Popular in Geological Hazards and Their Human Impact
Popular in Geology
This 17 page Class Notes was uploaded by Ashley Thompson on Sunday September 20, 2015. The Class Notes belongs to GEOL105010 at University of Delaware taught by McGeary,Susan in Spring 2015. Since its upload, it has received 14 views. For similar materials see Geological Hazards and Their Human Impact in Geology at University of Delaware.
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Date Created: 09/20/15
Plate Tectonics I 91815 948 AM Processes that Formed the Ocean Basins 0 Static Earth Model 0 Oceans and continents formed early in history of Earth 0 Spatial relationships constant since that time 0 Dynamic Earth Model 0 Initial placement of oceans and continents very different from present 0 Spatial relationships continually changing on geological time scales Development of Plate Tectonics Theory 0 1666 Francios Placet Coastlines match 0 1858 Antonio SniderPellegrini Fossils match Development of Dynamic Model 0 Began in the 19th Century 0 World maps became more accurate People began to notice jigsaw fit of the continents o Oceanographic expeditions provided better bathymetric information concerning the world ocean 0 Fit continents together and matched Geological features Paleoclimates FossHs o In 1912 proposed that continents broke apart from original land mass and moved away from each other 0 Published an original hypothesis of continental drift in 1915 in The Origins of the Continents and Oceans 0 Continental Drift Evidence Good jigsaw fit of continents n Especially with new data from oceanographic expeditions on contours of continental shelves Ifjigsaw puzzle is pieced together several geological features match up n Ancient coral beds a Continuity of rock structures a Glacial deposits Continuous range of fossil biota across fittogether continents n Cynognathus land reptile n Mesosaurus freshwater reptile n Lystrosaurus land reptile n Glassopteris fern Correlation of mountains with nearly identical rocks and structures Glacial features of the same age restore to a tight polar distribution 0 Wegener s hypothesis Lots of circumstantial evidence accumulated over more than a decade But lacked a plausible forcing mechanism Idea was generally rejected by geologists after initial stir Whole concept of continental drift ignored for next 40 years Birth of Plate Tectonics Credited to German scientist Alfred Wegener o Concluded that over long periods of time continents move 0 Considered the possibility that present day distribution of fossils has resulted from the splitting up of a larger land mass Called the landmass Pangaea meaning all landquot 0 Wegener s Evidence 0 Apparent shoreline fit of continents across the North and South Atlantic 0 New information on offshore contours obtained by recent oceanographic expeditions o Suess s Glossopteris fossils 0 Areas of erosion apparently caused by the same glacier in tropical areas now widely separated S Africa India Australia 0 Shackleton s 1908 fossilized remains of tropical plants in Antarctica Wegener s Proposal o The MidAtlantic Ridge zone in which the floor of the Atlantic as it keeps spreading is continuously tearing open and making space for fresh relatively fluid hot sima rock that forms the continuous lower layer of the Earth s crust rich in Si and Mg from depth Sima is now known as the mantle Wegener s Expanded Idea 0 Proposed mechanism to account for hypothetical drift Heavy continents were slung toward the equator on a spinning Earth by centrifugal effect Force tidal drag on continents from combined effects of sun and moon Phenomenon of drifting continents Static vs Dynamic Earth Model 0 Discovery of subsea mountain ranges led to theories of continental drift and plate tectonics 0 via a research cruise on the Vema a research vessel based at LamontDoherty Earth Observatory Mid Ocean Ridges Midocean ridge system discovered in every ocean Although the ridge runs down the middle of the Atlantic Ocean not all ridges located in center of the ocean Determining the way ocean basins were created turns out to be the key to unraveling the mystery of how Wegener s theory Mid Atlantic Ridge Detail 0 Valley of 31 miles wide amp 06 miles deep runs along the crest line of the ridge 0 the floor of this valley found to be composed of freshly crystallized young basalt Valley floor bounded by prominent normal faults or steep rock faces 0 Amount of heat escaping from Earth s interior at the ridge s crest is 10Xs greater than for most of Earth MidAtlantic ridge cut by long linear transform faults and fracture zones Earthquakes are common and originate at shallow depths of less than 22 miles along the ridge crests and transform faults Evidence for SeaFloor Spreading MidOcean Ridges 0 