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Earth's bulk density is about 5.5 grams per cubic centimeter while its surface rocks are near 3. What does that gap show?
ADense material sits in the core
BThe crust is thicker than thought
CThe mantle is mostly molten
DThe oceans add most of the mass
Answer & Solution
Correct answer: A. Dense material sits in the core
1. Bulk density is the mass of the whole planet divided by its whole volume.
2. Surface rock measures about 3 grams per cubic centimeter.
3. The whole planet averages about 5.5 grams per cubic centimeter, which is far higher.
4. An average can only exceed the surface value if denser material lies deeper down.
5. That denser material is the metal core, and the sorting that produced it is called differentiation.
6. Thickening the crust cannot help, because crust is the light material, not the heavy kind.
7. Melting the mantle changes its state but not its composition, so the average would barely move.
8. Water is less dense than rock, so oceans pull the average down rather than up.
_Source: OpenStax Astronomy (CC BY 4.0), section 8.1 The Global Perspective_
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