Composition of Earth

Crust (Continental + Oceanic)

The Earth’s crust is dominated by silicate minerals. Elements occur mostly in oxides, silicates, carbonates, and sulfates.

Major elements (by weight %):

• Oxygen (O): ~46.6%
• Silicon (Si): ~27.7%
• Aluminium (Al): ~8.1%
• Iron (Fe): ~5.0%
• Calcium (Ca): ~3.6%
• Sodium (Na): ~2.8%
• Potassium (K): ~2.6%
• Magnesium (Mg): ~2.1%

Mantle

Dominated by ultramafic rocks (peridotite) with silicate minerals like olivine and pyroxene.

Major elements (by weight %):

• Oxygen (O): ~44%
• Magnesium (Mg): ~23%
• Silicon (Si): ~21%
• Iron (Fe): ~6%
• Minor: Al, Ca, Na

Core

Made mainly of Fe–Ni alloy with light elements (S, O).

Major elements:

• Iron (Fe): ~85%
• Nickel (Ni): ~5–6%
• Sulfur (S), Oxygen (O): minor % (help explain lower density of core than pure Fe–Ni).

Atmosphere

The atmosphere is dominated by gases.

Major elements (by volume):

• Nitrogen (N): ~78%
• Oxygen (O): ~21%
• Argon (Ar): ~0.93%
• Carbon dioxide (CO₂, C + O): ~0.04%
• Minor gases: Ne, He, CH₄, Kr, H₂, O₃

Hydrosphere (Oceans)

The oceans are dominated by H₂O and dissolved salts.

Major elements (by weight % of seawater):

• Oxygen (O): ~85.8%
• Hydrogen (H): ~10.8%
• Chlorine (Cl): ~1.9%
• Sodium (Na): ~1.05%
• Magnesium (Mg): ~0.14%
• Sulfur (S, as SO₄²⁻): ~0.09%
• Calcium (Ca): ~0.04%
• Potassium (K): ~0.04%