General Characteristics 0 Rift valleys at top of ridge 50 km wide 1km deep Valleys bounded by steep walls with prominent normal m Slippage of valley walls in vertical plane 0 Also transform faults Slippage along valley flanks in horizontal plane Results in zigzag pattern of rift valleys Two Distinct Kinds of Faults 0 Normal form in vertical directionand hanging wall drops down 0 Strikeslip forms when walls move horizontally they slip along the strike Evidence for SeaFloor Spreading MidOcean Ridges vs Continental Mountain Ranges 0 Compressional forces create continental mountain ranges The Rise of Plate Tectonics 0 WW II and the Cold War Military Spending o US Navy mapped seafloor with echo sounding sonar to find and hide submarines Generalized maps showed oceanic ridges submerged mountain ranges fracture zones cracks perpendicular to ridges trenches narrow deep gashes abyssa plains vast flat areas seamounts drowned undersea islands 0 Dredged rocks of the seafloor included only basalt Gabbro and serpentinite no continental materials 0 Meanwhile US military developed new advanced seismometers to monitor Soviet nuclear tests 0 By the late 19505 seismometers had been deployed in over 40 Allied countries and were recording 24 hrsday 365 daysyear 0 Besides the occasional nuclear test they recorded every moderate to large earthquake on the planet With these highprecision data seismologists found that activity happens in narrow bands 0 Found ridges had magnetic propertiesWHAT SeaFloor Spreading 0 Essentials of Wegener s hypothesis were revived in 1960 when Princeton University geophysicist named Harry Hess proposed hypothesis of sea floor spreading o What evidence led to this new hypothesis Evidence for SeaFloor Spreading MidOcean Ridges 0 General Characteristics Wider extent of ridges discovered in 19305 I Found in all ocean basins a Huge horizontal extent 60000 km a All connected MidAtlantic Ridge traces shape of bordering continents 0 Magnetic Properties Seafloor spreading combined with geomagnetic polarity reversals creates magnetic anomaly stripes In this figure normal is blue and reversed is grey Lecture 4 Slide 43 The Rise of Seafloor Spreading In 19621962 geologists realized that the patterns are symmetrical across oceanic ridges Dating of the rocks showed the youngest rocks at the ridge Magnetism of Earth s Crust Magnetism of earth s crust is measured on each side of the ridge 0 Called Anomalies Found to be u 1 Parallel to ridge n 2 Symmetrical on each side of ridge and rate of seafloor spreading can be calculated using age and distance from ridge crest of any magnetic anomaly stripe Paleomagneticism Earth has a magnetic field and seafloor provides evidence of geomagnetic polarity reversals Polarity of Earth s magnetic field reverses at intervals ranging from 105 to 106 years 0 Average 45 x 105 years 0 Cause probably related to disruptions in fluid dynamics in molten layers of mantle and outer core 0 Magnetic minerals in newly formed rock at spreading center of ridge assume polarity of earth at time of their formation 0 Changes polarity of Earth s magnetic field are captured as these rocks move away from the spreading center 0 This process is called paleomaqnetization 0 Changes in polarity of rocks as one moves away from rift valleys support idea of sea floor spreading Important Findings 0 Oceanic crust that lies on either side of the ridge moves apart 0 Separation produces tensional forces that create normal faults and rift valleys at ridge crests Spreading of ocean floor implies basaltic crust becomes increasingly older with distance from the ridge line 0 Basaltic crust eventually transported down ridge flank and becomes covered with sediment Characteristics of Spreading Zones 0 New oceanic crust generated 0 Shallow earthquakes 0 Mountain chains Silicapoor rock 30 gcmA3 Spreading rates can be measured 0 12 cmyr slow 0 615 cmyr fast Summary of Evidence Jig Saw fit of continents Matching geological features ofjigsaw fit continents 0 Ancient coral beds 0 Continuity of rock structures 0 Glacial deposits 0 Continuous range of fossil biota across fittogether continents Huge horizontal extent of midocean ridges o MidAtlantic Ridge traces shape of bordering continents o Rift valley walls have normal ie vertical faults 0 Valley flanks have transform ie horizontal faults 0 Continental mountain ranges have evidence of compressional forces 0 Forces pushing material together Midocean ridges have evidence of tension forces 0 Forces puing material apart 0 Magnetic anomalies 0 Long bands of basalt parallel to axis of rift valleys 0 Changes in polarity of basalt from one band to the next as observer moves away from axis in either direction 0 Large and varied set of evidence provides overwhelming support 0 