geology (Fach) / Tectonic plates, Mid ocean ridges and plate cooling model (Lektion)

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• What are the different types of plate boundaries? DIVERGENT  This involves the spreading motion, creating a constructive effect (creating oceanic lithosphere). It creates a ridge or a rift and there is volcanic activity present. CONVERGENT  This causes a subduction motion within the plates and causes a destructive effect - i.e. the oceanic lithosphere is destroyed. The topography created is a trench and volcanic activity is present. TRANSFORM  This causes a lateral sliding. This causes a conservative effect. i.e. lithosphere is neither created nor destroyed. This causes no major effect on topography and no volcanic activity.
• Are there Volcanoes and earthquakes along divergent plate boundaries? Yes
• What is the main difference between the lithosphere and asthenosphere? The lithospehre is rigid, the asthenospshere deforms like a viscous liquid 
• Give a range for the relative rates of present day plate movements 10's to 100's mm/yr 
• What is the name of the plate tectonic force that pushes the slab downwards? Trench pull
• What are mid-ocean ridges? A mid -ocean ridge is a seafloor mountain system formed by plate tectonics. It typically has a depth of -2600 metres and rises about 2km above the deepest portion of an ocean basin.  There are reversed or magnetic anolmalies on the seafloor 
• How are we able to deduct seafloor age? Geomagnetic polarity reversals - i.e. switching of the orientation of the geomagnetic field. This occurs within varying frequenices which take place over 3000-5000 years.  Plate tectonic reconstructions can be deduced from seafloor age maps. 
• What are the rates of spreading ? Fast---> 10-20 cm/yr medium --> 5-10cm/yr slow ---> 1-5cm/yr 
• Draw a slow spreading ridge draw
• draw a fast spreading ridge draw 
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• Describe the slow spreading ridges (i.e. variation in magma budgets) -There is variable architecture and crustal thickness , there is segmented ridges -There is a well developed axial rift valley and rugged topography. -There is prevalent faulting that may be present and may root in brittle ductle transition  -There may be large, long-lived and fault controlled vent fields  -There are peridodite -hosted hydrothermal vent systems and serpentination. 
• Describe fast spreading ridges -These are thin and narrow (1-2km) wide melt lens  -They are smooth topography, homogenous with crustal structure.  -They have a poorly developed axial rift valley  -There is focused venting, small vent fields - Alteration is likely limited to the upper crust 
• What is the moho? It is the boundary between the earth's crust and the mantle. It is defined by the distinct change in velocity in seismological waves as they pass through changing densities of rock. 
• What is a dike? It is a sheet of rock that is formed in a fracture in pre-exisitng rock body. Dikes can be either magmatic or sedimentary in origin 
• What is a peridodite? It is a very dense, coarse grained intrusive rock. it is derived from upper mantle. It is a very common component of the oceanic lithospehre. 
• What is serpentinazation It is the process where by rock is changed with the addition of water ito the crystal structure of the minerals found within the rock. The process involves reduction in heat and pressure. 
• What is a melt lens ? It is a body of rock that is thick in the middle and thin at the edges
• What are the sources of earth's internal heat? - heat remaining from the initial accretion of the earth  -gravitational energy released from the formation of the core - Radiogenic heating within the mantle and the crust 
• What are the main sources of radiogenic heat ? conduction, convection and advection
• How do you determine the heat flow ? q=KAT/D  Where.. K=the determining of the thermal conductivities of the rocks in between  d=  different depths AT  = the difference between 2 or more temperature readings 
• What does the measured heat flow reveal? It reveals important information concerning the nature of the rocks and the processes that affect the lithosphere. 
• When does heat flow decrease? As the newly formed lithosphere moves away from an oceanic ridge. The cooling and shrinking of the lithospehre results in an increase in its density -- due to isotasy it subsides into the athensophere therefore the ocean depth increases away from the ridge -- 2-3km -- ocean ridges -- 5-6km and abysall plains 5--6km 
• What are the two general models of the oceanic lithosphere the plate model -- GDH1 this assumes that the lithospehre is produced at a mid ocean ridge with constant thickness and that temperature at the base of the plate corresponds to its temperature of formation The boundary - layer model (HS) This assumes that the lithospehre does not have constant thickness but thickens and subsides as it cools and moves away from the ridge  The plate model fits data better for older lithosphere (60ma) suggests once lithospehre has cooled to a certain thickness it remians constant until it subducts  However, both models predict the observed linear variation between ocean depth and sqaure root of age of the lithospehre ---> the oceanic lithospehre cools and subsides as it ages away from the ridge.
• What is the age of crust formula? d= 2500+350t1/2 t=(d-2500/350)2
• What is the mean spreading rate ? V= distance / half spreading age distance 
• What is the main flaw of the theoretical models? The main flaw for the theoretical models for thermal evolution is that they are based on the assumption that heat is lost by conduction only, whereas in reality other mechanisms of heat trnasfer might be in operation, such as convection and advection 
• the base of the lithosphere? is an isotherm - 1300 degrees celcius 

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