50PLATE TECTONICSI. IntroductionA. General1. The theory of plate tectonics is a recent development in the geological sciences,really accepted by scientific community since the early 1960's. 2. Earlier in the century geologic paradigm was dominated by the belief that oceanbasins and continental land masses were permanent and fixed on the surface of theearth. 3. The theory of Plate Tectonics now recognizes that the positions of land masses arenot fixed and that they have moved about the earth's surface over geologic historya. Ocean basins/oceanic crust are continually being created and destroyedthrough tectonic processes.B. Terminology1. "Tectonics" - is a term that refers to the deformation of the earth's crust.2. "Plate" - refers to the subdivision of the earth's crust and lithosphere into a numberof tectonically coherent blocks 3. "Plate Tectonics" - refers to the formation and migration of these lithosphericplates, a. Problem: time spans of plate motion on order of 10' of to 100's of million's ofyears, (1) theory has beed deduced from evidence recorded in earth's rocks,often difficult to interpret and sometimes inconclusive.II. Overview of Earth InteriorA. Crust- a relatively thin outer layer1. oceanic crust- thin on order of several km's thicka. volcanic / basalt in composition2. continental crust-thicker on orde of 10's of kms thicka. plutonic/sedimentary/"granitic" in compositionB. Mantle- rocky layer located below the crust and having a thickness of 2885 km1. Mantle- dense, iron-magnesium silicate rocks512. "Moho"a. Mohorovicic discontinuity or Moho-seismic discontinuity in which velocity ofearthquake waves increases abruptly below a depth of 50 kmb. now known to be boundary between crust and upper mantle3. Asthenosphere- soft zone of partially melted rockC. Lithosphere- outer solid portion of the earth which includes the upper mantle above theaesthenosphere and the crust.D. outer core- 2270 km thick, possesses characteristics of mobile liquid1. Liquid, iron-rich, E. inner core- 1216 km thick, solid metallic sphere1. Core- thought to be composed of iron and nickel, very speculative, based on studyof meteorites and speculation that they represent the interior composition of earth.III. Historical Perspective on the Evolution of Plate Tectonic Theory: Continental Drift a Precursor toTectonic TheoryA. Continental Drift - Alfred Wegner (German earth Scientist) proposed a hypothesis in early1900's that the world continents have been drifting about on the earth's surface1. Supercontinent of "Pangaea" existed 200 M.Y. ago in which all of major worldscontinents were once amalgamated together, and have since broken apart andmigrated or drifted to their present positions/configurations.2. Evidence for Wegner's hypothesis of Continental Drifta. Jig-saw puzzle fit of the Continents: b. Fossil Evidence (1) Mesosaurus which is only found on east coast of South America andwest coast of Africa.(2) How did these critters migrate across the ocean basins? c. Similar Rock Types and Structural Rock Deformation across ocean basind. Paleoclimatic Evidence(1) Evidence for glacial conditions 250 m.y. ago are found in similaraged rocks from southern Africa, South America, India and Australia523. Problem with Wegner's ideasa. not widely acceptedb. suggested on the continents were "drifting" not ocean basinsc. did not have a viable mechanical explanation as to how continents would"drift"IV. Modern Plate Tectonic TheoryA. Basic Model - Based on early work by Wegner, more recent mapping of seafloor,magnetic surveys of earth's magnetic field, and observation of earth's seismic activity orearthquake activity.1. Plates- Plate tectonics model suggests that the outer, rigid lithosphere of the earthconsists of about twenty rigid segments known as "plates". a. Plate Mobility - it is recognized that each moves as a distinct rigid unit inrelation to other plates. (1) These plates move on top of a semi-plastic aesthenosphere, andinteract with one another along their boundaries. 2. Plate Boundaries and Nature of Interaction between Platesa. 3 types of plate boundary interaction: Divergent, Convergent, or Transformfault boundariesb. Divergent Boundaries - boundary condition in which tectonic plates moveapart, resulting in upwelling of magma and volcanic material to create newseafloor: i.e. creation of new crust.(1) located at crests of mid-oceanic ridges, where plates move apart andmolten rock is injected and cooled to form new seafloor.(2) Seafloor spreading- process of plate divergence and injection ofmagma. (a) E.g.Atlantic ocean basin, has undergone seafloor spreadingover last 165 m.y. at avg. rate of 6 cm/year. (b) e.g. Red Sea in Middle East is an example of a very youngocean basin that is just beginning the process of seafloorspreading.(3) Continental Rifting(a) Pulling apart of continental crust by faulting(b) Incipient seafloor spreading center53c. Convergent Boundaries- plate boundaries in which two plates move towardone another or collide. (1) Collision of one plate into another results in downbending of one plateand descent of that plate beneath the other,(2) subduction zone- a zone of plate convergence in which where anoceanic plate descends into the upper mantle beneath the overridingplate.(3) trenches - zone where subducting slab dives beneath over-ridingplate(4) volcanic arc - an arcuate chain of volcanoes on continental crust thatresult from subduction of oceanic crust beneath continental crust.(a) Cascade mountains in U.S., Andes in SAM, Sierra Mtns in CAare eroded core of volcanic arc.(b) Forearc region - zone in front of arc, towards trenchi) e.g. Willamette Valley, west of Cascades(c) Backarc region - zone behind the arc, away from trenchi) e.g. central Oregon, east of Cascades(5) Types of convergent boundaries(a) Oceanic-Continental Plate Convergence-i) e.g. W. Coast of North America (b) Oceanic-Oceanic Plate convergencei) e.g. Japan(c) Continent-Continent Plate Convergencei) e.g. India / Asia / Himalayasd. Transform Fault Boundaries: condition where plates slide horizontally pastone another along a fault (or fracture along which there is movement) (1) Crust is neither consumed nor destroyed. (2) Transform faults connect convergent and divergent boundaries into aworldwide network of interconnected plate boundaries. (a) (e.g. San Andreas Fault in Ca).V. Evidence to Support Modern Plate Tectonic TheoryA. Magnetism and Paleomagnetism (result of search for German submarines in WWII)1.
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