The major portion of Earth crust is mainly composed of which two chemical elements by mass?

Introduction / Context:
Geology and general science often require basic knowledge about the composition of Earth crust. The crust is the outermost solid layer and is mainly made up of a limited set of chemical elements. Knowing which elements dominate the crust by mass helps in understanding rock types, minerals and the behaviour of the lithosphere. This question asks for the pair of elements that together make up the largest portion of the crust.

Given Data / Assumptions:

- The focus is on the composition of the Earth crust by mass.
- Options list different pairs of elements: oxygen and iron, oxygen and silicon, silicon and iron, silicon and aluminium.
- We assume standard approximate composition as reported in basic geology texts.
- Only one pair corresponds to the two most abundant elements in the crust.

Concept / Approach:
Earth crust is mostly composed of silicate minerals, which are combinations of oxygen and silicon with other elements. Oxygen is the most abundant element by mass in the crust, and silicon is the second most abundant. These two together form a large percentage of the total crustal mass. Other elements such as aluminium, iron, calcium, sodium and potassium are present in smaller amounts. Therefore, when asked which two elements dominate the crust, the correct pair is oxygen and silicon.

Step-by-Step Solution:
Step 1: Recall that silicate minerals such as quartz and feldspar dominate the crust.Step 2: Silicate minerals are built from silicon and oxygen atoms arranged in various structures.Step 3: Oxygen is the most abundant element in the crust by mass, followed by silicon.Step 4: Compare this information with the options given in the question.Step 5: The only pair that lists these two elements together is oxygen and silicon, so that is the correct answer.

Verification / Alternative check:
If you recall standard tables of crust composition, you might remember approximate percentage values where oxygen alone accounts for close to half of the crust by mass, and silicon adds another significant fraction. When you add these two percentages, they clearly surpass combinations involving iron or aluminium. This quick memory check confirms that oxygen and silicon are the most abundant pair among the options.

Why Other Options Are Wrong:
Oxygen and iron: Iron is important, especially in Earth core, but in the crust it is not as abundant as silicon. This pair does not represent the two leading crustal elements by mass.
Silicon and iron: This pair omits oxygen, which is the single most abundant crustal element, so it cannot be correct.
Silicon and aluminium: Aluminium is significant in many minerals but is not as abundant as oxygen. The pair fails to include oxygen and therefore does not represent the main crustal composition.

Common Pitfalls:
Students may associate iron strongly with Earth because of its role in the core and in common metals, leading them to overestimate its abundance in the crust. Another confusion arises between the composition of the whole Earth and that of the crust alone. To avoid these mistakes, focus on the fact that crustal rocks are dominantly silicate minerals, and the term silicate directly reminds you of silicon and oxygen as the primary elements.

Final Answer:
The major portion of Earth crust is mainly composed of Oxygen and Silicon.