Density of Earth's layers

Crack open a boiled egg and you find three distinct layers — shell, white, yolk — each settled into its place. Earth is built the same way, only on a planetary scale. Slice the planet open and you would find a thin rocky crust, a thick rocky mantle, and a dense metallic core, arranged from lightest to heaviest as you go inward. That arrangement is not an accident of geology; it is a direct consequence of density, the same idea used to compare a marble and a lump of clay. This article uses the density and relative-density formulas from Chapter 9 of CBSE Class 8 Chemistry to explain why the young, molten Earth sorted itself into layers, and why the sorting order runs crust, then mantle, then core, from least to most dense.

The core insight

Because the young Earth was molten and its materials could move freely, denser substances such as iron sank toward the centre while lighter rocky material rose toward the surface, leaving Earth's crust, mantle and core arranged from least to most dense.

The core idea

Density is mass packed into a unit volume:

\text{Density} = \frac{\text{Mass}}{\text{Volume}}

where: Mass is measured in grams (g) or kilograms (kg), and Volume in cubic centimetres (cm³) or cubic metres (m³), so density is usually expressed in g/cm³ or kg/m³. To compare a material's density against a fixed reference, Class 8 Chemistry uses relative density, which compares a substance's density with that of water:

\text{Relative density} = \frac{\text{Density of the substance}}{\text{Density of water at that temperature}}

where: Density of water at that temperature is usually taken as $1\ \text{g/cm}^3$ , so relative density is a pure number with no unit — it simply tells you how many times denser (or lighter) a substance is compared with water. A denser solid sinks below a less dense liquid, and when different materials are mixed and left free to move, the denser ones settle to the bottom. The young Earth, billions of years ago, was largely molten — hot enough that its material behaved like a thick liquid, with particles free to move past one another. In that mobile state, the same settling rule that sorts a shaken jar of sand and pebbles took over on a planetary scale: the densest material sank towards the centre, and the least dense material rose towards the surface. This single idea explains why Earth today has three broad layers, ordered by density from the surface inward.

Crust — the thin, outermost, rocky layer we live on, made mainly of comparatively light minerals such as quartz, mica, and calcite. It has the lowest density of the three layers.
Mantle — a thick middle layer of hot, dense rock lying below the crust, with density greater than the crust's, made of minerals whose particles are more tightly packed.
Core — the innermost and by far the densest layer, believed to be made mostly of metals such as iron, far denser than the crust or mantle.

Two effects push density higher as you go deeper. First, composition: metals like iron are intrinsically denser than rock, so wherever metal settled, density jumped. Second, pressure: the weight of every layer above presses down on the material below, squeezing its particles closer together and raising its density further, on top of whatever composition already provides. Composition explains the big jumps between crust, mantle and core; pressure explains why density keeps climbing gradually within each layer too.

Figure

Cutaway of Earth showing three concentric layers, crust mantle and core, labelled with approximate density in grams per cubic centimetre increasing from the thin outer crust to the dense inner core

Derivation AComparing densities predicts which material sinks

The goal of this derivation is to show, starting purely from the definition of density as mass divided by volume, that comparing two materials occupying equal volumes reduces exactly to comparing their densities, which is the algebraic reason a denser material always settles below a less dense one regardless of the size of the blobs involved, and it is the reasoning that explains how Earth's molten interior sorted itself into layers.

Step 1. Write the density definition for two materials A and B: $\text{Density}_A = \dfrac{m_A}{V}$ and $\text{Density}_B = \dfrac{m_B}{V}$ , where both materials are imagined occupying the same volume $V$ , as would happen if equal-sized blobs of rock and metal were floating side by side in the molten early Earth.

Step 2. Rearrange each definition for mass: $m_A = \text{Density}_A \times V$ and $m_B = \text{Density}_B \times V$ . For the same volume, the mass of each blob depends only on its density — a denser material packs more mass into that identical volume.

Step 3. Compare the two masses by taking their ratio: $\dfrac{m_A}{m_B} = \dfrac{\text{Density}_A \times V}{\text{Density}_B \times V} = \dfrac{\text{Density}_A}{\text{Density}_B}$ . The volume $V$ cancels completely, so the mass ratio equals the density ratio no matter how large or small the equal blobs are.

