Oil is a mixture of many different chemicals. Not only does each of these have its own toxicity, but each also behaves differently in the environment. Some components are much more volatile than others, and so they tend to evaporate more rapidly when oil is spilled. Some are more easily broken down by microbes on the beaches.
Sunlight can also degrade oil components.
In Prince William Sound, our chemists tell us that the remaining Exxon Valdez oil ranges from very weathered to relatively fresh . Oil Composition Graphs Here are graphs showing the chemical composition of the fresh Exxon Valdez crude oil and two different oil residues found at our study sites. One residue sample, which bears little resemblance to the original oil, shows a highly weathered material. The other, still quite different from the cargo oil, nonetheless retains more of the compounds found in the original product and so is considered as less weathered. In general, we have found that the more exposed to the elements the oil is, the more rapidly it weathers .