Seismic Waves & Earth's Interior
Distinguish between P-waves (compression, travel through solids and liquids) and S-waves (shear, cannot pass through liquids); explain why a seismic shadow zone exists on the far side of an earthquake; describe how seismologists use wave refraction and reflection to infer that Earth has a solid inner core, liquid outer core, mantle, and crust
The lesson
Every earthquake sends two main kinds of waves racing through the planet: P-waves and S-waves. Seismographs around the world catch both and record exactly when each one arrives.
P-waves push and pull, squeezing rock the way you'd squeeze a slinky. They move through solid rock and through liquid rock, and they travel fastest, so they always arrive first. S-waves shake side to side instead. They can only move through solids. A layer of liquid stops an S-wave completely.
After a big earthquake, stations across the globe listen for the waves. Directly opposite the earthquake, stations pick up P-waves but no S-waves at all. That empty ring is called the shadow zone. Since S-waves can't cross liquid, the shadow zone is proof that a liquid layer sits deep inside Earth: the outer core.
P-waves also bend at the boundaries between layers, the way light bends entering water, and they speed up again once they reach the solid inner core. By timing these bends and the shadow zone together, scientists mapped a solid inner core, a liquid outer core, a thick mantle, and a thin crust, all without drilling anywhere near that deep.
S-waves get blocked by Earth's liquid outer core, and that blocked spot, the shadow zone, is how scientists mapped the planet's hidden layers without ever drilling there.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.