The Dynamic Geosphere and Plate Tectonics

1. The DynamicGeosphere andPlate Tectonics Chapter 3

2. Origins of plate tectonic theory—how scientists developed and tested it Driving mechanism for geosphere movements and how plate tectonics work How continents have split apart, moved, and reassembled How Earth system interactions at plate boundaries are related to earthquakes, volcanoes, mountains, and ocean basins How tectonic processes affect people by creating both natural hazards and vital natural resources You will learn

3. Continental Drift—An Early Idea • Early evidence in 16th century—continental outlines fit (like Africa and South America) • Coal deposits and other strata on separate continents having similar fossils such as Glossopteris • Suggestion that modern day continents now separated were joined in Earth’s past. Figure 3-2 Distinctive fossil seed fern Glossopteris found in southern continents or Gondwanaland.

4. Continental Drift and Alfred Wegener • First to formulate a detailed, global explanation of how continents assumed their present locations and shapes • Laid groundwork for theory of plate tectonics Figure 3-3 Alfred Wegener Hypothesized continental drift

5. Wegener’s Evidence for Continental Drift if Continents Reassembled • Widely separated but very similar sequences of sedimentary rocks containing same fossils (Gondwana sequence) • Glacial deposits appeared to cluster around Antarctica and had associated ice flow directions radiating out from a central point • Ages of rocks similar and mountain belts matched Figure 3-4 Fossil Evidence Examined for Wegener’s ContinentalDrift Hypothesis

6. The Supercontinent Pangaea • Pangaea =Greek for “all land” • Collisions combined the continents into one giant landmass ≈ 250 million years ago (MYBP) (at top) • Since then, continents positioned atop tectonic plates have split and moved apart to their present positions (at bottom) Figure 3-5

7. The Saga of Alfred Wegener • Wegener’s arguments for continental drift failed because he did not present a viable mechanism for moving the continents. • Wegener erroneously proposed that tidal forces generated by Earth’s spin tugged continents through the oceanic crust, thereby moving them—basic physics says this is impossible. • Although Wegener’s continental drift idea was deemed a failure and even ridiculed by the scientific community, the idea, however, challenged geologists to reconsider much of what they had come to believe about how a dynamic Earth worked.

8. Explaining “Moving Continents”— The Theory of Plate Tectonics • Wandering Magnetic Poles • Exploring the Ocean Basins—More Discovery: • Seafloor Spreading • Magnetic Stripes • Earthquakes provide another test Figure 3-6 Apparent Polar Wander Curves

9. EXPLORING THE OCEAN BASINSMapping the Seafloor—Clues for Plate Tectonics Figure 3-7 Major Ridges and Trenches of the Seafloor (a) PostWorld War II mapping revealed that the seafloor contains features such as ridges (mountain chains) and deep trenches. (b) A computer-generated map of part of the East Pacific Rise, a mid-ocean ridge. Red = highest elevations, blue = lowest.

10. SEAFLOOR SPREADING—Harry Hess’ Contribution to Plate Tectonics • Harry Hess, a Princeton Univ. geologist, in 1962 published a paper titled History of the Ocean Basins—proposed hypothesis to explain “continental drift.” • Hess recognized a mechanism to explain continents being separated via spreading in one part of ocean crust and a sinking in another part (at trenches). • At mid-ocean ridges (MORs), new oceanic crust forms as lithosphere pulls apart and magma from mantle wells up, cools, and solidifies. • Volcanoes and a central rift valley are common along fast-spreading ridges like the Mid-Atlantic Ridge. Figure 3-8 Seafloor Spreading and Creation of New Ocean Crust

11. A Magnetic Signature on the Seafloor—A “Giant Barcode” • In addition to Hess’ discovery, Vine and Matthews, two geophysicists from Cambridge Univ., in 1963 proposed magnetic stripes corresponded to past when Earth’s magnetic field was either normal or reversed • Additional data from magnetic mapping of the seafloor also further helped to unravel the mystery of how plates moved • Magnetic field stripes are arranged symmetrically about the center of MORs Figure 3-9 Magnetism of the Ocean Crust Along Mid-Atlantic Ridge south of Iceland.

12. Matching Magnetic Signatures—Stripes on Land and on the Seafloor • Analysis of well-dated rocks on land, like these 5-million-year-old basalt flows on Kauai, a Hawaiian island (a), made it possible to construct a timetable of magnetic polarity reversals. • The timing and duration of these reversals (b)could be correlated with the magnetic stripe patterns on the seafloor (c). Figure 3-10 Volcanic Rock Sequences Were Used to Date Magnetic Polarity Intervals

13. Age of Seafloor • Not only is there symmetry of the magnetic signature on the seafloor but there is also a similar symmetry with respect to age of the volcanic rocks making up the seafloor relative to the MOR • Youngest ocean crust is at MOR (red) and the oldest ocean crust is generally furthest from the MOR (blue, green, and cooler colors) Figure 3-11 Age of the Seafloor MORs are extremely young by geological standards—no more than 2 million years old. Rocks of the seafloor become progressively older as one travels away from MORs.

14. Earthquakes—Another Test For Plate Tectonics • Earthquake studies of oceanic ridges and trenches provided key evidence needed to further support the emerging theory of plate tectonics. • In 1965, a Canadian seismologist, J. Tuzo Wilson, made a major contribution by noting lateral sliding movement at faults near MORs connecting the spreading movements taking place at adjacent ridge segments (Fig. 3-12b) = transform faults. FIgure 3-12 Transform Faults(a) At Mid-Atlantic Ridge, long linear breaks connect spreading movements on adjacent MOR segments. (b) Motion of plates toward or away from each other transformed into motion of plates sliding past each other.