Concepts of continental drift and seafloor spreading are universally accepted by the scientific community 0 Overall process is called plate tectonics Plate Tectonics 0 Basic idea depends on structure of upper layers of Planet Earth 0 Continental crust o Oceanic crust o Mantle Lithosphere n Cold and brittle Asthenosphere a Hot and molten Lower mantle The mantle derives its heat from the even hotter core layers of planet earth 0 Sources of heat at earth s core 0 Gravitational pressure 0 Energy from initial earth formation radioactive decay 0 General View 0 Heat source comes from deep in the planet 0 Drives convective flow in asthenosphere o Molten basat upwes at divergent points in flow 0 Upwelling fow ruptures thin oceanic crust aong axis of mid ocean ridges 0 Cooling by ocean water forms spanking new oceanic crust 0 New crust carried away from axis by frictional drag associated with convective flow in asthenosphere Spreading and Subduction Zones 0 Occur along Opposite Edges of the same plate 0 Driving mechanisms for plate movement 7 Major Tectonic Plates 0 African plate Antarctic plate Eurasian plate IndoAustralian plate North American plate Pacific plate and South American plate Rates of Geologic Processes 0 Many geologic processes including plate movements occur over immense time scales 0 Rule of thumb on average lithospheric plates move at about the rate that your fingernails grow about 2 inyr Took about 200 million years for Atlantic Ocean to reach its present dimensions by the way the Maritimes were connected to Africa just before this time Plate Tectonics II 91815 948 AM Outline for Today s Lecture 0 Plate tectonic model 0 Characteristics of plate boundaries 0 Plate mechanics 0 Dynamics of mantle plumes 0 Seismic activity away from plate boundaries Plate Tectonics 0 Basic idea depends on structure of upper layers of Planet Earth 0 Continental crust o Oceanic crust o Mantle Lithosphere cold amp brittle Asthenosphere hot amp molten Lower mantle 0 General View 0 Heat source comes from deep in the planet Drives convective flow in asthenosphere Molten basalt upwells at divergent points in flow Upwelling flow ruptures thin oceanic crust along axis of mid ocean ridges Cooling by ocean water forms spanking new oceanic crust 0 New crust carried away from axis by frictional drag associated with convective flow in asthenosphere Plate tectonics o Plates are driven by cooling of earth convenction Heat within Earth comes from residual heat and radioactive decay of naturally occurring elements 0 Gravity provides additional force to move plates 0 What are the Tectonic Plates 0 Lithospheric plate There are a dozen large plates some of continents some don t All are in motion a The 100km thick surface of the Earth a Contains crust and part of the upper mantle n It is rigid and brittle n Fractures to produce earthquakes 000 O o What is the Asthenosphere o The hotter upper mantle below the lithospheric plate 0 Can flow like silly putty o It is viscoelastic solid NOT LIQUID Subduction and Seismicity 0 Creation and destruction of oceanic crust are cataclysmic processes 0 Occur in conjunction with high seismic activity 0 Earthquakes o Volcanoes 0 Heat flow from interior 0 Activity concentrated along plate boundaries 0 Seafloor spreading zones 0 Subduction zones Plate Tectonic Model Assumptions 0 Continental crust is thick and cool with low density rock 0 Oceanic crust is thin and cool with mgh density rock 0 Lithospheric plates are rigid and brittle and are spatially discrete o Asthenosphere is viscous hot and dynamic o Lithospheric plates above it move along with movement of asthenosphere because of frictional drag 0 General Features of Model 0 New crust is formed at spreading zones 0 Crust becomes part of lithospheric plates 0 Crust and plates move away from point of origin in response to convection in asthenosphere 0 Therefore lithospheric plates must be moving around 0 So what happens at the plate boundaries Three basic types of plate boundaries In Divergent Stages of Development 0 A Upwarping of lithosphere and continental crust B Upwelling of magma from asthenosphere and formation of new oceanic crust in rift valley o C Formation of more oceanic crust and Hnearsea 0 Red Sea is a geologically new spreading zone 0 D Formation of midocean ridge and extensive new oceanic crust n Convergent9Plate Boundaries Collision of two plate boundaries 0 One plate slides under the other subduc on Crustal material in subducting plate is destroyed melted and becomes part of asthenosphere Types 0 I Oceanic plate collides with oceanic plate Oceanic plate subducted and destroyed Trenches formed II Oceanic plate collides with continental plate Oceanic subducted and destroyed Continental crust is uplifted III Continental plate