Cracking is a type of conversion process in oil refining, besides combining and modifying. Cracking essentially breaks large hydrocarbon molecules into lighter ones. It was initialized by the high demand of gasoline or other middle distillates in 1913. Until now, cracking has been a crucial process for gasoline production. Cracking is divided into thermal and catalytic cracking. Thermal cracking uses high temperature and pressure only, while catalytic cracking involves catalysts to speed up the process or enable the process to run in lower temperatures.
Downey. M. (2009) Oil 101. Wooden Table Press.