In the internal structure of planet Earth, below the rocky mantle, the central core is mainly composed of which one of the following elements?

Introduction / Context:
Understanding the internal structure of the Earth is a key part of physical geography and earth science. The planet is divided into crust, mantle, and core, each with different composition and properties. This question focuses on the composition of the core, which lies below the mantle and extends to the centre of the Earth, and asks which element dominates in that region.

Given Data / Assumptions:

• The region discussed is the core, lying beneath the mantle.
• We are asked about the main element that makes up this core.
• The options are Aluminium, Chromium, Iron, and Silicon.
• We assume the standard scientific model of the Earth interior based on seismic and density evidence.

Concept / Approach:
The Earth core is extremely dense and has properties best explained by a composition dominated by iron, with a smaller amount of nickel and some light elements. Seismic wave behaviour, density calculations, and comparison with meteorites all support this conclusion. Aluminium and Silicon are abundant in the crust and mantle in the form of silicate minerals, not in the metallic core. Chromium exists in minor amounts but is not a major constituent. Therefore, the most appropriate single element to describe the core composition is iron.

Step-by-Step Solution:
Step 1: Recall the three main layers of the Earth: crust, mantle, and core. Step 2: Understand that the core must be made of very dense materials to account for the overall density and moment of inertia of the planet. Step 3: Seismic studies show a metallic inner core and outer core, indicating that the material is not silicate rock but metal. Step 4: Iron, sometimes together with nickel, is the metal that best fits the observed density and seismic data for the core. Step 5: Aluminium and Silicon are more common in the crust and upper mantle as parts of aluminosilicate minerals, not in the core. Step 6: Chromium is present only in minor quantities and does not dominate the core composition. Step 7: Therefore, iron is the correct answer as the main element that makes up the Earth core.

Verification / Alternative check:
Standard earth science references describe the Earth as having an iron rich core, often using the term iron nickel core. Evidence comes from seismic wave speeds, the behaviour of P waves and S waves, laboratory experiments on iron at high pressures, and the composition of iron meteorites that are thought to resemble core material. All these independent lines of evidence consistently point to iron as the principal element in the core, confirming the answer.

Why Other Options Are Wrong:

• Aluminium: A light metal mainly concentrated in the crust and upper mantle, not dominant in the dense core.
• Chromium: Present in some minerals and alloys but not in sufficient quantity to be the main core component.
• Silicon: Abundant in rocks of the crust and mantle as silicate minerals, not the primary element in the metallic core.

Common Pitfalls:
Students sometimes confuse the composition of the crust, which is rich in Silicon and Aluminium, with the composition of the core. Others may remember the phrase iron nickel core but then become unsure when only single elements are listed in the options. To avoid confusion, keep in mind that the crust and mantle are silicate rock layers dominated by Oxygen, Silicon, and Aluminium, while the core is a dense metallic layer dominated by iron.

Final Answer:
The core beneath the mantle is mainly made up of Iron.