Question

Explain how the core is only one-sixth of Earth's volume, yet it is onethird of Earth's mass.

Short answer

The core is denser and made of heavier elements, contributing to one-third of Earth's mass despite being only one-sixth of its volume.

Step-by-step solution

Understanding Earth's Structure

Earth consists of several layers: the crust, mantle, and core. The core itself is subdivided into the inner and outer core. These layers are not only distinct in their composition but also vary greatly in their state (solid or liquid) and density.

Volume and Mass Definition

Volume refers to the amount of space an object occupies, while mass is the quantity of matter within it. The distribution of mass within Earth's layers depends on the density and volume of each layer.

Relating Density to Mass and Volume

Density is defined as mass per unit volume, expressed mathematically as \( \text{Density} = \frac{\text{Mass}}{\text{Volume}} \). This principle is key when discussing how different layers of Earth contribute more to its volume or mass based on their densities.

Comparing Core and Other Layers

The Earth's core, composed mainly of heavy elements like iron and nickel, is extremely dense compared to the mantle and crust. Although the core occupies about one-sixth (16.7%) of the total volume, its higher density means it contains a larger portion of Earth's mass.

Calculating Mass Contribution

Since the density of the core is much higher—more than twice that of the mantle—the mass contribution of the core is disproportionate to its volume. Specifically, it accounts for about one-third (33%) of the Earth's total mass, which aligns with our calculations based on density differences.

Key concepts

Earth's Structure

Earth is composed of distinct layers, each with its own unique properties. The outermost layer is the crust. It is relatively thin and is divided into continental and oceanic crust. Below the crust lies the mantle, which is much thicker and consists mainly of silicate rocks. Finally, the deepest part of Earth is the core, which is further divided into the inner and outer core. These layers differ not only in composition but also in physical properties, such as density and state of matter. While the crust and mantle are composed mostly of lighter elements like oxygen and silicon, the core predominantly contains heavier elements such as iron and nickel. This structure allows Earth to maintain its dynamics, from the movements of tectonic plates to the generation of its magnetic field.

Density

Density is a key concept when studying Earth's structure. It is the measure of how much mass exists within a specific volume. The formula to determine density is \( \text{Density} = \frac{\text{Mass}}{\text{Volume}} \). In simple terms, it tells us how packed or concentrated the matter is in a given space. This concept is crucial for understanding why different layers of the Earth have varying contributions to its overall mass and volume. For instance, the core of the Earth is denser than the crust. This means it has more mass packed into a smaller volume. As a result, even though the core takes up less space, it holds a significant portion of Earth's total mass.

Volume and Mass Relationship

The relationship between volume and mass stems from their connection through density. To fully understand this relationship, it’s important to comprehend that while volume measures the space occupied, mass measures the amount of matter present. These two are linked by density, as expressed by \( \text{Density} = \frac{\text{Mass}}{\text{Volume}} \). This equation implies that for a given volume, an increase in density leads to an increase in mass. In Earth's case, the core has a small volume relative to the whole planet, yet its high density translates to a large mass. Thus, even as the core constitutes just one-sixth of Earth's volume, its immense density enables it to account for one-third of the total mass.

Inner and Outer Core

The Earth's core is divided into two distinct parts: the inner core and the outer core. The inner core is solid and primarily composed of iron and nickel. One of the unique traits of the inner core is its high temperature and pressure, which results in a dense, compact structure. As a result, the mass of the inner core is considerable despite its volume being relatively small.
The outer core, on the other hand, is in a liquid state due to slightly lower pressure conditions compared to the inner core. This fluidity is crucial for creating Earth's magnetic field through the dynamo effect. Although both inner and outer core share compositional similarities, their physical states differ significantly. These difference contribute to the overall dynamics of the core, impacting not only Earth’s magnetic properties but also its geodynamical behavior.