The density of the earth's crust varies from

Analyze the question: It asks for the range of density of the earth's crust, the outermost solid layer of the earth.

Explain the concept: The earth has three main layers, and their densities increase with depth:

• Crust: the outermost layer, made of lighter rocks. Continental crust is mostly granite (the sial layer) at about 2.7 g/cm³. Oceanic crust is mostly basalt (the sima layer) at about 3.0 to 3.3 g/cm³.
• Mantle: denser, about 3.4 to 5.5 g/cm³.
• Core: the densest part, about 10 to 13 g/cm³ or more.

Solution:

1. Crust densities run from about 2.5 g/cm³ (lighter surface and sedimentary rocks) up to about 3.3 g/cm³ (dense basalt of the ocean floor).
2. Option A (17.0 to 19.5) is far too dense. It is above even the core's density.
3. Option B (8.0 to 10.9) and Option C (4.5 to 7.0) match deeper layers (mantle or core), not the crust.
4. Option D (2.5 to 3.3 g/cm³) matches the known density range of crustal rocks.

Answer: D. 2.5 g/cm³ to 3.3 g/cm³

Tip: Remember that density increases with depth: crust is about 2.5 to 3.3, mantle is about 3.4 to 5.5, and core is about 10 and above. Any option with high numbers can't be the crust. In the exam, choose the smallest values for the crust.