15. Plate Tectonics Today Modern tectonic plates move slowly, averaging several cm (a few inches) per year ≈ fast as your fingernail grows. Figure 3-13 The Geosphere’s Tectonic Plates Tectonic plates, many of which include both oceanic and continental lithosphere, come in many shapes and sizes. Arrows indicate some relative motions between plates.

16. East Africa-Arabia: An Example of Modern Rifting • Upwelling in asthenosphere is causing lithosphere to thin and weaken. As a result, the African continent is being separated from Arabia along the Red Sea and split apart along the East African rift valleys. • Africa is splitting apart, in the same way Pangaea did, starting some 250 million years ago (turns out Wegener was correct on how continents initially split apart). Figure 3-14 Africa Is Rifting Apart Today

17. Rifting—A Continent Is Split • Rifting starts when hot asthenosphere material begins to rise. • Upwelling in asthenosphere heats and weakens crust, resulting in lithospheric thinning and normal faulting. • Crustal extension proceeds until the continental crust becomes thin enough to split apart. • Pieces of continent migrate away from each other as mantle upwelling continues and oceanic crust forms where continents once joined. Figure 3-15 Rifting Splits Continents

18. Divergent Plate Boundaries in the Ocean—Spreading Centers New seafloor is formed at spreading centers Figure 3-17 Hot Springs Form on the Seafloor Near Spreading Centers Mineral-rich hot waters from hydrothermal vents form dark plumes called “black smokers.” Figure 3-16 Normal Faults Common at divergent plate boundaries.

19. Convergent Plate Boundaries—Subduction Zones and Reverse Faults • Convergent plate boundaries = two plates move toward each other. • Boundaries can be between 2 oceanic plates, an oceanic plate and continental plate, or 2 continental plates. Figure 3-19 Reverse Faults Figure 3-18 Consuming Lithosphere at Convergent Plate Boundaries

20. Making of the Himalayas—Convergence • Where continents converge, neither plate is dense enough to sink into the mantle—they hit and form a suture zone. • When two continental plates (in this case the Indian and Eurasian plates) come together in great collisions they form the world’s highest mountains. • Thrust faults are common in crustal shortening areas like at convergent plate margins or boundaries. Figure 3-20 The Making of the Himalayas

21. Convergent Margin—Aleutian Trench, Alaska Figure 3-21 The Alaska-Aleutian Convergent Plate Boundary Plate convergence gives rise to deep-sea trenches, long volcanic arcs, island arcs, and earthquakes. At Alaska-Aleutian convergent boundary (a), oceanic Pacific plate is sinking into mantle beneath North American plate and earthquakes originate in the inclined subduction zone (b).

22. Strike-Slip—Faults at Transforms • If two plates are not diverging or converging, they are sliding past one another along a transform plate boundary. • Most common fault along transforms is the strike-slip fault. Figure 3-22 Strike-Slip Faults

23. Surface Offset on San Andreas Fault • Nearly instantaneous movement of the land’s surface during earthquakes marks plate boundary as along the San Andreas Fault • Fence line (upper right) displaced dramatically during the Great San Francisco Earthquake of 1906 Figure 3-23

24. Fault Movements at Plate Boundaries San Andreas Fault, California Note offsets of stream valley (far left) and linear lakes along fault. Figure 3-24 Effects of Earthquakes and Fault Movements at Plate Boundaries

25. Plate Tectonics and Earthquakes • Distribution of earthquakes is a good guide to location of tectonic plate boundaries. • Most earthquakes occur along convergent and transform plate boundaries—the most devastating are around the Pacific Ocean rim. • Offshore quakes displace ocean waters generating seismic sea waves, or tsunamis (e.g. Indonesian tsunami that killed 250,000 people in 2004). Figure 3-25 Global Distribution of Earthquakes

26. Plate Tectonics and Volcanoes Figure 3-26 Global Distribution of Volcanoes Figure 3-27 Mt. Pinatubo A Volcano at a Convergent Plate Boundary

27. Plate Tectonicsand Mountain Building • Plate tectonics plays a role in building mountains in a number of ways such as by: • Accretion (adding pieces of the geosphere) along the continental margin (Fig 3-28). • Compression at convergent plate boundaries. • Collision of continents at convergent plate boundaries. • Magmatic processes also help form large mountains and long mountain ranges where oceanic plates converge and subduct beneath continental margins. Figure 3-28 The Chugach Mountains of Alaska Offscraping (accretion) of seafloor sediments against overriding plate at a convergent margin produced mountains.

28. Plate Tectonic Influence on Climate and Resources—Some Examples Figure 3-30 A Copper Mine in Utah Mineral deposits form in settings shaped by plate tectonics. The Bingham Canyon copper mine in Utah formed within subduction-related volcanoes. Figure 3-29 The Andes Mountains Rainshadow Rainshadow effect ultimately the result of subduction zone volcanism in satellite photo of South America. Moist air from Pacific Ocean moves eastward up slopes of the Andes and cools, condenses, and results in precipitation as it climbs the mountains.

29. SUMMARY • Persistent research, applications of new technology, and syntheses of diverse observations of 3+ generations of scientists during the 20th century developed theory of plate tectonics. • Earth’s rigid lithosphere is broken into pieces called tectonic plates. • Plates move apart (divergent boundary—creating new seafloor or rifting), come toward each other (convergent boundary—creating volcanic mountains and major earthquakes), or slide horizontally past one another (transform—creating earthquakes). • Plates localize volcanic and seismic activity and plate motion influences the biosphere, climate, and availability of resources.