Earth's Secret Signals

Earth's Secret Signals 🌍

Nobody has ever dug more than a few kilometres into the Earth — yet we know it has a **liquid** core. How? By listening to **seismic waves** from earthquakes. These waves travel right through the planet, and what arrives (and what does not) reveals what is inside.

P-waves ➡️

**P-waves** (primary waves) are **longitudinal**. They are the **fastest** seismic wave, so they **arrive first**. Crucially, P-waves travel through **both solids and liquids** — so they can pass right through the core and reach the far side of the Earth.

S-waves 〰️

**S-waves** (secondary waves) are **transverse**. They are **slower**, so they **arrive second**. The key fact: S-waves travel through **solids only** — they **cannot** pass through a liquid. Remember that, because it is the clue to the whole puzzle.

Match each wave property

Through the liquid core

Which type of seismic wave can travel through liquids, such as the liquid outer core?

• P-waves
• S-waves
• Both equally well
• Neither

Inside the Earth 🧅

The Earth is layered like an onion: • **Crust** — the thin, solid, rocky surface. • **Mantle** — mostly solid rock that can flow very slowly. • **Outer core** — **liquid** iron and nickel. • **Inner core** — **solid** iron and nickel (kept solid by huge pressure).

Order the layers

An interactive activity.

The S-wave shadow 🕵️

Here is the detective work. After an earthquake, S-waves are detected near the quake — but **not** on the **far side** of the Earth. There is an **S-wave shadow zone**. Since S-waves cannot travel through liquids, something in the middle of the Earth must be liquid, blocking them. That something is the **outer core**.

Diagnose the core

An interactive activity.

The signal rules

P-waves are _____ waves and travel through both solids and liquids. S-waves are transverse and travel through _____ only. Because S-waves are not detected on the far side of the Earth, the outer core must be _____.