collides with continental plate Uplifting and mountain formation 0 Characteristics of convergent zones 0 O O O O Crust lost or deformed Deep earthquakes Deep trenches with island arcs or continental mountain ranges Rock silicarich after remelt Major source of oil a Transform Plate boundaries 0 Differential movement of two adjacent plate boundaries 0 Crust and lithosphere are neither created nor destroyed o Crust is conserved EX San Andreas Fault 0 Implications of Plate Tectonics While ocean basins are created and destroyed continents are too light to be subductedso simply break apart and collide Earthquakes coincide with plate boundaries Volcanoes tend to occur along plate boundaries 0 How Fast are the Plates Moving o Plates move 110cm per year rate of fingernail growth Seismic Activity Away from Plate Boundaries Hotspot Volcanoes o Are produced by movement of plate over stationary magma plume from a point source of heat in the mantle o Formed independently from volcanoes at divergent and convergent boundaries and can actually occur in the stable interior of a plate 0 Heights of hotspot volcanoes decrease with increasing distance from point of active volcanism Mantle Plumes and Hotspots o Plumes and lithospherecrust move relative to each other 0 Hotspot the linear volcanic chains trace the drift path of a plate over a mantle plume Passive Continental Margins EX Atlantic Seaboard Margin Characteristics 0 Few earthquakes Few faults Wide continental shelf 20200km wide Deltas Amazon Coral reefs Australia Economically important oil gas fisheries OOOOO Active Continental Margins EX West coasts of North and South America Margin Characteristics 0 00000 Frequent earthquakes Active volcanoes faults Narrow steep shelf Troughs Faults Trenches Island arcs Earth Structure and Plate Tectonics 91815 948 AM Radioactive Decay Review 0 Amount of parent left at different half lives 0 0 half lives 100 11 0 1 half life 50 12 0 2 half lives 25 14 0 3 half lives 125 18 0 4 half lives 625 116 0 etc o How much of the parent element is left after 2 half lives 0 Answer 25 o How much of parent element is left after 3 half lives 0 Answer 18 Earth Structure and Plate Tectonics Plate Boundaries o Convergent Plate Boundaries Plates converge producing mountains volcanoes and earthquakes Subduction Zone Oceanic crust goes under another crust n OceanicOceanic or continentContinent a Process subduction oceanic lithosphere is destroyed Big earthquakes Explosive Volcanoes Volcanic Arcs Oceanic Trench Continental Collision If both are continental they collide a Process collision Mountain range 0 High plateau o Divergent Plate Boundaries move away from each other at midocean ridges MidOcean Ridges OceanicOceanic a Process sea floor spreading Small earthquakes Many many volcanoes Midoceanic ridges Oceanic crust forms Continental Rifts ContinentContinent a Process Continental Rifting o Transform Plate Boundaries Transform Faults plates move laterally past each other between seafloor spreading centers n OceanicOceanic OceanicContinental ContinentalContinental a Process translation 0 Big earthquakes No volcanoes 0 Plate Tectonic Summary Lithospheric plates move horizontally relative to each other across the surface of the planet The plates themselves are strong and rigid but the boundaries between plates are complicated and hazardous Earthquakes Volcanoes Tsunami n Convergent PB 0 Process of Subduction or Earthquakes big explosive volcanoes 0 Process of Continental Collision n Transform PB 0 Process of Translation Earthquakes no volcanoes n Divergent PB 0 Process of Sea Floor Spreading 0 Process of Continental Rifting Earthquakes nonexplosive volcanoes Convection 0 Hot less dense buoyant rises 0 Cold Denser rock sinks Mantle Liquid iron outer core Crust Solid iron inner core moiII hm 1 RIM Q Au 391 The Crust 670 0 Made up of oxygen silicon oxygen magnesium iron 0 Rocky9silicate materials 0 There are two kinds of crust continental and oceanic Continental crust underlies the continent9thicker than oceanic crust and has more aluminum and potassium Oceanic crust underlies the ocean basins9thin and has more iron and magnesium The Mantle o Rocky9silicate materials Solid not liquid 0 More iron and magnesium 0 Core 0 Outer core is liquid 0 Inner core is solid 0 Both made out of iron LithosphereAsthenosphere 0 We can also regard layering based on rock strength rheologyquot how a material behaves under pressure Lithosphere the outermost 100150 km of Earth Asthenosphere upper mantle below the lithosphere Largest Earthquake ever recorded since 1900 1960 Chile M95 0 Continental Margins 0 Where land meets the ocean Margins near plate boundaries are active Margins far from plate boundaries are passive
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