The pore spaces (known as vesicles) in pumice are a clue to how it forms. The vesicles are actually gas bubbles that were trapped in the rock during the rapid cooling of a gas-rich frothy magma. The material cools so quickly that atoms in the melt are not able to arrange themselves into a crystalline structure.Thus, pumice is an amorphous volcanic glass known as a  mineraloid .he abundant vesicles in pumice and the thin walls between them give the rock a very low specific gravity. It typically has a specific gravity of less than one, giving the rock an ability to float on water.
While an electromagnetic wave is called just a disturbance, a mechanical wave is considered a periodic disturbance. Mechanical waves are also called elastic waves as their propagation depends on the elastic properties of the medium through which the waves pass.
1. A mechanical wave needs a medium for its propagation. An electromagnetic wave can travel through vacuum and it has the highest speed as it moves through the vacuum. Sound wave is a mechanical wave and light is an electromagnetic wave. 2. A mechanical wave can be both transverse and longitudinal but an electromagnetic wave is a transverse wave. 3. The particles of the medium vibrate, when a mechanical wave passes through it. In an electromagnetic wave the oscillating parameters are the electric and magnetic field.