RockyPlanets_11feb08

RockyPlanets_11feb08 - Heat, Volcanism, Tectonics and...

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Heat, Volcanism, Tectonics and Planetary Surfaces The interior of the Earth is in constant slow motion. Heat is the driving force for this motion. These internal motions can distort the surface of a planet. This effect is called plate tectonics. Planets initially start off with a lot of heat and cool down as time passes and their internal activity slows down. Planets are formed by accumulation of small planetary bodies – accretion results in heating. This heating is proportional to KE (kinetic energy) which in turn is proportional to escape velocity – proprtional to size of planet.
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The consequences are many 1. Difference between rapid accretion and slow accretion. 1. Initial accretion of protoEarth and Final stages of accretion. 1. Difference between Heat and Temperature 1. Latent Heat of Fusion ~ 300 J/g for basalt.
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Temperatures of Small Bodies An important factor that affects the temperature of an object is the distance from the Sun. As one moves away from the Sun the energy gets diluted and it is less effective in heating a surface it strikes. Under ideal “black body” conditions the temperature t of a surface held perpendicular to Sun’s rays is given by: t = tsun*[sqrt(rs/a)] where tsun is temperature of Sun (~ 5700K), rs is the radius of the Sun (~ 6.96x10 5 ), a is distance of the object from the Sun. In reality this is violated for several reasons: Planetary surfaces are spheroidal, planets rotate, atmospheres complicate absroption and emission of radiation.
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How Rocks Melt ? A rock does not melt as soon as its specfied melting temperature is reached like pure ice. Minerals and rocks melt over a range of temperature. This is illustrated in Figure below. This is phase diagram for a very simple approximation to a rock. What does this diagram tell us? 1. The various fields tell us which phase(s) is stable as a function of temperature and composition. 2. Phase: It is a discrete material of homogeneous composition with well defined boundary (mineral, liquid or gas). 3. Mixtures can have any proportion of the end members.
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How Rocks Melt ? A rock does not melt as soon as its specfied melting temperature is reached like pure ice. Minerals and rocks melt over a range of temperature. This is illustrated in Figure below. This is phase diagram for a very simple approximation to a rock.
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RockyPlanets_11feb08 - Heat, Volcanism, Tectonics and...

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