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Jessica Hawthorn

47BIO21th Sept 12 Plate Tectonics Earth’s Lithosphere - upper lithosphere – crust o oceanic – mafic: rich in Mg and Fe; more dense o continental – felsic: rich in silica; less dense - lower lithosphere – uppermost part of mantle - moho discontinuity – boundary between crust and mantle, occurs within the lithosphere - lithosphere divided into plates; sits on top of asthenosphere (upper part of mantle) Plate Tectonics - tectonics: movement of earth’s crust and large-scale rock deformation - plate tectonics: o movement of plates (varying sizes) - deformation of rocks o metamorphosis at high pressures and temperatures  compressive forces  folding o folds and faulting  increase folding  overturned fold  overturned fold can break - types of faults: o normal: extensional motion o thrust: compressive motion o strike-slip: transform motion Continental Drift - older idea of plate tectonics - early scientists recognized relationship between fossils on continents separated by sea o most proposed a former connection via land bridges o observation that continents fit together goes back as far as 1858 (Antonio Snider-Pellegrini) though not popular back then - development of the idea that continents move across earth’s surface o Alfred Wegner (1910s) o Alexander du Toit (1920s and 1930s) - Wegner’s evidence: o Continents fit together o geological similarities o floral (plant life) and faunal (animal life) similarities - biostratigraphy: correlation between fossils o much work done by Du Toit o India, South America, Africa, Australia, Antarctica o geological correlation between rock sequences around the world - glaciation: orientation of glacial striations and till deposits on southern continents suggested they were linked - till deposits: where glaciers “dump their load” - no sound mechanism provided - Wegner’s (quickly discredited) possibilities: o centrifugal force caused by earth’s rotation? o precession of the earth (wobbling of axis)? o ‘tidal argument’ based on the tidal attraction of sun and moon? The Rise of Modern Tectonic Theory - recognition of Mid-Atlantic Ridges as site of landmass rupture and formation of Atlantic Ocean o first inferred in 1850 o actually discovered in 1872 o first mapped in 1950s - Harry Hammond Hess (1960’s) o Geopoetry: seafloor spreading; continents don’t plow through seafloor  entire crust moves o crust must be created and destroyed o driven by convection cells - convection: rotational flow of fluid resulting from density imbalance o material heated deep in asthenosphere rises o displaces cooler, denser material near surface - palaeomagnetism: magnetization of rocks resulting from earth’s magnetic field at the time of their formation - declinaton: angle that a compass needle makes with the line running to the geographic north pole; minerals lock in this orientation when they form rocks - a palaeomagnetic test of plate tectonics: o Vine and Matthews (1963) measured magnetization of rocks across Indian Ocean central ridge o found normal and reversed ‘stripes’ creating a mirror image - ridges: o hot rising mantle material rises to top of lithosphere and cools
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