Question

90% of the bulk Earth is constituted of Fe, Si, O and:

• A. Al
• B. Ca
• C. Mg
• D. Na

Hint:

Remember the acronym "Fe–O–Si–Mg" for bulk Earth composition. Al, Ca, Na are significant in the crust, but not in the whole Earth.

Answer 1

The Correct Option is C

Step 1: Composition of the bulk Earth.
Geochemical studies (including meteorite composition and seismic density models) indicate that the Earth is made primarily of four major elements: - Iron (Fe) - Oxygen (O) - Silicon (Si) - Magnesium (Mg) Together these four account for $\sim$90% of Earth's mass.

Step 2: Why Mg?
- In the mantle, the dominant minerals are olivine (Mg,Fe)$_2$SiO$_4$ and pyroxenes (Mg,Fe)SiO$_3$, showing the dominance of Mg. - The crust contains Al and Ca in feldspars, but their abundance in the whole Earth is much smaller compared to Mg. - Na is a trace constituent, not a bulk component.

Step 3: Supporting evidence.
The average bulk Earth composition (by weight): - Fe $\approx 35%$ - O $\approx 30%$ - Si $\approx 15%$ - Mg $\approx 14%$ Other elements (Al, Ca, Na, K, etc.) are present in much smaller amounts.

Final Answer:
\[ \boxed{\text{Mg}} \]