EESA05 LEC 2.docx

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Environmental Science
Mandy Meriano

THE TECTONIC CYCLE Bent and folded rock layers in the Himalayan mountains were distorted by the collision of two tectonic plates Movements of the earths tectonic plates create more of the major geologic features of this planet, including earthquakes, volcanoes, mountain chains etc and play a major role in driving climate, ocean and atmospheric circulation and evolution INTERNAL ENERGY Large impacts early in earths history were sufficient to melt the planet In the molten earth, density fractionation drew molten iron into the centre of the earth and the ices leaked towards the surface, resulting in the oceans and the atmosphere Gravitational setting of iron released more heat melting the rocky portions of the crust, resulting in the fractionation of the crust and mantle The result was an earth with an iron core (which sank to the middle because it was heavy), rocky crust and mantle, and a thick atmosphere and large ocean EARTHS STRUCTURE Earth can be divided into several layers, w mantle under a lot of pressure and very hot, some places more fluid then others plasticity allows the tectonic plates to move around lithosphere is the rigid layer on top of that is the crust which comprises of the oceanic and the continental moho boundaries: between the lithosphere and the upper mantle .. LITHOSPHEREIC PLATES The lithosphere is not a continuous layer, but is broken up into plates It is creates in some places and destroyed in others The rock material of the plates is constantly recycled as part of tectonic cycle Transform/passive plate margin: when two plates pass by each other; no new material being formed THE ROLE OF HEAT Heat is generated deep within the earth by the decay of radioactive elements Heat is released by radiation and by convection (which is more efficient) Convection of the mantle and the asthenosphere drives the tectonic cycle Convection: mass transfer of energy Convection of heat is driving the moving of the tectonic plates The collision of lithosphere plates causes most of the major features and activity on the surface of the earth Ability of water to hold on to latent heat is really high and for it to evaporate, you have to add a lot of energy This heat doesn’t disappear but its in the vapour phase in the atmosphere and some point it will turn to liquid and come back as rain/snow This change causes the energy to be released again (eg. When it snows it gets warmer) MOTIONS OF THE PLATES Lithospheric plates are still moving relative to one another- the atlantic ocean is growing wider by about 5 cm per year ; about the speed at which your fingernails grow PLATE BOUNDARIES Convection is ultimately responsible for the tectonic cycle, as it provides the forces by which lithospheric plates move horizontally These motions are accommodated by subduction If the plates are the same density: they converge and create mountains Divergent margin: when the plates move away from each other Transform margins DEVELOPMENT OF PLATE TECTONIC THEORY Similarities between the coastlines ofAfrica and southAmerica have been noted, and numerous hypothesis for these have been offered Most of these proposed that Africa and south America were joined and somehow became separated Little evidence for this though Thought that when the earth formed, it started to cool and expand, and this caused the continents to break apart Alfred Wegener proposed that the continents were once joined into a single large landmass called ‘pangaea’and one big ocean called ‘tethys’ Wegner proposed that continents has split apart and moved slowly to their present day position by a process termed continental drift He provided more evidence compared to before CONTINENTAL DRIFT Wegner saw similarities between rocks and fossils on either side of the atlantic ocean at corresponding points of similar coasts The plant fossils occur throughout the landmasses of Gondwana (currently southAmerica, African, india,Australia, and antartica) These similarities were most easily explained if these land masses were joined together Evidence was also found in the pattern of ancient glacial deposits This was most easily explained by a single ice cap covering a land mass lying across the south pole made up of the conjoined continents big difference between sea ice(just float on water) and land ice in the absence of continental drift, there would have had to have been several ice caps on different continents, ranging from polar to tropical latitudes what about the other continents? (Asian, northAmerica..) They were another landmass called Laurentia CONTINENTAL DRIFTAND PLATE TECTONICS Wegners ideas were rejected principally because no driving mechanism could be found If the continents were moving around, then the ocean floors around the continents should be in a constant state of turmoil Wegner was never able to reconcile the forced necessary to drive plate motions Understanding possible driving mechanisms required knowledge of the earths interior that did not exist in his day Plate tectonic combines wegners ideas of continental drift with driving mechanisms that are powered by heat produced within the earth by radioactive decay PLATE TECTONIS THEORY Heat is constantly generated within the earth which creates convection cells within the mantle Upwelling magma (where the convection cells are rising) thins the overlying crust, eventually melting through, creating long chains of volcanoes where new oceanic crust is created The lithosphere plates on either side of the rift zone move apart and new crust is created An equivalent amount of crust must be destroyed in subduction zones, where oceanic crusts descends into the mantle There is more sediment away from the mid ocean ridges which comes from the continents Closer to the continents means farther from the mid ocean ridge DIVERGENT MARGINS Where the plated are separating, magma wells up to the surface to create new oceanic crust- a process called seafloor spreading EVIDENCE FOR SE
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