The correct option is They exert pressure leading to stretching and squeezing of the material..
Explanation
Earthquake waves are broadly classified into Body Waves and Surface Waves. Primary waves (P-waves) are the fastest type of Body Wave and are the first to be recorded by seismographs. Their propagation characteristics are defined by the direction of particle vibration relative to the wave's movement.
Option Analysis:
- They create troughs and crests in the material. is incorrect: The creation of crests and troughs is a characteristic of transverse waves, where the displacement of the medium is perpendicular to the direction of propagation. This behavior is observed in Secondary waves (S-waves), not P-waves.
- They vibrate perpendicular to the direction of propagation. is incorrect: Vibration perpendicular to the direction of propagation defines transverse waves. P-waves are longitudinal waves, meaning the particles vibrate parallel to the direction of the wave.
- They exert pressure leading to stretching and squeezing of the material. is correct: P-waves propagate as longitudinal waves. As they move through a medium, particles vibrate back and forth in the same direction as the wave propagation. This action exerts pressure on the material, creating alternating zones of compression (squeezing) and rarefaction (stretching), thereby changing the density of the material.
- They can travel only through solid materials. is incorrect: P-waves are capable of traveling through solid, liquid, and gaseous materials. It is S-waves that are restricted to traveling only through solid materials.
Key Takeaway:
P-waves are longitudinal waves that vibrate parallel to the direction of propagation, causing stretching and squeezing (density changes) of the material. In contrast, S-waves are transverse waves that vibrate perpendicularly, creating distortions in shape.