Internal Structure of the Earth : geology's most wildly speculative topic.
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v 0015 of 'Internal Structure of the Earth' by Greg Pouch
at 2011-03-25 13:48:14 LastSavedBeforeThis 2011-03-25 13:48:03 18InternalStructure
3 Conclusions Figures
4 How do we know that?
5 Seismic Observations
6 Geophysics: a Quick Introduction
9 Isostasy (table)
10 How did the earth get this way?
This would look similar for gravity, in these cases.
Compositional (Density) Structure
Crustal composition is based on direct observations, mantle is based on seismic velocities, xenoliths, and indirect observations, and the core is based on seismology, indirect observations, and moment of inertia.
This is based on our observation of density structures, laboratory experiments determining mechanical properties of certain rocks at various pressure-temperature conditions, and on estimates of the variation of temperature with depth
The strange density situation is due to partial melting of peridotite. Generally, partial melts are more iron-rich than the source rock: the restite is typically more magnesium-rich. The liquids are less dense than the solids, but as they cool, they contract and the denser, more easily melted iron-rich product becomes denser. When the overlying solid is denser, it tends to sink.
Physics of Isostasy
The block at right is subject to two forces: its weight W and the buoyant force B on its bottom (pressure)
W=A solid T g =Area*weight_density_solid*thickness
B=A liquid d g =Area*weight_density_liquid *depth
When they are equal (W=B), the block is in isostatic equilibrium or
solid T=liquid d or solid/liquid = d/T
If there is a big density contrast, you get lots of relief for given variation in thickness.
If there is a big difference in thickness, you get a big difference in relief
The graph above is an Excel Chart object. Double-clicking it while in edit mode should open it in Excel, and let you see the formulas and values and play with the values.
Crustal columns usually have the same total mass: they float like blocks of wood in the liquid-like mantle. Mountain chains have low-density roots (they're like icebergs). Trenches have complex density structure.