Chapter 2

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Environmental Science
Jovan Stefanovic

CHAPTER 2: PLATE TECTONICS: Plate tectonic theory: surface of the earth is divided into large plates that change position/size -volcanic eruptions/ earthquakes occur at plate boundaries -continental drift+ sea-floor spreading = theory of plate tectonics WHAT IS PLATE TECTONICS Tectonics: study of the origin and arrangement of structural features of earths surface ex. Folds/ faults, mountain belts, continents etc. Geologic activity occurs at plate boundaries; plates move away, past or towards each other 8 big plates + small ones = outer shell of earth Plate tectonics explain: earthquake distribution, origin of mountain belts / sea-floor topography, distribution/composition of volcanoes Theory: 1960 Continental drift: continents move freely over the earths surface Sea-floor spreading: sea floor forms at the crest of mid-ocean ridges, move horizontally away from the ridge toward ocean trench HOW DID THE PLATE TECTONICS THEORY EVOLVE? THE EARLY CASE FOR CONTINENTAL DRIFT Idea of super continents around for more than 130 years Super continent called Pangea Laurasia ( northern super continent) + gondwamaland ( south + india) = Pangea Distribution of late Palaeozoic glaciations supports Pangaea Gonwanaland continents all have glacial deposits Paleoclimatology: study of ancient climate Glacial features indicate cold climate near north/south pole ; coral reef indicate warm water near equator Sedimentary rocks show where ancient poles/ equator were located Evidence for change in position of poles support Wegners concept of continental drift SKEPTICISM ABOUT CONTINENTAL DRIFT Wegener proposed continents ploughed through oceanic crust, crumbling up mountain ranges on the leading edge of the continent where they pushed against the sea floor though to have violated strength of rocks RENEWED INTEREST IN CONTINENTAL DRIFT: Revival of the idea continental drift/ sea-floor spreading into plate tectonics Concept investigation in 2 areas: 1) Study of the sea floor 2) Geophysical research exp relation to rock magnetism Study of the sea floor: - Ocean covers 70% + of earth surface - Rocks can be taken from sea floor: Broken from the sea floor by a rock dredge ( opens steel container dragged over ocean floor) Sampled with a corer Weighted steel pipe dropped vertically into mud/sand of ocean floor Sampled by sea-floor drilling ( cuts long rod like cores from the ocean floors) tool for studying: - single- beam echo sounder = measures water depth, draws profile of submarine topography - sidescan sonar: measures intensity of sound reflected back to the tow vehicle , provides detailed image of the sea floor , info about sediments/ bedforms - seismic reflection profiler: same as echo sounder but uses louder noise at lower frequency; records water depth, internal structure of the rocks/ sediments , folds/ faults, unconformities - deep-sea cameras: photograph rock/ sediment Geophysical Research: evidence of polar wandering came from the study of rock magnetism \ magnetic poles located near geographic poles; position of magnetic poles move year year but stays close to geo poles as they move rocks record strength/ direction of magnetic field magnetite: preserves a record of magnetic field when it cools below curie point; sedimentary rocks, red shale, iron oxides = record magnetic field paleomagnetism: study of ancient magnetic fields polar wandering paths are used to reconstruct continental movement over time single pole stood still while continents spilt apart and rotated as they move RECENT EVIDENCE FOR CONTINENTAL DRIFT: evidence came from: rock matches from now separated continents, identical contacts are found on south America & Africa , isotopic ages match both rocks on continents glacial striation show that during the late palozoic Ea continental glaciers moved from Africa towards the present ; many boulders in South America glacial deposits have been traced to Africa History of Continental Positions: paleomagnetic evidence shows the direction/ rate of drift pangea split 200 mil, continents have been in motion for 2 bil yearsWHAT IS SEA-FLOOR SPREADING? Sea-floor spreading: sea floor moves away from mid ocean ridges as a result of mantle convection sea floor moving like a conveyer belt away from crest of mid-ocean ridges, down flanks of ridges and across deep ocean basins then it disappears beneath a continent or island arc sea floor moving away from ridge crest spreading axis or spreading center Subduction: sliding of seafloor beneath continent or island arc Convection: circulation pattern driven by the rising of hot material and/or the sinking of cold material ; hot rises, cold sinks convection explains sea floor features and the young age of sea floor rocks hot mantle rock rising under mid-oceanic ridges mantle rocks move horizontally away from ridge crests on each side which creates tension at the ridge crest which cracks open oceanic crusts creating rift valleys and associated shallow-focus earthquakes mantle rocks moves move horizontally away from ridge crests, carries the sea; mantle ( hot) rock cools and becomes denser, sinking beneath the ocean surface downward plunge of cold rock accounts for oceanic trenches and their low heat flow values, large negative gravity anomalies associated with trenches interaction b/w moving sea-floor rock & stationary rock causes benioff zone of earthquakes associated
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