Lecture20 nov25-bb

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  • The 1960’s is relatively recent. Geology is a new science compared to Physics, Chemistry and even Biology. People were doing separate studies about earthquakes, volcanoes, age of sea floor, paleomagnetism. Someone put it all together. A new theory developed – Seafloor Spreading. There were a lot of evidence to support Seafloor spreading.
  • Note: Orange is shallow. At spreading centers, EQ only occur at the surface. At SZ, deep EQ occur, i.e. Japan, Indonesia, Philippines, Aleutians. How does seismic activity support plate tectonics? They occur along plate boundaries.
  • Note: Orange is shallow. At spreading centers, EQ only occur at the surface. At SZ, deep EQ occur, i.e. Japan, Indonesia, Philippines, Aleutians. How does seismic activity support plate tectonics? They occur along plate boundaries.
  • Convergent boundaries are where the plates move towards each other.
    There are three types of convergent boundary, each defined by what type of crust (continental or oceanic) is coming together.
    Therefore we can have: continent-continent collision, continent-oceanic crust collision or ocean-ocean collision….
  • When continental crust pushes against continental crust both sides of the convergent boundary have the same properties (think back to the description of continental crust: thick and buoyant). 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. For example, the European Alps and Himalayas formed this way.
  • 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 centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram).
    When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram).
    The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes.
    Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.
  • When two oceanic plates converge, because they are dense, one runs over the top of the other causing it to sink into the mantle and a subduction zone is formed.
    The subducting plate is bent down into the mantle to form a deep depression in the seafloor called a trench.
    Trenches are the deepest parts of the ocean and remain largely unexplored.
  • Manned or unmanned submersible vehicles (top right photo) have explored small parts of trenches discovering new species (like the fish photographed here) and amazing ecosystems.
  • The third type of boundary are transform boundaries, along which plates slide past each other.
    The San Andreas fault, adjacent to which the US city of San Francisco is built is an example of a transform boundary between the Pacific plate and the North American plate.
  • Figure 5.11: Cumulus clouds form as hot, invisible air bubbles detach themselves from the surface, then rise and cool to the condensation level. Below and within the cumulus clouds, the air is rising. Around the cloud, the air is sinking.
  • Heat flow provided the mechanism to move the lithospheric plates.

Transcript

  • 1. 1
  • 2. Hess was in World War II He was initially assigned to duty in New York City, where he was responsible for estimating the positions of enemy submarines in the North Atlantic. Hess was then assigned to active sea duty and eventually became commander of an attack transport ship. This vessel carried equipment for sounding the ocean floor, and Hess took full advantage of it. He mapped a large part of the Pacific Ocean, discovering in the process the underwater flattopped seamounts that he named guyots, in honor of A.H. Guyot, the first professor of geology at Princeton. 2
  • 3. Sonar • A device that bounces sound waves off underwater objects and then records the echoes of these sound waves. • Sonar mapped midocean ridges.
  • 4. Hypothesis of sea-floor spreading Molten rock (magma) oozes up from the Earth's interior along the mid-oceanic ridges, creating new seafloor that spreads away from the active ridge crest and, eventually, sinks into the deep oceanic trenches. 4
  • 5. Sea-floor spreading
  • 6. • Continental drift reexamined in 1960’s with new information • New theory developed – Seafloor spreading – Earthquakes – Volcanism – Age of seafloor – Paleomagnetism – Heat flow • Theory combining continental drift and seafloor spreading termed “Plate Tectonics”
  • 7. • The submersible, Alvin, found strange rocks shaped like toothpaste squeezed from a tube. Such rocks can form only when molten material hardens quickly after erupting under water.
  • 8. Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”. They hold a record of reversals in Earth’s magnetic field
  • 9. The earth’s magnetic field does not point exactly north
  • 10. The magnetic field is produced by the motion of electric charges, i.e., electric current 10
  • 11. The Earth's magnetic field appears to come from a giant bar magnet, but with its south pole located up near the Earth's north pole (near Canada). The magnetic field lines come out of the Earth near Antarctica and enter near Canada. 11
  • 12. The liquid iron appears to produce electric currents in the opposite direction of the earth rotation. The currents create the earth magnetism. 12
  • 13. The earth’s magnetic field direction The study of paleomagnetism has demonstrated that the Earth's magnetic field has changed over time.
  • 14. 15
  • 15. The portion of the ocean crust nearest the mid-ocean ridge must have what type of magnetic polarity? 1. normal 2. reverse 3. none 63% 28% 9%
  • 16. We don’t know!!! Can you find a solution to this tough scientific problem? 17
  • 17. Scientists did a drilling sample and found rocks that the farther away from the ridge the older the rocks were. The younger ones were in the center of the ridge.
  • 18. 19
  • 19. ocean floors - 200 million years land - billions of years Why is seafloor so young relative to continents? SUBDUCTION
  • 20. New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenches
  • 21. Subduction Ocean floor plunges into deep-ocean trenches. Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle 22
  • 22. Theory of Plate Tectonics
  • 23. The theory of plate tectonics Earth’s lithosphere is broken into giant plates that move laterally on top of asthenosphere. Most earthquakes and volcanic eruptions happen at plate boundaries. Three types of relative motions between plates: 24
  • 24. The plate refers to the pieces of the lithosphere. The plates of the lithosphere moves atop the asthenosphere
  • 25. Current Earth’s surface layers are divided into nine very large plates and several smaller ones
  • 26. Earthquake distribution matches plate boundaries
  • 27. Volcanoes match some plate boundaries; some are hot spots
  • 28. Convergent Divergent Transform 29
  • 29. Convergent Boundaries • There are three styles of convergent plate boundaries – Continent-continent collision – Continent-oceanic crust collision – Ocean-ocean collision
  • 30. Continent-Continent Collision • Forms mountains, e.g. European Alps, Himalayas
  • 31. Himalayas
  • 32. Continent-Oceanic Crust Collision • Called SUBDUCTION
  • 33. Earthquake depth indicates subduction zones
  • 34. Ocean-Ocean Plate 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. • The worlds deepest parts of the ocean are found along trenches. – E.g. The Mariana Trench is 11 km deep!
  • 35. Ocean-Ocean plate collision Modern example: Japan
  • 36. Transform Boundaries • Where plates slide past each other Above: View of the San Andreas transform fault
  • 37. San Andreas Fault near Gorman, California. The grey, metamorphic quartz monzonite on the left side of the fault are rocks of the Pacific Plate and the brown sandstone and siltstone on the right of the fault are rocks of the North American Plate. Photograph copyright by David Lynch. Plate 39
  • 38. • Plates move away from each other • New crust is being formed
  • 39. An example  Atlantic Ocean
  • 40. Divergent boundaries also can rip apart (“rift”) continents
  • 41. Continental Drift 1) Begins with view of Earth with continents in their present positions, 2) continents move back in time to reunite as Pangaea, 3) Pangaea label appears, 4) Locations of stratigraphic and fossil evidence that Wegener used to argue in favor of continental drift is added. http://www2.nature.nps.gov/geology/usgsnps/animate/A14.gif
  • 42. What are the driving forces of plate motion?
  • 43. Rayleigh-Benard Convection 45
  • 44. Atmospheric Convection Fig. 5-11, p. 118
  • 45. Street Clouds
  • 46. Street Clouds over City College of New York taken by Professor Gedzelman
  • 47. How does Earth work?
  • 48. Theory of Plate Tectonics
  • 49. How does convection work? No one knows but they aren’t afraid to propose models! Two mantle convection cells Complex convection
  • 50. Divergent Convergent Transform
  • 51. Transform Convergent Divergent