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.
Earth is the medium for earthquake waves. Waves that can travel only through a medium are called mechanical waves. Most waves, including water waves, sound waves, and earthquake waves, are mechanical waves. Electromagnetic waves are the most common kind of nonmechanical waves. READING TOOLBOX Compare After you read this section, make a chart comparing transverse and longitudinal waves.