Chapter 5 Lecture Outline. Plate Tectonics: A Scientific Revolution Unfolds

Transcription

1 Chapter 5 Lecture Outline Plate Tectonics: A Scientific Revolution Unfolds

2 From Continental Drift to Plate Tectonics Before 1960 most geologists saw the positions of ocean basins and continents as fixed A new model of tectonic processes developed A scientific revolution Tectonics study of large-scale deformation and structures in the outer portion of the Earth. Tectonic processes deform crust and create major structural features Plate Tectonics Theory relates such deformation to the existence and movement of rigid plates over a weak or partly molten layer in the earth s upper mantle It became the unifying theory of geology

3 Continental Drift: An Idea Before Its Time In 1915, Alfred Wegener outlined the hypothesis of continental drift Published :The Origin of Continents and Oceans Single supercontinent of all of Earth s land: Pangaea Fragmented ~200 mya and smaller landmasses drifted to their present positions

4 However, Wegener was not the first to suspect the continents were on the move. World maps in the 1600s suggested that South America and Africa fit together 1596 Dutch map maker Abraham Ortelius Thesaurus Geographicus Americas were, torn away from Europe and Africa by earthquakes and floods the vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three (continents).

5 Antonio Snider-Pelligrini geographer

6 Continental Drift: An Idea Before Its Time Geographic evidence Similarity between coastlines on opposite sides of the Atlantic Opponents argued that coastlines are modified through time by erosion and deposition Continental shelf is a better approximation of the boundary of a continent

7 Continental Drift: An Idea Before Its Time Fossil Evidence Identical fossils found in South America and Africa Paleontologists agreed: land connection necessary to explain fossil record

8 Continental Drift: An Idea Before Its Time Mesosaurus Small Permian aquatic freshwater reptile Found in eastern S. America and western Africa Glossopteris Seed fern Africa, Australia, India, S. America, and Antarctica Opponents explain fossil patterns by rafting, oceanic land bridges, and island stepping stones

9 Much Fossil Evidence

10 Continental Drift: An Idea Before Its Time Geologic Feature Evidence Rock types and mountain features match up 2.2 billion-year-old igneous rocks in Brazil and Africa Mountain belts end at coastlines and reappear across oceans

11 Continental Drift: An Idea Before Its Time Glacial Evidence Evidence for glaciation on continents now at tropical latitudes Can be explained by supercontinent located near the South Pole

12 Paleoclimate evidence

13 Fossil/Rock Sequences

14 The Great Debate Wegener s hypothesis of continental drift was met with criticism Objections were based on lack of mechanism for continental drift Wegener proposed that tidal forces of the Moon moved continents and that sturdy continents broke through thin oceanic crust Wegener incorrectly suggested that continents broke through the ocean crust, much like ice breakers plowed through ice Strong opposition to the hypothesis from the scientific community

15 The Fatal Flaw! Geophysicists found it in defiance of the laws of physics Sir Harold Jefferies what force could cause the continents to plow though the oceanic crust?

16 The Theory of Plate Tectonics Idea lay dormant for almost 50 yrs Technology out of WWII led to rebirth of the study of the ocean basins Oceanographic exploration increased dramatically following World War II Discovery of global oceanic ridge system Earthquakes at great depths in western Pacific ocean trenches No oceanic crust older than 180 million years Thin sediment accumulations in deep-ocean basins

17 Harry Hess - A New View! Professor of Geology, Princeton During WWII, did echo soundings of Pacific between battles!» Paper History of Ocean Basins 1962» Resulted in the hypothesis of Seafloor Spreading What Problems with Wegener s Model Did Seafloor Spreading explain? Continental crust was riding along on a spreading oceanic crust

18 The Theory of Plate Tectonics Lithosphere is the crust and uppermost (coolest) mantle Oceanic lithosphere varies in thickness Thin at ridges, up to 100 km thick Mafic composition More dense than continental lithosphere Continental lithosphere km thick Felsic composition Responds to forces by bending or breaking

19 The Theory of Plate Tectonics Asthenosphere is the hotter, weaker mantle below the lithosphere Rocks are nearly melted at this temperature and pressure Responds to forces by flowing Moves independently from lithosphere

20 The Theory of Plate Tectonics Lithosphere is broken into irregular plates Plates move as rigid units relative to other plates 7 major plates make up 94% of Earth

21 The Theory of Plate Tectonics Interaction between plates at plate boundaries Divergent boundaries (constructive margins) Two plates move apart Upwelling of hot material from mantle creates new seafloor Convergent boundaries (destructive margins) Two plates move together Oceanic lithosphere descends and is reabsorbed into mantle Two continental blocks create a mountain belt Transform plate boundaries (conservative margins) Two plates slide past each other No lithosphere is created or destroyed

22 The Theory of Plate Tectonics

23 Divergent Plate Boundaries and Seafloor Spreading Most divergent plate boundaries are along the crests of oceanic ridges New ocean floor is generated when mantle fills narrow fractures in oceanic crust

24 Divergent Plate Boundaries and Seafloor Spreading

25 Divergent Plate Boundaries and Seafloor Spreading Most divergent plate boundaries are associated with oceanic ridges Elevated seafloor with high heat flow and volcanism Longest topographic feature on Earth s surface (covers 20% of surface) Crest is 2 to 3 km higher than adjacent basin and can be 1000 to 4000 km wide Rift valley is a deep canyon along the crest of a ridge resulting from tensional forces

26 Divergent Plate Boundaries and Seafloor Spreading

27 Divergent Plate Boundaries and Seafloor Spreading Seafloor spreading is the process by which new seafloor is created along the ocean ridge system Average spreading rate is ~5 cm/year Up to 15 cm/year or as slow as 2 cm/year New lithosphere is hot (less dense) but cools and subsides with age and distance from the ridge system

