Theory of Plate Tectonics: Combined Seafloor Spreading and Subduction Lithosphere created at spreading ridges and consumed at subduction zones Continents are along for the ride! Mechanisms? Three types of plate boundaries Plate boundaries are defined by locations of Earthquakes. Plate Boundaries ~15 plates Only 3 types of boundaries Divergent, convergent, transform 1
Divergent Boundaries: Plates spread apart Mantle rises, melts and forms new lithosphere OCEANS are formed Continents rift ultimately resulting in mature ocean basin (e.g. Atlantic). animation Transform Boundaries: plates slide past each other Very common in ocean b/c they link ridge segments Required because They also link trenches to ridges, trenches to trenches, etc. spreading is perpendicular to linear axes & Earth is spherical, and Spreading rates vary along ridges Show animation 3rd Type - Convergent Margins Where plates collide continents eventually grow Two types: collision zones (e.g. Himalaya) Evolve from subduction zones Build large mountains stuff-up or stop subduction with continental mass that cannot subduct into mantle. Subduction zones (e.g. Japan, Andes, Aleutians) Build island arcs (e.g. Japan, Aleutians) and continental arcs (e.g. Andes) Consume lithosphere Grow continents by addition of island arcs. Animation 2
1. Slab Pull: Subducting slab pulls lithosphere along Issue: No Atlantic subduction So, need additional mechanisms 2. Ridge Push: Sloped density contrast (Lith vs. Asth) is unstable Horizontal force works to flatten this slope to stable configuration Demo: putty with plates Plate Tectonics mechanisms 3. Asthenosphere convection: Regional flow of asthenosphere shears plate Forces plate along No Atlantic Subduction, thus no slab pull in Atlantic Except for small Scotia and Puerto Rico subduction zones Plate Motions How can we understand plate motions: their velocities and paths over time? 3
Hot-spots and rates of plates: Absolute Velocities What is a hot-spot (or plume)? Anomalously hot mantle (from core-mantle boundary) Rises Buoyantly (plume) Melts on assent (adiabatic decompressive - see volcanoes lecture) Feeds volcanoes on Lithosphere How to make a hot-spot track Plume burns through lithosphere Plate motion continues New volcanoes form Example: Candle burning a piece of paper. Animation - good more cartoons! 4
Create hot-spot track See the hot-spot tracks? Samoa, Hawaii-Emperor, Ninety-East Ridge, Tristan/Walvus Ridge, etc. (they are everywhere!) Should plate velocities from hot-spot tracks agree with spreading rates? A. Yes B. No 2.4 Ma 0.7 Ma Absolute vs. Relative plate vectors Do they always agree? Blue vs. red vectors Not always, spreading rate is plate motion relative to divergent boundary. Hot-spot track/absolute velocity is velocity of plate relative to fixed reference 5
Plate Tectonics & Wilson Cycle ~15 plates interact at margins Created at MOR and consumed at Subduction Zones Wilson Cycle: Tuzo Wilson Ocean basins open & close over geologic time Continents grow by Accumulation of sediment in intercontinental basins Accumulation of island arcs as SZs collide with continents Supercontinents form when all continents collect at a SZ, then they break apart Wilson cycle: watch oceans Pangaea! Panthalassic & Tethys 6
Open Tethys rift Pangaea, open Atlantic, Pacific: Close Tethys, open Atlantic, begin closing Pacific: Today: 7
Review Questions Describe how the three mechanisms driving plate motions operate to move plates. Why do earthquake locations define the margins of plates? What are the three basic types of plate boundaries? Compare and contrast collision zones and subduction zones. Which type of plate boundary produces an ocean. Consider an ocean rimed by subduction zones (similar to today s Pacific Ocean). What is necessary for the ocean to close (reduced area)? Contrast transform margins with fracture zones. Describe the Wilson Cycle? How do hotspot tracks indicate plate velocities? Why don t spreading or convergence rates agree with absolute plate velocities? Under what circumstances might a mid-ocean ridge or a subduction zone migrate? 8