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ERTH 2415 (21)
Chapter 3

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Earth Sciences
ERTH 2415
Claire Samson

Chapter 3 Plate Tectonics and Earthquakes Plate TectonicsThe lithosphere of Earth is broken into pieces called plates These gigantic pieces pull apart during seafloor spreading at divergence zones slide past at transform faults or collide at convergences zones The study of the movements and interactions of the plates is known as plate tectonicsThe tectonic cycle first melted asthenosphere flows upward as magma and cools to form new lithosphere on the ocean floor Second the new lithosphere slowly moves laterally away from the zones of oceanic crust formation on top of the underlying asthenosphere seafloor spreading Third when the leading edge of a moving slab of oceanic lithosphere collides with another slab the denser slab turns downward and is pulled by gravity back into the asthenosphere subduction while the lessdense more buoyant slab overrides it Lastly the slab pulled into the asthenosphere is reabsorbed It takes 250 million years to complete this cycleDevelopment of Plate Tectonic ConceptIn 1620 Francis Bacon of England noted the parallelism of the Atlantic coastlines of South America and Africa and suggested that these continents had once been joinedDuring the 1800s Eduard Suess presented evidence in support of Gondwanaland a supercontinent composed of South America Africa Antarctica Australia India and New ZealandAlfred Wegener presented his ideas about continental drift and that all continents had once been united in a supercontinent called PangaeaEvidence of Plate Tectonics from Seafloor SurveysAs the lava erupted from a volcano cools at the surface of Earth minerals begin to grow as crystals Some of the earliest form crystals incorporate iron into their structures After the lava cools below the Curie point at a out 0C ato s in ironbearingineralseco eagnetized in the direction of Earths magnetic field at that time and placeThe atoms point towards the magnetic pole of their time the north magnetic pole normal polarity or the south magnetic pole reverse polarityLava flows pile up as sequences of stratified rock and the magnetic polarity of each rock can be measuredVolcanic rock contains minerals with radioactive elements that allow determination of their ageThe oldest rock on the ocean floors are 200 million years old Evidence of Plate Tectonics from Earthquakes and VolcanoesA popular way of forecasting the locations of future earthquakes uses the seismicgap method If segments of one fault have moved recently then it seems reasonable to expect that the unmoved portions will move next and thus fill the gaps One segment of a fault can move two or more times before an adjoining segment moves onceA majority of active volcanoes are found on the edges of tectonic plates The creation of new lithosphere and the destruction of old lithosphere is generally accompanied by volcanic activity Recycling Earths Outer Layers
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