Which element is the most abundant in Earth's crust?

• A. Oxygen
• B. Silicon
• C. Aluminum
• D. Iron

Answer: (A)

The main idea is that Earth's crust is dominated by silicate minerals, so oxygen ends up as the most abundant element there. Most crust rocks are made of silicates and oxides, and oxygen is a major part of their chemical formulas (for example, in SiO4 units and various oxide groups), which tiny atoms pack into a huge amount of mineral mass. That combination makes oxygen account for roughly half of crust material by mass, far more than any other element. Silicon is the next most abundant because it forms the backbone of silicate minerals, but aluminum and iron occur in smaller amounts in the crust. (Iron is actually a larger part of the mantle and core than of the crust.)