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Thus far it sounds like a fairly simple proposition to estimate the variation in density of the earth due to local changes in geology. There are, however, several significant complications. The first has to do with the density contrasts measured for various earth materials.
The densities associated with various earth materials are shown below.
| Material | Density (gm/cm^3) |
|---|---|
| Air | ~0 |
| Water | 1 |
| Sediments | 1.7-2.3 |
| Sandstone | 2.0-2.6 |
| Shale | 2.0-2.7 |
| Limestone | 2.5-2.8 |
| Granite | 2.5-2.8 |
| Basalts | 2.7-3.1 |
| Metamorphic Rocks | 2.6-3.0 |
Notice that the relative variation in rock density is quite small, ~0.8 gm/cm^3, and there is considerable overlap in the measured densities. Hence, a knowledge of rock density alone will not be sufficient to determine rock type.
This small variation in rock density also implies that the spatial variations in the observed gravitational acceleration caused by geologic structures will be quite small and thus difficult to detect.
A density unit gaining currency is the tonne per cubic metre, (t/m^3), which is numerically the same as g/cm^3, and conforms to SI conventions. This is also a useful unit in relating "visual" volumes with masses. The formal SI unit is the kilogram per cubic metre.
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