Inside lava, there are volcanic gases in the form of bubbles, which develop underground inside the magma. When the lava erupts from inside the volcano, it is full of a slush of crystalline minerals (such as olivine). Upon exposure to air the liquid freezes and forms volcanic glass. Different types of lava have different chemical compositions, but most have a high percentage of silicon and oxygen in addition to smaller amounts of elements such as magnesium, calcium and iron.
Internal heat and geological activity - M.teachastronomy.com
Lava-bending is a specialized sub-skill of earth-bending that allows the user to manipulate molten earth. This rare ability allows the bender to phase-change earth into lava, lava into earth, and otherwise manipulate existing lava with great dexterity. The use of earth in a liquid state turns this bending into something closer to water-bending, requiring more fluid movements and allowing a more supple combat style. Due to its own nature, lava can spread and cause more damage when left unattended.
Bolin vs Ghazan - Youtube.com