Plate Tectonics: A Scientific Theory Unfolds

1. Plate Tectonics: A Scientific Theory Unfolds

2. Objectives After reading, studying, and discussing Chapter 5, you should be able to: • List the evidence that was used to support the continental drift hypothesis. • Describe the theory of plate tectonics. • Explain the differences between the continental drift hypothesis and the theory of plate tectonics. • List and describe the evidence used to support the plate tectonics theory. • Explain the difference between divergent, convergent, and transform fault boundaries. • Describe the models that have been proposed to explain the driving mechanism for plate motion.

3. Continental Drift: An Idea Before Its Time • Alfred Wegener published The Origin of Continents and Oceans in which he set forth his hypothesis of continental drift. • Pangea (“all land”): a supercontinent that began breaking into smaller continents about 200 million years ago.

5. Evidence Supporting Continental Drift • The Continental Jigsaw Puzzle • Fossils Match Across the Seas • Rock Types and Structures Match • Ancient Climates

6. Coal beds stretch across the eastern U.S. and continue across southern Europe. Matching plant fossils are found in South America, Africa, India, Australia, and Antarctica. Matching reptile fossils are found in South America and Africa. Matching early mammal fossils are found in South America and Africa. Fossils in South America and Africa are found in rocks of identical age and type. Matching rock types and mountain belts occur in North America and the British Isles, and Africa and South America. Evidence of glaciers is present in regions with warm, dry climates. Continents that are close to the equator today were once closer to the South Pole in the distant past. Evidence Cont’d

10. Seafloor Spreading • Wegener could not explain the crust spreading, but later it was found that the this was the cause. • States that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches. • Proposed by Harry Hess

11. Sea Floor Spreading was the missing link needed by Wegener to complete his model of continental drift! • THIS GIVES RISE TO THE THEORY OF PLATE TECTONICS!

12. Video on Alfred Wegener http://www.teachersdomain.org/asset/ess05_vid_wegener1/

13. Video Discussion Questions 1. What did Wegener propose as a method of explaining the close fit of the continents on either side of the Atlantic Ocean? 2. What was so unusual about the fossil remains of Mesosaurus? 3. How did clues found in South Africa and Arctic islands support Wegener's theory of continental drift? 4. Why wasn't Wegener's theory accepted at the time?

14. Plate Tectonics • The Earth’s crust is divided into 12 major plates which are moved in various directions. • This plate motion causes them to collide, pull apart, or scrape against each other. • Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. • The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

15. World Plates

16. Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle. Below the lithosphere is the asthenosphere. Tectonic Plates

17. Lithosphere • Fragmented into many pieces • Divided into two parts based on density- continental and oceanic • Continental crust-composed of lighter felsic minerals and rock like granite • Oceanic Crust-made up of denser mafic minerals, and rocks such as basalt

18. Inside Earth • CORE- densest layer composed of Fe with smaller amounts of Ni and others • Inner core- solid • Outer core- liquid • Mantle-thick shell of dense, molten and rocky material that surrounds core • Crust- thinnest & outermost layer • Consists of rocky matter that is less dense than mantle rock

20. In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents or on the ocean floor. As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity. Seafloor spreading occurs along oceanic ridge systems, creating new seafloor. Continental rifting occurs when divergent plate boundaries develop within a continent. Divergent Boundaries

22. Iceland is located right on top of a divergent boundary. In fact, the island exists because of this feature. As the North American and Eurasian plates were pulled apart (see map) volcanic activity occurred along the cracks and fissures (see photographs). With many eruptions over time the island grew out of the sea!

23. Convergent Boundaries • There are three styles of convergent plate boundaries • Continent-continent collision • Continent-oceanic crust collision • Ocean-ocean collision

24. When continental crust pushes against continental crust both sides of the convergent boundary have the same properties. Neither side of the boundary wants to sink beneath the other side, and as a result the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges. Example: the European Alps and Himalayas Continent-continent collision

25. India plate struck the Eurasian plate -2 continental plates collided and formed huge mountains-the Himalayas!

26. At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust. This is called a Subduction Zone. The oceanic crust descends into the mantle at a rate of centimeters per year. This oceanic crust is called the “Subducting Slab” (see diagram). When the subducting slab reaches a depth of around 100 kilometers, it dehydrates and releases water into the overlying mantle wedge The addition of water into the mantle wedge changes the melting point of the molten material there forming magma which rises up into the overlying continental crust forming volcanoes. Continent-Oceanic Collision

27. Subduction=volcanoes!

28. Oceanic-oceanic Collision • When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. • The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. • Adjacent to the trench, volcanic island arcs form in the same way that continental volcanoes form.

