Bitumen, dense, highly viscous, petroleum-based hydrocarbon that is found in deposits such as oil sands and pitch lakes (natural bitumen) or is obtained as a residue of the distillation of crude oil (refined bitumen). In some areas, particularly in the United States, bitumen is often called asphalt, though that name is almost universally used for the road-paving material made from a mixture of gravel, sand, and other fillers in a bituminous binder. Bitumen is also frequently called tar or pitch—though, properly speaking, tar is a byproduct of the carbonization of coal and pitch is actually obtained from the distillation of coal tar. Bitumen is defined by the U.S. Geological Survey as an extra-heavy oil with an API gravity less than 10° and a viscosity greater than 10,000 centipoise. At the temperatures normally encountered in natural deposits, bitumen will not flow; in order to be moved through a pipe, it must be heated and, in some cases, diluted with a lighter oil. It owes its density and viscosity to its chemical composition—mainly large hydrocarbon molecules known as asphaltenes and resins, which are present in lighter oils but are highly concentrated in bitumen. In addition, bitumen frequently has a high content of metals, such as nickel and vanadium, and nonmetallic inorganic elements, such as nitrogen, oxygen, and sulfur. Depending on the use to which bitumen is put, these elements may be contaminants that have to be removed from the finished product. By far most refined bitumen is used in paving asphalt and roofing tiles, as is a large amount of natural bitumen. However, most of the bitumen extracted from Canada’s oil sands is upgraded into synthetic crude oil and sent to refineries for conversion into a full range of petroleum products, including gasoline.