Growth • Formation of Pangaea • Use data from continents to make models of what Earth may have looked like before Pangaea • A large continental mass stretched between the south pole & the equator  Gondwana • made of smaller landmasses (S. America, southern Europe, Africa, the Near East, India, Australia, New Zealand, & Antarctica) • Other small landmasses ranged across the globe • Gondwana moved northward and converged w/ the other landmasses to form Pangaea

37. Plate Movements & Continental Growth • Break up of Pangaea • 2 separate landmasses • Gondwana • Laurasia • Over time both broke into smaller landmasses which began to resemble the continents today • Pangaea is still breaking up… • The formation & break up may have happened many times in the past Break up of Pangaea animation

39. Plate Movements & Continental Growth • Plate Tectonics & Continental Growth • P.T. affects shape of continents as well as position • Added rock materials to the margins of ancient continent cores (cratons) • ~2.5 billion years ago (bya) core continental material stabilized • Ex. North American craton • at surface in most of eastern Canada  Canadian Shield • Some of the oldest rock material (~4 billion years old) • Rest of the craton is buried under a platform of sediments • Remainder of the continent is material added to the craton

41. Plate Movements & Continental Growth • Sources of Growth Material • Deep-sea sediments • Added when an oceanic plate plunges under a continental plate at a subduction boundary • Sediments from ocean floor scraped off & left behind on edge of continent • Igneous Rock • Magma rises beneath surface & cools  pluton • Volcanoes at subduction boundaries add to edges

42. Plate Movements & Continental Growth • River Sediments • Weathered & eroded materials deposited at edges • Terrane • large block of lithospheric plate that has been moved & attached to continent • 3 characteristics to ID a terrane • 1. bounded on all sides by faults • 2. rocks & fossils found in terrane do not match those of neighboring terranes • 3. magnetic record of terrane does not match surround terranes Growth of a Continent Animation

43. HW • Read pgs 182-187 • Answer # 3