OK so what s the speed of dark When everything is coming your way you re obviously in the wrong lane OCNG 251 Oceanography Tuesday Nov 11th 2008 Defining Boundaries 2 Plate Tectonics II 1 Review 2 Plate Tectonics The dynamic Earth Who laughs last thinks slowest slowest Defining Boundaries 1 Marine Provinces 2 Intro to Plate Tectonics 1 Marine Provinces Continental Margins Shelf Shelf Slope Rise Rise Deep Ocean Basins Abyssal Abyssal Plains Abyssal Hills Seamounts Trenches Trenches Mid Oceanic Ridges 2 Intro to Plate Tectonics Differentiation and formation of distinct layers Isostasy Ocean vs Continental Crust Continental Drift and Sea Floor Spreading Evidence Evidence An Introduction to Plate Tectonics Originally Originally the elements segregated during the formation of Earth density fractionation fractionation Core 31 5 Ni Fe Differentiation and formation of distinct layers seemed to have taken very little time 100My Mantle 68 1 Mg Fe Si O Crust 0 4 Al Si O Physical differentiation Not all of the core is molten As Pressure melting point at higher internal pressures Fe melts at 2000 2000 C So liquid liquid iron only occurs where temperatures are high enough but pressures are low enough But what does it mean to have a liquid core As the result of the Earth Earth s rotation the molten iron flows continually around the solid inner core forming a magnetic field The movements of liquid metal in the outer core eddies generate a process akin to a dynamo that generates a magnetic field on Earth and reversals reversals And how does that change our lives First of all it protects us from solar winds winds But then it provides evidence for differentiation and molten core Also provides evidence for Plate Tectonics What does it mean to have a plastic Mantle Principle of Isostasy It It s a matter of balance Ocean Crust vs Continental Crust Continental Drift He further proposed that this ancestral supercontinent had begun breaking up approximately 200 million years earlier into a northern portion and a southern portion He noted that land fossils 150 Ma from different continents were similar whereas fossils 150 Ma were different and had evolved separately Continental Drift Alfred Wegener 1880 1930 German meteorologist advocated the theory of continental drift in the early XXth Century Drawing on several lines of evidence he rejuvenated the idea that all the continents were once joined as one landmass which he named Pangaea Pangaea Continental Drift However the mechanisms he proposed rotation of the Earth and tidal forces did not stand the test of science and his theory theory was disregarded Evidence for Sea Floor Spreading In the 1950s as people began creating detailed maps of the world s ocean floor they discovered a mid ocean ridge system of mountains nearly 60 000 km nearly 40 000 mi long This ridge goes all the way around the globe However Global Plate Tectonics A matter of balance Sea floor spreading and Plate Tectonics New material is brought to the surface in the center of Mid Oceanic ridges Divergent Divergent boundaries boundaries How Old material sinks below the surface at subduction zones Convergent Convergent boundaries boundaries Evidence for Sea Floor Spreading American geologist Harry H Hess proposed that this mountain chain was the place where new ocean floor was created and that the continents moved as a result of the expansion of the ocean floors Also during the 1950s physicists in England became interested in the observation that certain kinds of rocks produced a magnetic field field In 1966 Walter Pitman Columbia U published his work in Science Magnetic Anomalies over the Pacific Antarctic Ridge Ridge The paper showed the symmetry of the magnetic anomalies along the Pacific Antarctic Ridge It confirmed the seafloor spreading hypothesis and caused the unification of the geological world around the Theory of Plate Tectonics Tectonics There comes a sense of evidence but how does it work Geomagnetic Field Paleomagnetism Earth Earth s geomagnetic field two poles Intensity strongest and vertically oriented at magnetic poles Orientation of minerals in solidifying lava Geomagnetic field has flipped back and forth over time Evidence for Sea Floor Spreading The oddest and most problematic feature of the mid oceanic ridges pertains to their magnetic properties Magnetic profiles across midoceanic ridges showed alternating high magnetic readings positive and low magnetic reading negative These stripes run parallel to the ridge and are symmetrically distributed about the ridge axis Geomagnetic Field Paleomagnetism Geomagnetic field has flipped back and forth over time That process and its discovery gave geophysicists evidence of Sea Floor Spreading Additional Evidence Increased age of the oceanic crust away from mid ocean ridges Additional Evidence Heat flow from the Earth interior to the crust decreases as the distance from the ridge center and crustal age increase Additional Evidence Increased thickness and age of sediments away from mid ocean ridges Sea floor spreading and Plate Tectonics Divergent boundaries Convergent boundaries Sliding boundaries oceanic and continental faults Additional Evidence Mid Oceanic Rift Valleys Faulting Faulting Additional Evidence Deep Earthquakes 100 km Subduction zones Surface Earthquakes 0 100 km Seismic epicenters epicenters Shallow earthquakes Not randomly distributed on the surface of the Earth arranged in orderly pattern Additional Evidence Deep Earthquakes 100 km More Physical Evidence Hot spots spots of mantle plumes i e Hawaii and Emperor Seamount Chain More Hotspots More than a hundred hotspots beneath the Earth s crust have been active during the past 10 million years Most of these are located under plate interiors for example the African Plate but some occur near diverging plate boundaries Some are concentrated near the mid oceanic ridge system such as beneath Iceland the Azores and the Galapagos Islands Old er Source USGS http pubs usgs gov gip dynamic hotspots html More Evidence Yet Seafloor spreading and the odyssey of the green turtle turtle Source A Carr and P J Coleman 1974 Seafloor spreading theory and the odyssey of the green turtle Nature Vol 249 p 128 130 Evidence for Sea Floor Spreading Paleomagnetic signatures of oceanic crust Increased thickness and age of sediments away from mid ocean ridges Heat flow from the Earth interior to the crust decreases as the distance from the ridge center and crustal age increase Age of the