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ECU GEOL 1500 - Plate Tectonics - Notes + Diagrams - Fall 2014

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OVERVIEWPLATE BOUNDARIES AND PLATE MOTIONB. Secondary ProcessINCLUDEPICTURE "http://pubs.usgs.gov/gip/dynamic/graphics/FigS1-1.gif" \* MERGEFORMATINETPLATE TECTONICS A UNIFYING THEORY AND MODELOVERVIEWPlate tectonics provides a unifying theory that can be used to explain the origin and past and present geographic locations of both continental and oceanic terranes. In addition major Earth processes, such as mountain building, earthquakes, and volcanism, can be explained within the context of Plate Tectonics. TheTheory of Plate Tectonics states that the Earth’s outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material. Thus, Plate Tectonics is the study of the movement and deformation of the Earth’s lithosphere. The Earth’s lithosphere (defined immediately below) is divided into seven large plates and several smaller ones, which move at rates ranging from about 2 to 14 cm/year. As a plate moves, everything on it moves. If the plate is capped partly by oceanic crust and partly by continental crust, then both the ocean floor and the continent move at the same rate and in the same direction. The old term “continental drift” is still used sometimes to describe movement of the continents, but actually everything on a plate moves, not just the continents.In addition to compositional layering within the Earth, there are other changes at depth due to changes in Physical Properties, such as changes in rock strength. Changes in physical properties are largely controlled by changes in temperature and pressure rather than rock composition. One location within the Earth where changes in physical properties do not coincide with compositional boundaries is the lithosphere-asthenosphere boundary in the upper mantle where the balance between Pressure and Temperature conditions is such that the rocks in the asthenosphere are hot, weak, and plastic whereas those in the lithosphere are cold, strong, and rigid. The Lithosphere (Rock Sphere), which extends from the Earth’s surface to depths of 70 to 100 km, is composed of cooler, stronger, and more rigid/brittle materials than the asthenosphere immediately below. The Lithosphere is composed of the uppermost part of the mantle and all of the Earth’s crust. The Asthenosphere (Weak Sphere) extends from the base of the Lithosphere (70-100 km) to about 400 km depth and is hotter, weaker, and more plastic than the materials immediately above or below this depth range due to the balance between pressure and temperature within this depth range. Parts of the asthenosphere are partially molten, and it lies completely within the Upper Mantle.PLATE BOUNDARIES AND PLATE MOTIONDue to the escape of heat energy from Earth’s interior, movement occurs along narrow zones between plates or the boundaries between the Earth’s lithospheric plates. There are three types of plate movements and corresponding boundaries. These are: (1) Divergent Plate Boundaries where two plates move apart; (2) Convergent Plate Boundaries where two plates move toward each other; and (3) Transform Plate Boundaries where two plates move horizontally past each other in opposite directions.Global Distribution of Earthquakes Defines the Plate BoundariesSummary Diagram of Plate Boundaries (be able to label this diagram if the labels were missing!)Divergent Plate Boundaries (Constructive) A. Major Processes1. Continental Rifting 2. Seafloor Spreading B. Secondary Processes1. Earthquakes2. VolcanismC. Primary Geographic/Physiographic Features1. Rift Valleys/Fissures (e.g., East Africa Rift Valley)2. Oceanic Ridges with Rift Valleys/Fissures along their axes (e.g., Mid-Atlantic Ridge)(East Pacific Rise)Continental Rift Zone TypeResults: Young plate boundaries, rift valleys that may eventually fillwith ocean water creating a new ocean and then it becomes an oceanic-oceanic divergent boundary/mid-ocean ridge. Volcanism, Tensional Stress, Normal faulting, weak Earthquakes. Ex. Great RiftValley East Africa, Lake Superior and Reelfoot Rift in North America (ancient/dormant rift).Ocean Ocean Rift Zone Type (Bottom)Results: Major process is Seafloor Spreading with Mid-Ocean ridgessurrounded by new oceanic crust, the newest crust is closest to therift zone. Basaltic Volcanism, Tensional Stress, Normal faulting, weak Earthquakes. ex. Mid-Atlantic Ridge, East Pacific Rise. Animations: http://www.wiley.com/legacy/college/media_dev/exploring/tsunami/ch13_animations/divergent_plates.html Map of the Ocean Floor Showing Ocean Ridge-Rise FeaturesIndicating Divergent BoundariesConvergent Plate Boundaries (Destructive) A. Two Major Types1. Subduction Zone Type – Two sub-types a. Major Processab i. Subduction of Oceanic Crust under Continental Crust (Oceanic-Continental Convergence = Continental-type or Andean-typee.g., Andes, Cascade Range in US Pacific NW) Results: Subduction zone, basaltic oceanic crust is more dense than granitic continental crust. Deep Ocean Trenches, Andesitic Volcanism dominant, Stratovolcanoes and Volcanic Mountain Range on Land. ex. Cascade Mountains in the Pacific Northwest US and the Andes Mountains in South America. Compressional Stress, Reverse Faulting, strong Earthquakes.Example of Oceanic-Continental Subduction Zone Type Convergent Boundary ii. Subduction of Oceanic Crust under other Oceanic Crust (Oceanic-Oceanic Convergence = Island Arc-type – e.g., Philippines, Japan)Results: Subduction zone. Deep Ocean Trenches, Andesitic Volcanism dominant, Volcanic Island Arcs. ex. Mariana Trench, Japan, Aleutian Islands (Alaska), Philippines, Sumatra (2004 Tsunami). Compressional Stress, Reverse Faulting, strong Earthquakes.Example of Oceanic-Oceanic Subduction Zone Type Convergent BoundaryAll of Northwest Pacific (see diagram above)b. Secondary Processesi. Earthquakes ii. Volcanismc. Primary Geographic/Physiographic Featuresi. Deep Sea Trench (e.g., Peru-Chile Trench, Philippine Trench) “Ring of Fire” <http://pubs.usgs.gov/publications/text/fire.html>ii. Volcanically active mountain range parallel to Subduction Zone-Trench(e.g., Andes Mountains, Cascade Mountains)2. Collision Zone-Type a. Major Processi. Continent-Continent CollisionResults: Folded Mountains on Land, but no Volcanism. ex. Himalayan Mts., the Alps in Europe, the Appalachian Mountains (ancient orogenys or mountain building


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