PLATE TECTONICS
advertisement
THE THEORY OF PLATE TECTONICS INTRODUCTION Tectonics- large scale deformational features of the crust Plate tectonics – Earth’s outer shell divided into plates – Plates move & change in size thru time Activity at plate boundaries Combines: – Continental drift – Sea-floor spreading – Paleomagnetism INTRODUCTION Ideas – – – Continental drift- Alfred Wegener Sea-floor spreading Paleomagnetism Early Case for Continental DriftAlfred Wegener (F.B. Taylor & H.H. Baker) Continental coastlines fit together – 1620 Sir Francis Bacon: Africa and S. America Rocks – & structures indicated that continents joined Pangea- supercontinent of the late Paleozoic » Separated into Laurasia & Gondwanaland Fossil evidence- Glossopteris & Mesosaurus Late Paleozoic glaciation Skepticism – about Continental Drift Problem of driving mechanism Continental Drift Alfred Wegener 1912 Pangaea (ALL EARTH) Evidence: – Continents FIT together like the pieces of a puzzle – Fossils – Rocks & structures – Paleoclimate Continental Drift Alfred Wegener 1912 Pangaea (ALL EARTH) Evidence: – Continents FIT together like the pieces of a puzzle – Fossils – Rocks and structures – Paleoclimate Continental Drift Alfred Wegener 1912 Pangaea (ALL EARTH) Evidence: – Continents FIT together like the pieces of a puzzle – Fossils – Rocks and structures – Paleoclimate INTRODUCTION Tectonics- large scale deformational features of the crust Plate tectonics – Earth’s outer shell divided into plates – Plates move & change in size Activity at plate boundaries Combined: – Paleomagnetism – Sea-floor spreading INTRODUCTION Tectonics- large scale deformational features of the crust Plate tectonics – Earth’s outer shell divided into plates – Plates move & change in size Activity at plate boundaries Combined: – Paleomagnetism – Sea-floor spreading Paleomagnetism Iron becomes magnetized below the Curie Point (600oC) Magnetite and hematite aligns on existing magnetic field Dip indicates old magnetic pole position Apparent motion of north magnetic pole through time – Split in path – indicates continents split apart Paleomagnetism Magnetite aligns on existing magnetic field Dip indicates old magnetic pole position Apparent motion of north magnetic pole through time – Split in path – indicates continents split apart SEA-FLOOR SPREADING Magnetic anomalies – 1950’s detection of 10-50km wide strips symmetrical about ocean ridges – Vine and Matthews: magnetic reversals Sea-floor moves away from mid-oceanic ridge Plunges beneath continent or island arc- subduction Plate movement rate of 1 to 20 cm/year, 5 cm/yr average Driving force – – Mantle convection Ridge Push- Slab Pull forces SEA-FLOOR SPREADING Explanations – Mid-oceanic ridge » Hot mantle rock beneath ridge High heat flow Basalt eruptions » » Rift valley Shallow-focus earthquakes SEA-FLOOR SPREADING Explanations – Oceanic trenches » » » » – Low heat flow Negative gravity anomalies Benioff zone earthquakes Andesitic volcanism Age of sea floor » » » Young age of sea floor rocks (oldest 160 my) Implies youngest should be at ridges, oldest at trenches Explains pattern of pelagic sediment How do we know that plates move? Marine – magnetic anomalies Vine-Matthews Hypothesis » » » » – – Anomalies Reversals Normal and reverse polarity Positive and negative anomalies Measuring the rate of sea floor spreading Predicting sea floor age Plates and Plate Motion Plate – – Entirely sea floor or continental and oceanic Lithosphere – – Crust & uppermost mantle Thickness increases away from ridge Asthenosphere – – Low seismic velocity zone behaves plastically Plates and Plate Motion Plate – – Entirely sea floor or continental and oceanic Lithosphere – – Crust & uppermost mantle Thickness increases away from ridge Asthenosphere – – Low seismic velocity zone behaves plastically History of Continental Positions Pangea split up 200 m.y. Continents in motion for at least 2 billion years How do we know that plates move? Fracture Zones & Transform Faults – Pattern of earthquakes at ridges and fracture zones – Transform fault Measuring plate motion directly – Use of satellites Plates and Plate Motion Interior of plates relatively inactive- Cratons Activity along boundaries – Trenches (zone of subduction), melanges (complex of shear rock), accretionary prism (sedimentary and volcanic wedges separated by high angle faults) – e.g., earthquakes, volcanoes, young mountain belts Plate tectonics a unifying theory for geology Boundaries – Divergent – Convergent – Transform DIVERGENT BOUNDARIES During break up of a continent – Rifting, basaltic eruptions (Flood Basalts), uplifting – Extension- normal faults, rift valley (graben) forms – Shallow focus earthquakes Continental crust separates – Fault blocks along edges – Oceanic crust created – Rock salt may develop in rift East African Rift System – early stages of rifting – continental rifting Red Sea Rift Red Sea Gulf of Eilat Dead Sea Linear Seas TRANSFORM BOUNDARIES Two plates slide past each other Usually between mid-oceanic ridge segments – Can also connect ridge and trench – Or trench to trench Origin of offset of ridges CONVERGENT BOUNDARIES Plates move toward each other One plate overrides the other – Subduction zone CONVERGENT BOUNDARIES Continental-Continental – convergence Two continents approach each other and collide » » » » – Crust thickened » – Sea floor subducted on one side Ocean becomes narrower and narrower Continent wedged into subduction zone but not carried down it Suture zone Two thrust belts Mountain belt in interior of continent Plate Size 104 km2 to 108 km2 New sea floor added to trailing edge of plate – e.g. North American plate growing at midAtlantic ridge Oceanic plate might get smaller as continetal plate overrides it – e.g. Eastward moving Nazca plate subducted beneath westward moving South American plate Intra-Plate Features Thermal Plumes Explains – Yellowstone volcanism – Hawaiian volcanism – Aseismic ridges What Causes Plate Motions? Slab push-pull Convection in mantle –Deep mantle convection – Two-layer convection What Causes Plate Motions? Convection – in mantle Convection a result of plate motion » » » Ridge push Slab pull Trench suction
