GEOL 120 1st Edition Lecture 4Outline of Last Lecture I. Wegner’s observationII. Harry HessIII. Seafloor spreading and subductionIV. Oceanic ridges V. SeamountsVI. Oceanic TrenchesVII. Seafloor spreading hypothesisVIII. Earth magnetic FieldOutline of Current Lecture I. Plate Tectonic TheoryII. What is a Tectonic Plate?III. How are plates distributedTypes of continental MarginsIV. Lithospheric StructureV. Plate BoundariesVI. Subduction ZonesVII. Subduction zones Earthquakes VIII. Transform Plate BoundariesIX. Hot SpotsX. What makes plates move?Current LectureI. Plate Tectonic Theorya. Evolved in the 1960sb. The earth’s lithosphere is divided into platesII. What is a Tectonic Plate?a. Tectonic plates are comprised of crust and uppermost mantlei. The lithosphere1. Thickness variesIII. How are plates distributeda. The location of plate boundaries is largely defined by the distribution of earthquakesb. Plate boundaries may or may not occur at continental marginsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.IV. Types of continental Marginsa. Activei. Near plate boundariesb. Passivei. Far from a plate boundary1. Continental crust transitions to oceanic crustV. Lithospheric Structurea. Continental platesi. Made up of mostly felsic rocks1. Less dense than mafic rocks2. Float higher on the asthenosphereb. Oceanic plates i. Made of mafic rocks1. More dense than continental crust2. Pulls ocean floor downVI. Plate Boundariesa. Divergenti. Plates move away from one another1. Dominated by volcanism, crustal stretching and thinningii. Two main types1. Continental riftsa. Embryonic2. Oceanic Ridgesa. Mature and fully developediii. Formation of new oceanic crust1. Gabbro rocks form in the “crystal mush” of magma chamber2. Basalts form as lava breaches the ocean floor and cools3. Thinned lithospheric mantleb. Convergenti. Plates move toward one another1. When oceanic plate and continental plate coverage, the oceanic lithosphere always sinks because of higher density2. Oceanic lithospherea. Recycled at subduction zonesc. Transformd. Each defines a different type of motioni. They are complimentaryVII. Subduction Zonesa. Locations where Oceanic Lithosphere sinks back into the mantleb. Unique features of subduction Zonesi. Accretionary prismsii. Volcanic arcs and back arc basin are characteristic featuresVIII. Subduction zones Earthquakesa. Movement of subducting plate “grinding” against overriding plate causes earthquakes.i. Shallow earthquakes are highest magnitudeb. Death of subduction zonei. Occurs when the last bit of oceanic lithosphere is subductedIX. Transform Plate Boundariesa. Plates move past one another without creating or destroying crustX. Hot Spotsa. Long-lived volcanic centers that are not associated with plate boundariesb. How they formi. Stationary plume in the mantle that periodically erupts through an overriding plateXI. What makes plates move?a. Early ideas suggested regular convection cells pulled plates along form underneathb. How fast plates movei. Anywhere from 1 to 15 cm per year1. Absolute and relative motions can differ by a