Pennsylvania Esthetician State Board Practice Exam

Oxygen is the most abundant element in the Earth's crust, comprising approximately 46% of its weight. This high abundance is primarily due to the formation of silicate minerals, which are made up of silicon and oxygen. Oxygen combines easily with other elements, forming various compounds such as silicates, oxides, and carbonates.

In the geological composition of the Earth, silicates, which contan both silicon and oxygen, account for a significant portion of crustal materials. Therefore, while silicon is also abundant and plays a crucial role in the crust's composition, oxygen remains the primary element.

Additionally, while carbon and iron are important elements in the Earth's crust, their concentrations are much lower compared to oxygen and silicon. Carbon is primarily found in organic materials and certain minerals, and iron is commonly found in various ores but does not approach the abundance of oxygen in the crust. Thus, the selection of oxygen as the answer reflects its dominant presence in the crust's composition.