EPS 102 lecture 11

EPS 102 lecture 11 - EPS 102 Lecture 11 Tuesday February...

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EPS 102 Lecture 11 Tuesday February 24 th , 2009 We are going to start the transition now to talking about Planet Earth. We know quite a bit about the structure of Earth. since we live on Earth, we know hundreds or thousands/millions times more details about Earth than any other planet. Our planet has 3 zones to it: a metallic zone or the core surrounded by a rocky zone or the mantle + crust, and then the outer fluid envelope or the hydrosphere= atmosphere + oceans. There is water in the atmosphere and oceans. The hydrosphere has a fuzzy boundary. Shouldn't we include rivers, lakes, groundwater? Boundary between hydrosphere and the solid earth is somewhat diffuse. The crust acts like a sponging material that lets water through by pore spaces. How much water is inside the Earth? The rocky part of the Earth/ mantle might have abundances of water or Hydrogen of the order of a fraction of a % or 1-2%; if that is the case that there is that much water in the mantle, then that means that most of the water is in the mantle and not the oceans anymore. Trace amounts of water inside the large solid earth could be a whole lot. The radius of the Earth is under 6400 km; the core mantle boundary is roughly halfway down; the core itself comes in an outer thick fluid shell part, and the interface between the solid and inner core is another halfway down. because the radial distance is halfway down, the volume is reduced by 10%. How do we know about the interior? Most of the information comes from seismology, but other lines of evidence include: for the shallow part of the Earth, it comes from rock samples that come directly from the mantle brought up through volcanic processes. Xenoliths are the foreign rocks that come up; basaltic lavas can bring up rocks. How do we know that this rock comes from the mantle? It turns out that there is a subset of these xenoliths that are brought up in violent explosions. In particular, the kind of volcanic eruption that we have not seen in modern times that brings up a material that is uniquely signature to the mantle. This mineral is diamond. Diamond is a high pressure form of carbon that is brought up in a small subset of the xenotliths that come to the surface. We can go into the laboratory and start with graphite, squeeze it with high pressures to make diamond. Higher pressures cause graphite to convert to diamond. Similarly, diamonds taken to lower temperatures and pressures converts it to graphite. We can take this laboratory diamond, and relate it to pressure and depth. At depth, the rock flows on geological time scales so that pressures in the mantle are equalized over time. Going deeper, the change in pressure = density*gravitational acceleration* change in height or distance. Density changes a little bit with depth, and 'g' can be approximated as constant in the mantle as well. Distances down in the Earth are related to pressure changes. Density is going to be about 2- 3*10^3kg/m^3. And 'g' = 10 m/s^2. This gives you a means of relating changes in
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EPS 102 lecture 11 - EPS 102 Lecture 11 Tuesday February...

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