29. Transform Fault Boundaries • Transform boundaries-plates slide past each other. • Ex. San Andreas Fault- boundary between the Pacific plate and the North American plate • If this movement continues, the part of California west of the fault zone could become an island • Triggers Earth quakes and mudslides

30. More Evidence of Plate Tectonics • Ocean drilling: found that the age of the sediment increases with increasing distance from the ridge crest, supporting seafloor-spreading hypothesis which predicted the youngest oceanic crust would be found at the ridge crest and the oldest would be at the continental margins.

31. More Evidence of Plate Tectonics • Hot spots: existence of island chains that formed over hot spots provide a frame of reference for tracing the direction of plate motion

32. More Evidence of Plate Tectonics • Paleomagnetism: rocks that formed thousands or millions of years ago and contain a record of the direction of the magnetic poles at the time of their formation • As basaltic lava cools, the iron-bearing minerals become oriented parallel to Earth’s magnetic field. It is locked in place and provides record of Earth’s magnetic field at that time • Isochron- line on map that connects areas of same age. • Magnetic reversal -change in Earth’s magnetic field • Detected by a magnetometer

34. Young crust is near the ridges!

35. Hawaii Hot Spot Video • http://www.teachersdomain.org/asset/ess05_vid_hawaii/ • Do all islands form in this way? How are the volcanoes on the Hawaiian Islands different from other volcanoes? • What is magma called when it erupts on to the Earth's surface? • What is considered the best explanation for the origin of the magma that forms the Hawaiian Islands, and explains why all other volcanoes in the Hawaiian Island chain (to the northwest) are now extinct or dormant? • Mauna Loa and Kilauea are the main volcanoes on the big island of Hawai'i. Which is the largest? Which is the oldest? Where is most of the active extrusion of lava occurring now on the island of Hawaii? • Loihi is the youngest of the Hawaiian volcanoes, although it is not yet an island. Where is it located?

36. Why do the plates move? • Two related ideas are widely accepted: • Slab pull: Denser, colder plate sinks at subduction zone, pulls rest of plate behind it. • Mantle convection: Hotter mantle material rises beneath divergent boundaries, cooler material sinks at subduction zones. • So: moving plates, earthquakes, & volcanic eruptions are due to Earth’s loss of internal heat.

37. http://www.teachersdomain.org/asset/ess05_vid_plateintro/ • http://www.teachersdomain.org/asset/ess05_vid_wegener2/

38. HYDROTHERMAL VENTS • Seafloor spreads at mid-ocean ridge-hot water • Analogy- geysers(old faithful) on land

39. HYDROTHERMAL VENTS • When seafloor spreading occurs, the water near the crust becomes extremely hot. • The hot water mixed with the cold bottom water produces shimmering, upward-flowing stream rich in sulfur & magnesium.

40. 3 types of Hydrothermal vents • Black smokers • White smokers • Clear smokers • Smoker-chimney • Vent glows- “chemical light” not visible light

41. Vent Communities Giant tube worms • Rely on bacteria (chemosynthetic) rather than photosynthetic Miniature Lobster

42. Review • Who is credited with developing the continental drift hypothesis?

43. Answer: Alfred Wegener Next Question: What was Pangea?

44. Answer: A supercontinent that began breaking apart into smaller continents about 200 million years ago. Next question: What type of boundary occurs when plates move apart, resulting in upwelling of material to create new seafloor.

45. Answer: Divergent boundary Next question: How many plates make up the earth?

46. Answer: 12 Next question: What is the lithosphere made up of?

47. Answer: the crust and the uppermost part of the mantle. Next question: What are the three types of convergent plate boundaries?

48. Answer: Continental-continental, oceanic-continental, and oceanic-oceanic