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+ Evidence for Plate Tectonics Chapter 9.4 Objectives • Summarize • Evidence • that led to the theory of plate tectonics • the discovery of seafloor spreading. • Explain • Significance of magnetic patterns on the seafloor • their relationship to seafloor spreading • the theory of platetectonics. + Continental Drift - Review Wegener - Continental drift hypothesis Evidence Continental puzzle Fossil records Matching mountain ranges Ancient climate What part of his hypothesis was not supported? He had no ? + Theory of Platetectonics The earths lithosphere is fragmented into a dozen or more large and small plates which ride a top the asthenosphere. + Theory of Platetectonics Lithosphere Earth's rigid outer shell • Consists of several plates Plates are moving slowly Largest plate is the Pacific plate Plates are mostly beneath the ocean Asthenosphere • Exists beneath the lithosphere • Hotter and weaker than lithosphere • Allows for motion of lithosphere + Evidence to support Plate Tectonics 5 main scientific developments led to the Theory 1) Ruggedness and youth of the ocean floor 2) Volcanic activity along trenches and mountain ranges 3) Paleomagnetism 4) Seafloor spreading 5) Hot spots + 1. Rugged and young ocean floor Before the 19th century The ocean floor was considered flat 1955 first soundings chart published (*Sonar) Showed underwater mountains Oceanic crust less than 200 million years old (my) *Improved Technology – Radiometric Dating Ocean Rocks and Sediments • Analysis of deep-sea rocks and sediments produced two important discoveries. 1. The ages of seafloor rocks vary in different places 2. The age of oceanic crust consistently increases with distance from a ridge. 3. The thickness of ocean-floor sediment was, in general, much less than expected 4. The thickness of the sediments increases with distance from an ocean ridge. Evidence emerges as Technology improves • The development of echo-sounding methods allowed scientists to study the ocean floor in great detail. • Sonar uses sound waves to measure water depth by measuring the time it takes for sound waves to travel from the device and back to a receiver. Ocean Floor Topography • The maps made from the data collected by sonar and magnetometers showed underwater mountain chains called ocean ridges. • The same data showed that these underwater mountain chains have counterparts called deep-sea trenches. • These two topographic features of the ocean floor puzzled geologists for over a decade after their discovery. + 2. Earthquakes and volcanic activity along ocean trenches and mountains World’s earthquakes and volcanic activity Precise documentation that it is concentrated along Ocean trenches Ocean mountain ranges *Improved technology - seismology Greatly advanced knowledge in 1960’s The World wide standardized seismograph network (WWSSN) Established to monitor activities related to the 1963 treaty banning above ground testing of nuclear weapons + 3. Repeated changes of earth’s magnetic field Confirmation of repeated changes of the earth’s magnetic field in the geologic past In the 1950’s scientists using magnetometers (WWII subs) began noticing odd magnetic variations across the ocean floor A magnetometer is a device that can detect small changes in magnetic fields, allowing scientists to construct magnetic maps of the seafloor. Magnetism • Rocks containing iron-bearing minerals provide a record of Earth’s magnetic field. Paleomagnetism is the study of Earth’s magnetic record. • Basalt, because it is rich in iron-bearing minerals, provides an accurate record of ancient magnetism. Magnetism The Geomagnetic Time Scale – Towing magnetometers behind ships to measure the magnetic field of the ocean floor revealed an interesting pattern. • In places where the magnetic readings of the ocean floor matched Earth’s present field, a stronger-than-normal reading (+) was recorded. • In places where the magnetic data were reversed in relation to Earth’s present magnetic field, a lower-than-normal reading (–) was recorded. Magnetism The Geomagnetic Time Scale – Studies of continental basalt flows in the early 1960s revealed a pattern of magnetic reversals over geologic time. – A magnetic reversal is a change in Earth’s magnetic field. • A magnetic field that is the same as the present has normal polarity. • A magnetic field that is opposite to the present has reversed polarity. Seafloor Spreading Magnetism Magnetic Symmetry – The positive and negative areas of the seafloor form a series of stripes that were parallel to ocean ridges. – The magnetic pattern on one side of the ridge is a mirror image of the pattern on the other side of the ridge. Magnetism Magnetic Symmetry – The magnetic data collected from the ocean floor matched the pattern of magnetic reversals that had been found in basalt flows on land. – From this match, scientists were able to determine the age of the ocean floor from a magnetic recording and quickly create isochron maps of the ocean floor. – An isochron is a line on a map that connects points that have the same age. Magnetism + 4. Emergence of seafloor spreading hypothesis Emergence of the seafloor spreading hypothesis and associated recycling of the ocean crust The seafloor spreading hypothesis was supported by the paleomagnetic data Seafloor Spreading • An American scientist named Harry Hess proposed the theory of seafloor spreading. Seafloor spreading states that new ocean crust is formed at ocean ridges and destroyed at deepsea trenches. – Magma is forced toward the crust along an ocean ridge and fills the gap that is created. Seafloor Spreading Seafloor Spreading – When the magma hardens, a small amount of new ocean floor is added to Earth’s surface. – Each cycle of spreading and the intrusion of magma results in the formation of another small section of ocean floor, which slowly moves away from the ridge. + Intraplate volcanism Activity within a tectonic plate Associated with plumes of heat in the mantle Form localized volcanic regions in the overriding plate called a hot spot Produces basaltic magma sources in oceanic crust (e.g., Hawaii and Iceland) Produces granitic magma sources in continental crust (e.g., Yellowstone Park) Oldest island furthest from hot spot Helped clear up the mystery of intraplate volcanic activity