Q1.
Did you know that our planet, Earth, is composed of several layers, such as crust, upper mantle, lower mantle, outer core, and inner core, each with its particular range of density?
Answer
Yes — and the layers are arranged in a clear order: the density increases as you go from the surface towards the centre. The outermost layer, the crust, is the lightest of them all.
| Layer (from outside inwards) | Density compared with the others |
|---|---|
| Crust | Lightest — least dense |
| Upper mantle | Denser than the crust |
| Lower mantle | Denser still |
| Outer core (liquid) | Much denser |
| Inner core (solid) | Densest of all |
Why it happens: As you go deeper, both the pressure and the temperature rise sharply. The enormous pressure of everything lying above squeezes the material into a smaller volume without changing its mass, and Density = Mass/Volume, so the density goes up. The deeper layers are also made of heavier materials to begin with. Compressing matter like this is exactly the effect described in Section 9.5.3 — only here it is happening on the scale of a planet.
Tip: Notice that temperature and pressure pull in opposite directions here. Heat alone would expand the material and lower its density; the pressure inside the Earth is so great that it wins comfortably.