The salt is reacting only vibration of the base platform, not to the sound itself. The base platform is reacting to the sound.
The platform has a specific resonant frequency...which basically means...the frequency at which it will vibrate when exposed to a pure sinusoidal wave.
Each frequency has harmonics.
As the pitch of the sine wave reaches harmonics of the base resonant frequency, the base vibrates in a different way. It is the evenness of the vibrations that creates the different pattern of salt.
And who says a B.Mus. in Music Production and Engineering is an impractical degree!!!