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Earth and Ocean Sciences
EOSC 116
Louise Longridge

MODULE E Planetary Engineering: Mesozoic Tectonics Lesson 17 Plate Tectonics 101 Introduction - Plate Tectonics = Grand Unifying Theory explaining relationships between processes within Earth From Theory of Continental Drift - Alfred Wegener in 1915 The Theory of Plate Tectonics 1) Earths surface consists of many lithospheric plates including crust (continental or oceanic) and immediately underlying mantle, cold and rigid 2) These plates are presently moving around on Earths surface and interacting with one another -some lithospheric plates consist of both continental and oceanic floor ex) N.American plate = N.American continent and western half of Atlantic Ocean Indian-Australian plate = continental rocks of India, Australia and oceanic floor Plate Motion - plate motions random rates and directions - rates of 2-18 cm/yr quite slow, but with perspective of geological time in terms of million years, rates of 100s of km/million years means entire oceans can be destroyed in tens millions years Significance of plate tectonics 1) Almost all earthquakes, most volcanoes, occur where lithospheric plates are interacting 2) Main mineral and hydrocarbon resources occur in specific tectonic settings 3) Plate tectonic processes happening at depth responsible for occurrences at Earths surface: size and shape of oceans, nature and distribution of landforms, general climatic conditions of each of Earths regions Plate Boundaries - map of epicenter of significant earthquake, M > 3.5 - active boundaries marked by zones of earthquakes and volcanoes - earthquakes closely associated with active plate boundaries Land forms associated with plate boundaries -mountain belts occur along many plate boundaries - N. America: main mountain belts stretch from Alaska to northwest N. America all the way to south along western edge of N. and S. America Oceanic forms associated with plate boundaries - topography of ocean floors is very irregular, including what would be high mountain chains and deep canyons (if they werent covered by water) - much of Earths recent tectonic history recorded on ocean floor, so oceans just as important and interesting as land surfaces Paleogeography - study of nature of landforms and evolution of landforms through time - critical for all life all Earths surface ex) if tectonic processes cause uplift of part of continent, may change from low-lying swamp area covered by shallow seas that is suitable for many landforms, to a high barren desert incapable of supporting significant life Evolution of the western margin of North America - following description of figures outline how western margin of N. America evolved from Early Jurassic (~180 Ma) until end of Cretaceous (~65 Ma; dinos died) - subaerial = land exposed above sea level (brown) - submarine = areas covered by shallow water (pale blue) - mountainous areas = bumpy, deeper submarine areas = dark blue, red dots = individual volcanic centers 1) Early Jurassic: ~180 Ma - large block of crust (Wrangellia) was converging with western edge of N. America and would eventually collide 2) Middle Jurassic: ~160 Ma - a series of volcanic islands fringing western edge of N. America and Wrangellia continuing to move towards continent - a shallow sea now covered central part of N. American continent 3) Late Jurassic: ~145 Ma - Wrangellia actively colliding with southern edge of N. America and process caused mountain belt growth along collision zone - shape and extent of shallow water interior sea has changed considerably since 160 Ma 4) Early Cretaceous: ~125 Ma - Wrangellia now collided with most of western margin - mountain chain along most of western edge of N. America - shallow interior seaway extends from Arctic Ocean south to southern Wyoming and another in Northern Mexico 5) Late Cretaceous: ~85 Ma - interior seaway (called Cretaceous Seaway) extended full length of N. America and separated from Pacific Ocean by a continuous mountain chain 6) Cretaceous/Tertiary boundary: ~65 Ma - interior seaway, where many dinos had flourished, had largely dried up - dinos extinct Plate tectonics: Basic concepts - 2 main types lithospheric plates: oceanic (thinner, denser) and continental - differ in composition, thickness, density - as a result, oceanic crust usually submarine (underwater) whereas continental crust usually subaerial (high-standing, well above sea level) - 3 main lithospheric plate interactions 1) Divergent boundaries plates moving apart from one another 2) Convergent boundaries plate moving towards one another 3) Transform boundaries plates sliding along one another Divergent Margins - stages of process that breaks/rifts a continent apart, ending up with an ocean basin separating two continental fragments 1) Extension 2) Rift valley formation 3) Opening of ocean continent tears in two; continent ridges faulted and uplifted; basalt magma erupts from oceanic crust 4) Young ocean widens with developing mid-ocean ridge The Red Sea a continent being rifted apart - red sea, separating African and Arabian plate, an example of what happens when a single continental block is rifted into two - in this case, a spreading center or ridge is forming along boundary between the two plates and new oceanic crust is created as two continental plates spread apart Convergent Margins - when oceanic crust subducts under another plate (oceanic or continental), large volumes of molten rock (magma) produced - magma rises to shallow levels in crust of overriding plate and some erupts to surface as lava, forming volcanoes - since magmas produced along a linear plate margin, volcanoes form a linear or more commonly arcuate line (forming an arch) - 3 types of convergent margins 1) Oce
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