Stress in the Earth8

# Stress in the Earth8 - 12.005 Lecture Notes 8 Assertion...

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12.005 Lecture Notes 8 Assertion: most of the stress tensor in the Earth is close to "lithostatic," τ ij ~ - ρ gd δ ij , where ρ is the average density of the overburden, g is gravitational acceleration, and d is the depth of the point under consideration. Consider the following table of the lithostatic pressure at various points of interest: depth (km) Pa bars ocean ridge crest 2.5 25 MPa 250 abyssal plain 4 40 MPa 400 deep sea trench 12 120 MPa 1.2 kbar base of crust 30 1 GPa 10 kbar transition zone 600 20 GPa 200 kbar Core-mantle bndry 2900 140 GPa 1.4 Mbar Center of Earth 6400 360 GPa 3.6 Mbar Assertion: Typical "tectonic" stresses have magnitudes in the range 0.3 - 300 MPa (3- 3,000 bars). Conclusion: In considering tectonic problems, it is usually useful to subtract some isotropic reference state of stress. There are 2 common choices - the lithostatic stress, which leaves the nonlithostatic stress tensor, and the pressure field, which leaves the deviatoric stress field. But since we need to know the actual stress tensor in order to calculate the pressure, which requires knowing the solution to the problem in advance, it is most common to use the lithostatic stress field as the reference. In order to place these comments in context, let's look at the situation described in T&S

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Stress in the Earth8 - 12.005 Lecture Notes 8 Assertion...

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