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.
In an electromagnetic wave the motion of the electric and magnetic fields is transverse to the direction of propogation of the wave. So the E, B and k vectors are perpendicular to each other. Also unlike a mechanical wave, an electromagnetic wave does not require a medium to travel. It can travel in completely empty space. The matter wave or de Broglie wave comes from the wave particle duality of a quantum mechanical object such as an electron or an atom etc. This wave can also travel in empty space.