In basic earth science and world geography, which one of the following elements is present in the largest amount, in terms of percent by mass, in the Earth crust?

Introduction / Context:
Composition of the Earth crust is a standard topic in physical geography and basic earth science. Many competitive exams ask which element or oxide is most abundant by mass in the crust. This question focuses on the major rock forming elements and tests whether you remember which one dominates by mass in the outer solid shell of our planet.

Given Data / Assumptions:

• We are talking specifically about the Earth crust, not the whole Earth.
• The comparison is by percent of mass, not by volume or by number of atoms.
• The options given are Silicon, Oxygen, Carbon, and Calcium.
• Standard average crustal composition from geology is assumed.

Concept / Approach:
Rocks in the continental and oceanic crust are made up mainly of silicate minerals. These minerals are combinations of Oxygen with Silicon, Aluminum, and other metals. Because Oxygen atoms are relatively light but very numerous in these minerals, Oxygen contributes the highest percentage by mass in the Earth crust. Silicon is also abundant but ranks second by mass. Carbon and Calcium are important but occur in much smaller proportions compared with Oxygen and Silicon.

Step-by-Step Solution:
Step 1: Recall that the crust is dominated by silicate minerals such as quartz, feldspars, micas, and amphiboles. Step 2: Note that all silicate minerals contain large numbers of Oxygen atoms bonded to Silicon and other cations. Step 3: Standard tables of average crustal composition show Oxygen at roughly about forty five percent by mass, making it the single most abundant element in the crust. Step 4: Silicon usually appears next, with an approximate mass percentage of around twenty five to thirty percent, so Silicon is abundant but not the largest. Step 5: Carbon is present in carbonates and organic matter but forms only a very small fraction of the crust by mass. Step 6: Calcium is important in plagioclase feldspars and limestones but still contributes far less mass than Oxygen overall. Step 7: Therefore, the correct answer is Oxygen as the element present in the largest amount by mass in the Earth crust.

Verification / Alternative check:
Geology and geography textbooks consistently list the major elements of the crust in descending order by mass as Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, and Magnesium. Oxygen is therefore clearly ranked first. This pattern is linked to the dominance of oxides and silicates in crustal rocks. Cross checking multiple sources confirms that Oxygen holds the top position by mass, not Silicon, Carbon, or Calcium.

Why Other Options Are Wrong:

• Silicon: It is the second most abundant element in the crust by mass, but it does not exceed Oxygen.
• Carbon: Present mainly in carbonates and organic materials, but its overall mass fraction is very small compared with Oxygen and Silicon.
• Calcium: An important component of some minerals and limestones, yet its share of crustal mass is far below that of Oxygen.

Common Pitfalls:
Students sometimes assume that because rocks are called silicates, Silicon must be the most abundant element. This is a misunderstanding of chemical composition. Silicate minerals are built from silicate tetrahedra that contain one Silicon atom and four Oxygen atoms, so Oxygen actually dominates by mass. To avoid mistakes, remember that in the Earth crust, Oxygen is first and Silicon is second in abundance by mass.

Final Answer:
The element present in the largest amount by mass in the Earth crust is Oxygen.