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Natural Science II – ERTH 1040 Plate Tectonics J. D. Price Mid ocean Ridge Again – our understanding of the earth falls off with depth. In the 1920’s, Alfred Wegner cited evidence that the continents were joined (as recently 60 Ma). The coastlines of South America and Africa match. Rocks and fossils show similar environments across several continents. © Wiley, 2002 19-385 Bathymetry: mapping the topography of the sea floor. Initial done with ship-board sonar soundings, now complimented by satellite measurements. Q: What prompted mapping of the ocean floors? The Mid-Atlantic Ridge The Mid-Atlantic Ridge Roughly in the center of the Atlantic ocean, there is a topographic rise. This is an area marked by volcanism (mostly subaqueous, with exceptions like Iceland) Figure 19.22 19-384 Radiometric dating reveals the youngest crust (red) is at the ridges. The oldest crust (blue) is only 160 Ma and is furthest from the ridges. While observing bathymetric measurement, scientists found that the rocks on the seafloor recorded a series of reversals of the earth’s magnetic field. These roughly parallel ocean ridges. U.S.G.S. The earth's magnetic field The movement of the core and interaction with the mantle is thought to make the planet behave like a giant bar magnet. It protects the surface from the solar wind Atoms produce magnetic fields But atoms are randomly oriented – so fields cancel each other. In some materials, the atoms form domains, which may align in a field. Basalt A rock containing abundant Fe, Mg, Ca, Si, O Univ. North Carolina, Atlas of rocks, minerals, and textures website Magnetite (Fe3O4) A magnetic mineral abundant in basalt. http://webmineral.com/specimens/Magnetite.jpg Q: How is the Earth’s magnetic field recorded in rocks? Magnetic field recorded The earth’s field flips orientation irregularly (10 K years to 1 M years). Magnetic north moves south. Flips may take 1 K years in some cases. Magnetite grains in lavas preserve the earth’s field at the time they crystallize and cool. Figure 19.21 19-383 Reflection of seafloor spreading Orientation Figure 19.23 N S N 19-385 Q: Why is the magnetism recorded on both sides of the ridge? S The major plates of the lithosphere Source:After W. Hamilton, U.S. Geological Survey Q: What are the motions of the plates? Direction and magnitude of motion for the plates Arrows indicate direction and relative magnitude of each plate’s motion If the lithosphere (crust & uppermost mantle) is in movement, but all of the fragments are moving in different directions with different accelerations then •Some pieces are moving apart from each other Divergence - extension •Some pieces are moving into each other Convergence - compression •Some pieces are moving obliquely to each other Transform - strike slip Divergent Boundary Oceanic Rift Q: What is a probable cause of spreading? Divergent Boundary Continental Rift http://jsc.nasa.gov STS-32 Q: What will happen to East Africa with time? Example: the East African Rift Hot Spot Pacific plate motion Example: Hawaiian chain Hawai’i Transform margin Example: San Andreas Fault © Cornell Geology Continental collision Example: the Himalaya Q: Why doesn’t one of the plates subduct? Subduction: Continental Arcs Example: the Cascades Q: What is the deepest part of the oceans? Important point - the subducted slab doesn’t melt to produce the volcanism seen in the arc. It simply doesn’t get warm enough. Melting under subduction volcanoes is most likely related to fluid release deep in the mantle. Fluids move through networks to flux mantle, promotes melting at 1200ºC. Subduction: Island Arcs Example: Indonesia Gunung Semaru and Bromo Caldera, Java Q: What causes mantle melting in Island Arcs? If the materials are able to move, convection may occur Note: this works for most materials for most relative heats Cold lithosphere eventually sinks under its own weight. The pull drags plate, extends spreading center. A deeper mechanism is probably the ultimate source of motion - convection. Plate tectonics results from heat transfer The brittle lithosphere plates of the Earth “floats” on the hotter and more ductile asthenosphere Plates are constructed by volcanism at divergent margins Plates slide past each other along transform margins Oceanic plates cool, becoming heavier, and sink at convergent margins. Important Question: Are continents new or old? Mid ocean Ridge The upper mantle has a “depleted” composition. Crust + depleted mantle is roughly that of the lower mantle and chondrites The lanthanides (La-Lu) are also known as the rare earth elements. A G E S Shield Platform Orogen Extended Basin oldest rocks, 2.5 - 3.9 Ga oldest sediments, 2.5-0.6 Ga compressed,< 0.6 Ga <0.6 Ga <0.2 Ga