Step 4. Since gravity pulls harder on whichever blob has more mass for the same volume, and Step 3 shows that this is exactly the blob with the higher density, the denser material always sinks below the less dense one when both are free to move — the outcome depends only on density, never on the size of the sample. Applied to the molten young Earth, this is why iron-rich material sank to form the core while lighter rock rose to form the crust and mantle, regardless of how the material happened to be distributed at the start.

Worked example

A geology kit has two rock pieces of equal volume, 10 cm³ each: one is crust rock with mass 27 g, the other is core-like iron material with mass 79 g. Find the density of each, use relative density to compare them with water (density of water = 1 g/cm³), and state which would sink below the other if both were melted and mixed.

Step 1. Density of the crust rock sample: $\text{Density} = \dfrac{\text{Mass}}{\text{Volume}} = \dfrac{27\ \text{g}}{10\ \text{cm}^3} = 2.7\ \text{g/cm}^3$ .

Step 2. Density of the iron sample: $\text{Density} = \dfrac{79\ \text{g}}{10\ \text{cm}^3} = 7.9\ \text{g/cm}^3$ .

Step 3. Relative density of each, compared with water: $\text{Relative density of crust rock} = \dfrac{2.7}{1} = 2.7$ , and $\text{Relative density of iron sample} = \dfrac{7.9}{1} = 7.9$ .

Step 4. Apply the sinking rule from the derivation: since the two samples have equal volume (10 cm³ each), comparing their densities is enough — 7.9 g/cm³ is nearly three times 2.7 g/cm³, so if both were melted and left free to move, the iron sample would sink well below the crust rock, exactly as metal sank to form Earth's core while rock rose to form its crust and mantle. The iron sample (7.9 g/cm³, relative density 7.9) is denser and would settle below the crust rock (2.7 g/cm³, relative density 2.7).

Competitive stretch

A third sample, mantle-like rock, has mass 33 g in a volume of 10 cm³. Rank all three samples — crust, mantle, core — by density without recomputing crust and iron. Since volume is the same 10 cm³ for all three, density ranking follows mass ranking directly: mantle's density is $33/10 = 3.3\ \text{g/cm}^3$ , which sits between the crust rock's 2.7 g/cm³ and the iron sample's 7.9 g/cm³. Ranking, least to most dense: crust rock (2.7) then mantle rock (3.3) then iron/core sample (7.9) — matching the real order of Earth's layers.

Common traps

Assuming all of Earth's layers are made of the same material and differ only because of pressure — composition (rock versus metal) matters just as much as pressure.
Thinking the crust is the densest layer because it is "solid ground" — the crust is actually the least dense of Earth's major layers; density increases with depth.
Forgetting that pressure alone can increase density even without a change in composition — deeper layers are denser partly because the weight above compresses them.
Mixing up mass and density when comparing layers — it is the core's much higher density, not its size, that determines it settles below the mantle.

Ask the tutor: Want to work through more equal-volume density comparisons, or see how this same settling idea explains layering in other mixtures? Ask the AI tutor to guide you through it.

If pressure increases density, why does the crust not become as dense as the core over time

Pressure does raise density within each layer, but it cannot turn rock into metal — composition sets a ceiling. The crust is made of comparatively light minerals like quartz, mica and calcite, and even under whatever pressure it experiences, it never reaches the density of an iron-rich core. The core is denser mainly because of what it is made of, with pressure adding a further boost on top of that composition. So composition decides the big jumps between layers, while pressure explains smaller density increases within a single layer.

Is the whole Earth still molten like it was when the layers first formed

No. Only parts of Earth's interior are molten today, chiefly parts of the outer core and pockets within the mantle; the crust and much of the mantle are solid rock. The sorting into crust, mantle and core happened long ago, while the young Earth was largely molten and material could move freely enough for density to sort it. Once the outer layers cooled and solidified, that large-scale settling essentially stopped, freezing the layered structure we study today into place.

How do we know the core is denser than the mantle if nobody has drilled that deep

Nobody has physically sampled the core, but density can be inferred indirectly. Scientists know Earth's total mass and total volume, which gives an average density for the whole planet; that average is much higher than the density of surface rocks alone, so something denser must lie deeper inside. Combined with the reasoning in this article — that denser material settles below less dense material when free to move, plus evidence about pressure and composition — this points to a mostly metallic, iron-rich core far denser than the rocky mantle above it.

Density of Earth's layers

The core idea

Worked example

Competitive stretch

Common traps

Before this

Where this leads