The oceanic crust of the Earth mainly consists of silica and which other major component, often summarised together as sima in basic geology?

Introduction / Context:
Earth science and basic chemistry often overlap when we discuss the composition of the Earths crust. The crust is divided into continental and oceanic types, each with characteristic chemical makeup. This question asks about the main components of oceanic crust, focusing on the common shorthand terms used in school level geology, such as sial and sima. Understanding these terms helps students connect elemental chemistry with large scale geological structures.

Given Data / Assumptions:

• The crust of the Earth is divided into continental crust and oceanic crust.
• Continental crust is often described as rich in silica and aluminium (sial).
• Oceanic crust is often described as rich in silica and another major element (sima).
• The question asks specifically for the second major component of oceanic crust along with silica.

Concept / Approach:
In school level geology, the terms sial and sima are used to summarise the composition of different parts of the crust. Sial refers to silica (SiO2) and aluminium rich rocks that dominate continental crust. Sima refers to silica and magnesium rich rocks that dominate oceanic crust. The oceanic crust is mainly composed of basaltic rocks, which contain significant amounts of magnesium and iron silicates. However, in the traditional shorthand, the emphasis is on silica and magnesium, hence the term sima (silica plus magnesium). Therefore the correct answer is magnesium.

Step-by-Step Solution:

Verification / Alternative check:
Basalt, which is the dominant rock type forming the oceanic crust, contains minerals such as pyroxenes and olivine that are rich in magnesium and iron silicates. While iron is also important, the traditional classification emphasises magnesium when defining sima. Textbooks typically state that the oceanic crust is mainly made of silica and magnesium rich rocks, confirming that magnesium is the intended answer. Thus, the combination of silica and magnesium correctly summarises the average chemistry of the oceanic crust at basic study level.

Why Other Options Are Wrong:

• Iron: Although oceanic crust does contain iron bearing minerals, the standard sima term is derived from silica and magnesium, not silica and iron.
• Manganese: Present only in minor amounts and not used as a defining component of crustal composition.
• Sulphur: Mostly associated with volcanic gases and sulphide minerals, not a primary component of crustal rocks.
• Calcium: Found in certain minerals such as plagioclase feldspar but not the main element used in the sima classification.

Common Pitfalls:
Students sometimes confuse the roles of magnesium and iron in basaltic rocks and may think that iron should be the second main component. Another common error is mixing up sial and sima, forgetting which one is associated with continental crust and which with oceanic crust. A simple memory aid is that sial has al for aluminium and sima has ma for magnesium. Keeping this relationship clear helps in answering similar questions correctly.

Final Answer:
The oceanic crust mainly consists of silica and magnesium, commonly summarised together as sima.