P-waves are longitudinal, seismic waves.

The idea

P-waves are longitudinal, which means the rock particles oscillate backwards and forwards parallel to the direction of energy transfer, squeezing and stretching the rock into compressions and rarefactions. That is the same motion as a sound wave in air, which is why P-waves are sometimes described as sound waves in rock. Because the material only has to resist being compressed, and both solids and liquids do resist compression, P-waves travel through solid rock and through the liquid outer core. They are the faster of the two body waves, which is why they are called primary waves and why they are always the first arrival on a seismogram. In the mantle their speed is several kilometres per second and it changes with depth.

What earns the marks

• Stating that the oscillations are parallel to the direction of energy transfer.
• Saying that P-waves can travel through both solids and liquids.
• Stating that P-waves travel faster than S-waves and so arrive first.
• Identifying the motion as compressions and rarefactions in the rock.

Common mistakes and how to put them right

• Writing parallel to the wave without saying parallel to the direction of energy transfer or wave travel.

• Swapping the definitions over, so calling P-waves transverse.

• Saying P-waves travel only in solids, when they pass through the liquid outer core as well.

• Describing the rock particles as travelling along with the wave.

Exam tips

• Learn the pair of definitions word for word; the marks are given for parallel and perpendicular.
• Link the letter to the property: P for primary, first to arrive, and it passes through liquids.
• If asked to compare P and S waves, answer in matched pairs rather than as two separate lists.