26: A[1] The average density of the rocks making up Earth’s crust is 2.7 – 3.0 g/cm 3 . Is this range greater than, less than, or equal to (includes) the average density of Earth? B[1] What might this (re: question 26) indicate about the interior composition and density of the Earth? C[3] As you’ve learned in Module 2, planetary differentiation results in the formation of compositionally distinct layers, thus the Earth is a layered body. Although it may be a little more complicated, let’s assume that the Earth is composed of three layers: crust, mantle and core. Their average densities are: crust (2.7 – 3.0 g/cm 3 ); mantle (3.4 – 5.6 g/cm 3 ); core (9.9 – 13.1 g/cm 3 ). Now compare these values to the densities you calculated in Table 1. Given this information, what materials do you think these layers of the Earth are most likely composed of? (Look for the densities that fall within these ranges, or are the closest to them)
The Correct Answer and Explanation is :
Answer to 26A:
The average density of the Earth’s crust (2.7 – 3.0 g/cm³) is less than the average density of Earth. The average density of Earth is approximately 5.5 g/cm³. Since the density of the crust is lower than that of the whole Earth, this indicates that the Earth is made up of materials that are denser than the crust in its deeper layers.
Answer to 26B:
The fact that the crust is less dense than the average Earth density suggests that the Earth’s interior must be composed of denser materials in the deeper layers. Specifically, the mantle and core, with densities ranging from 3.4 – 5.6 g/cm³ for the mantle and 9.9 – 13.1 g/cm³ for the core, would account for the higher average density of the planet. The core, being the densest layer, is likely composed of iron and nickel, while the mantle, which is denser than the crust but less dense than the core, might be made up of silicate minerals that contain heavier elements.
Answer to 26C:
Given the average densities of the Earth’s layers (crust, mantle, and core), we can infer the following likely compositions:
- Crust (2.7 – 3.0 g/cm³): The crust is the least dense layer and is primarily composed of silicates, particularly minerals like granite (found in continental crust) and basalt (found in oceanic crust). These rocks are lighter compared to the deeper layers of the Earth.
- Mantle (3.4 – 5.6 g/cm³): The mantle is denser than the crust and is likely composed of peridotite, a dense, iron- and magnesium-rich silicate rock. The mantle also contains minerals like olivine and pyroxene that have a higher density compared to the minerals found in the crust.
- Core (9.9 – 13.1 g/cm³): The core is the densest layer, and it is likely composed of iron (Fe) and nickel (Ni), with small amounts of lighter elements. The inner core is solid, while the outer core is liquid, but both are primarily metallic and very dense.
In conclusion, the differences in density between the crust, mantle, and core reflect the varying compositions of these layers, with denser materials (like iron and nickel) located deeper within the Earth, and lighter materials (like silicates) forming the outer layers. This differentiation has occurred over geological time, with the heavier materials sinking to form the core, and the lighter materials remaining closer to the surface as the crust.