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Download Lecture 1 Plate Tectonics
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Transcript
Plate Tectonics and Deformation Landforms Unifying Theory Plate Tectonics affects: Earthquakes Volcanoes Oceanic circulation and global weather patterns Natural resource distribution Distribution of organisms Early Hypothesis Alfred Wegener (1915) Developed the hypothesis of continental drift This suggested that all of the continents were once connected in a supercontinent called Pangaea His ideas were rejected: Due to the lack of mechanism All of his evidence came from the southern continents Classical Evidence The continents fit, especially along the continental slope regions where erosion is minimal Matching rock sequences and mountain ranges were found across continents Africa and South America have similar mountain ranges Rock type the same Rock ages the same Appalachian and European mountain ranges line up Classical Evidence Glacial evidence Scouring patterns show landmasses were connected Glaciers mass Fossil move outward from center of the ice evidence Glossopteris Mesosaurus Plate Tectonic Theory The lithosphere is divided into plates, which move because of heat transfers deep in the Earth This is a unifying theme in geology There is overwhelming evidence to support it It explains: Mountain building Earthquake activity Volcanism Distribution of life Natural resource distribution Supercontinent Cycles Theory by J. Tuzo Wilson (sometimes called Wilson Cycles) Supercontinents form, break up and reform throughout Earth’s history Plate Boundaries Divergent Plate Boundary Plates separate and move in opposite directions Crust is extended and fractured New ocean crust is formed Large fractures, shallow-focus earthquakes and basaltic pillow lavas are features found in these areas Plate Boundaries Old boundaries: Look for faults, dikes, sills, specific sediment series in rift valleys Pillow lavas Aulacogens This is where a rift valley was starting The failed rift valley is called an aulacogen Scientists supposed that the New Madrid fault zone is an aulacogen Plate Boundaries Convergent Plate Boundary Ocean – Ocean One plate is subducted, usually the older plate You end up with a subduction complex: Wedge of scraped-off sediments and lithosphere Volcanic arc’s can also occur: These are curved chains of volcanoes Plate Boundaries Ocean – Continent Oceanic plate is subducted Result in a continental volcanic arc that is andesitic Continent – Continent Continental plates collide and are welded together This results usually in mountain building Plate Boundaries Old Boundaries: Look rock for igneous rocks, intensely deformed Chaotic mixture of folded, faulted and metamorphosed rocks Ophiolites A series of rocks that contains slices of oceanic crust welded to the continental crust Plate Boundaries Transform Plate Boundary Plates move parallel, in opposite directions Mostly occur on the seafloor Results in earthquakes and fractures Old boundaries Evidence of old transform plate boundaries is sparse except for large displacements of rock systems. Intraplate Features Hot spots: The mantle plumes are stationary The plate moves over the plume and leaves behind a trail of progressively older volcanoes This can be used to determine direction and rate of plate movements Mechanisms of Plate Tectonics The lithosphere is divided into plates, which move because of heat transfers Continents and ocean floors move together A convection current is hypothesized Subduction occurs where cells descend Spreading occurs where cells ascend Deformation Deformation is how a rock body responds to tectonic changes Why is this important? This becomes a record of past events Helps us to find and recover resources Helps us to plan structures like bridges and dams Important Terminology Stress Application of force or pressure Types: Compressional Tensional Shear Important Terminology Strain Deformation Can caused by stress be: Elastic rocks return to original shape Plastic rock bend or fold Brittle rock fault High temperature and pressure plastic Near Earth’s surface (low temp and pressure) brittle Important Terminology The following are used to describe the orientation of rock. REMEMBER: sediments accumulate in horizontal layers!!! Strike Direction of rock layer intersecting a horizontal plane Dip Angle of deviation from horizontal Perpendicular to strike Types of Deformation Folds Monocline Simple bend or fold Anticline Limbs dip away from axis Oldest rocks are at the core Syncline Limbs dip toward axis Youngest rocks are at the core Types of Deformation Inclined fold Asymmetrical limbs Overturned Limbs dip in same direction because one limb is rotated > 90˚ Recumbent fold Axial planes are horizontal Plunging fold fold Axis is not horizontal (see usually as anticline or syncline) Types of Deformation Domes and Basins These are oval or circular equivalents of anticlines and synclines Domes- oldest rocks are in the center (anticline) Basin – youngest rocks are in the center (syncline) Types of Deformation Joints Fractures along which NO movement has occurred Faults Fracture along which movement HAS occurred Faults Hanging Foot With wall wall a fault relative movement is described, one or both walls may have moved. Faults Types: Dip-slip Faults vertical movement in fault plane Normal Fault Hanging wall moves down, and foot wall moves up Produced by tensional stress (divergent areas) Reverse Fault Hanging wall moves up, and foot wall moves down Produced by compressional stress Special kind of reverse Thrust fault: reverse with dip less than 45˚ Faults Strike-slip Faults horizontal movement Left or right lateral movement is shown Produced by shear stress (transform) Oblique-slip Faults both vertical and horizontal movement has occurred Block Diagrams A block diagram is a representation of a piece of the Earth. We analyze block diagrams for relative time order (i.e. what happened first, second, third… last) Block Diagrams A few rules to remember to analyze basic block diagrams: The oldest rocks are on the bottom (Principle Igneous intrusions or faults are younger than the rock it cuts through (Principle of Superposition) of Cross Cutting Relationships) Rock fragments in a layer are older than the rock itself (Principle of Inclusions) Practice 1 5 (youngest) Principle of Superposition & Principle of Inclusions & Principle of Cross Cutting Relationships 3 Principle of Superposition & Principle of Inclusions 4 Principle of Cross Cutting Relationships 2 Principle of Superposition 1 (oldest) Principle of Superposition Practice 2 5 6 4 7 (Reverse Fault) 3 2 1
