Full Year Lectures for BIO211

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Jessica Hawthorn

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BIO211 Sept 7 th Earth Origin and Structure Origin of Universe - age of the universe: big bang approx 15 billion years ago - planets formed near time of suns formation o about 4.6 billion years ago - outer planets composed of volatile elements - inner planets are rocky Origin of Earth - earth materials differentiated o densest at the center o less dense silicates rose to surface; cooled to form crust - meteorite impacts increased concentrations of some elements in upper earth o crust (0-40 km) o mantle (40-2890 km) o liquid iron core (2890-5150 km) o solid iron core (5150-6370 km) Origin of Moon - moon formed from impact of a protoplanet with earth - proportions of iron and magnesium differ from earths mantle o contribution from the colliding protoplanet? Origin of Continents - earliest crust was made up of relatively dense oceanic basalts (fine-grained volcanic rocks) - felsics (rocks rich in silicate materials) differentiated to form nuclei of continental crusts o modern analogue: Iceland border between 2 tectonic plates - felsics: high in silicate minerals oxygen and silicone - mafic: high in magnesium and iron - oceanic crust: denser and heaver, more mafic - continental: lighter, less dense, felsic - oldest rocks on earth are small archean fragments o high heat flow limited continental thickness o zircon crystals: 4.1-4.2 billion years old o Canadian shield: 3.8-4.0 billion years old - continental accretion: deep marine sediments accreted to continent, forming wedges between continental masses o taking a free ride from the oceanic crust and getting scraped off and put on the continental crust as the oceanic crust is subdued o continental crust stays on top because it is less dense than the oceanic crust thus oceanic goes underneath o if 2 oceanic together, one goes down (depending on thickness) o 2 continental collide: mountains Earths Structure Core - solid inner core: 6370-5150km deep - molten outer core: 5150-2890km deep - mostly iron Mantle - 2890-40 km deep - comprises most of earths volume - rocky, less dense than core - upper portion is called the asthenosphere: viscous, low density, behaves plastically - above the asthenosphere is called the lithosphere: ultramafic (very low in silicate materials, high in Mg and Fe), behaves elastically o uppermost part of the mantle (including the crust) - mohorovicic: boundary between the mantle and the crust Crust - 40-0km deep (0 at volcanoes) - continental: thicker, less dense - oceanic: thinner, more dense - together with the uppermost mantle, makes up the lithosphere Earth is an Archive - geologic record archives earths history - results from the interaction of complex systems within the planet - the lithosphere (specifically the sedimentary rocks of the crust) contains the record of life on earth 47BIO211 Sept 12 th Plate Tectonics Earths 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 earths 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 earths surface o Alfred Wegner (1910s) o Alexander du Toit (1920s and 1930s) - Wegners 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 - Wegners (quickly discredited) possibilities: o centrifugal force caused by earths 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 (1960s) o Geopoetry: seafloor spreading; continents dont 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 earths 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 o oceanic crust is formed o bends away from center to form ridge Plate Motion - why plates move: o convection cells in asthenosphere create drag on plate o elevation at ridge pushes plates away - can be measured using fixed points such as hot spots o hot spot: small stationary area with intense heating within crust, where a thermal plume rises from mantle area is stationary but the plates above it move o ex. Hawaiian hot spot: thermal plume creates volcano plate moves away from plume stranded volcanoes cool, leaving an island chain chain indicated direction and rate of plate motion - GPS can also measure plate motion: o Earth-orbiting satellites identity motion and transmit back to ground-based receivers o average rate ~5cm/yr - passive margins: tectonically inactive areas that accumulate sediment (ex. eastern South America) - active margins: zones of tectonic deformation and igneous activity (usually subduction zones ex. western South America) - divergent plate boundaries plates diverge at spreading zones
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