28 Divergent Plate Boundaries and Seafloor Spreading Continental rifting occurs when divergent boundaries develop within a continent Tensional forces stretch and thin the lithosphere Brittle crust breaks into large blocks Eventually become ocean basins

29 Divergent Plate Boundaries and Seafloor Spreading

30 Divergent Plate Boundaries and Seafloor Spreading

31 Convergent Plate Boundaries and Subduction Convergent plate boundaries occur when two plates move toward each other Convergence rate is equal to seafloor spreading Characteristics vary depending on subducting crust Subduction zones Lithosphere descends into the mantle Old oceanic crust is ~2% denser than asthenosphere Continental crust less dense than asthenosphere

32 Convergent Plate Boundaries and Subduction Deep ocean trenches Long, linear depressions Result of subduction Angle of subduction varies Nearly flat to nearly vertical Depends on density of crust Older crust is cooler and denser

33 Convergent Plate Boundaries and Subduction Characteristics of convergent plate boundaries vary depending on type of crust being subducted Oceanic + continental Oceanic + oceanic Continental + continental

34 Convergent Plate Boundaries and Subduction Oceanic Continental Convergence subduction of oceanic lithosphere Continental lithosphere is less dense Water from descending oceanic crust triggers partial melting of asthenosphere at ~100 km Molten material is less dense and rises Continental volcanic arcs

35 Convergent Plate Boundaries and Subduction

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37 Convergent Plate Boundaries and Subduction Oceanic-Oceanic Convergence One slab subducts under another Volcanism because of partial melting Generates volcanic island arcs Volcanic cones underlain by oceanic crust

38 Convergent Plate Boundaries and Subduction

39 Convergent Plate Boundaries and Subduction Continent-Continent Convergence Neither plate subducts Continental crust is buoyant Folding and deformation of rocks Mountain building

40 Convergent Margins: India-Asia Collision I

41 Transform Plate Boundaries Transform plate boundaries form when two plates slide horizontally past one another Transform faults No lithosphere is produced or destroyed Connect spreading centers and offsets oceanic ridges Linear breaks in the seafloor are fracture zones Fracture zones are inactive Active faults occur between offset ridge segments

42 Transform Plate Boundaries

43 A few Transform Faults,the San Andreas fault and the Alpine fault of New Zealand, cut through continental crust

44 Transform Plate Boundaries

45 Testing the Plate Tectonics Model Evidence from Deep Sea Drilling Project Collect sediment and oceanic crust Date fossils in sediment Sediment age increases with distance from ridge Sediment is thicker with increased distance from the ridge Oldest seafloor is 180 million years old

46 Sedimentary Evidence DSDP -Glomar Challenger JOIDES Resolution IODP Chikyu

47 Testing the Plate Tectonics Model

48 Testing the Plate Tectonics Model Volcanoes in the Hawaiian Island-Emperor Seamount Chain increase in age with distance from Hawaii A cylinder of upwelling hot rock (mantle plume) is beneath Hawaii A hot spot is an area of volcanism, high heat flow, and crustal uplift above a mantle plume A hot-spot track formed as the Pacific Plate moved over the hot spot

49 Testing the Plate Tectonics Model

50 Paleomagnetic Evidence

51 Testing the Plate Tectonics Model Today North and South magnetic poles align approximately with geographic North and South poles Iron-rich minerals influenced by magnetic pole Basalt erupts above the curie temperature, so magnetite grains are nonmagnetic Grains align to magnetic field during cooling Rocks preserve a record of the direction of magnetic poles at the time of formation Paleomagnetism or fossil magnetism

52 Position of paleomagnetic poles appears to change through time because of continental drift

53 Testing the Plate Tectonics Model Magnetic field reverses polarity during a magnetic reversal Rocks with same magnetic field as today have normal polarity Rocks with opposite magnetism have reverse polarity Polarity of lava flows with radiometric ages was used to generate a magnetic time scale Divided into chrons ~1 million years long Finer-scale reversals within each chron Vine and Matthews (1963) suggested stripes of normal and reverse polarity are evidence of seafloor spreading

54 Vine and Matthews Observed reversals in the earth s magnetic field

55 Testing the Plate Tectonics Model

56 Testing the Plate Tectonics Model

57 Testing the Plate Tectonics Model

58 Magnetic Patterns of the Sea Floor

59 What Drives Plate Motions? Mantle is solid, but hot and weak enough to flow Convection occurs as hot, less dense material rises and surface material cools and sinks During slab pull, cold, dense oceanic crust sinks because it is denser than the asthenosphere During ridge push, gravity causes lithospheric slabs to slide down the ridge Drag in the mantle also affects plate motion

60 Convection

61 What Drives Plate Motions?

62 What Drives Plate Motions? Mantle convective flow drives plate motion Subducting plates drive downward component of convection Upwelling of hot rock at oceanic ridges drives upward component of convection Convective flow is the heat transfer mechanism from Earth s interior

63 What Drives Plate Motions? Multiple models for convective flow: Whole-mantle convection Cold oceanic lithosphere sinks and stirs entire mantle Subducting slabs sink to core-mantle boundary Balanced by buoyant mantle plumes Layer cake model Subducting slabs do not sink past 1000 km

64 What Drives Plate Motions?

65 How Do Plates and Plate Boundaries Change? Total surface area of Earth is constant Size and shape of individual plates changes African and Antarctic Plates are growing Surrounded by divergent boundaries Pacific Plate is being consumed Surrounded by convergent boundaries) Plate boundaries move and change through time

66 How Do Plates and Plate Boundaries Change? New ocean basins were created during the breakup of Pangaea

67 How Do Plates and Plate Boundaries Change? Present plate motions can be used to predict future continental positions 50 million yrs 250 million yrs