When an oil spill occurs over a water surface it undergoes a number of chemical and physical changes, collectively termed weathering. Most of the processes, such as evaporation, dispersion, dissolution and sedimentation, lead to the disappearance of oil fro the surface of the sea, weheras others, particularly the formation of water-in-oil emulsion ("mousse") and the accompanying increase in viscosity, promote its persistence. Non-persistent oils, such as petrol and kerosine, dissipate quickly. Persistent oils, such as heavy fuel oils, dissipate slowly and usually require a clean-up response. Ultimately, the marine environment assimilates spilt oil through the long-term process of biodegradation. The rate of spreading and direction depends on the type and amount of oil, prevailing sea and climatic conditions and the time response and resources of emergency services. Extensive oil spills and those that reach coastal lands cause most environmental damage. Both scenarios can be lessened or avoided by responding rapidly to an oil spill in order to contain it.
There are various oil spill techniques: booms and skimmers, in-situ burning, dispersants, shoreline clean-up, bioremediation and disposal. The choice of the most appropriate is crucial and will depend upon the exact circumstances of the incident.