Tar sands, sometimes referred to as "oil sands," are layers of sedimentary rocks or sediments that hold vast reserves of petroleum in the form of bitumen. Also called asphalt, bitumen is a very thick liquid or a sticky black solid that contains a complex mixture of crude oil and other petrochemicals along with small amounts of various metals and other inorganic substances. It typically accounts for about five to 15 percent of tar sands deposits, which consist mostly of rock, sand, clay and water.
The largest of the world's numerous tar sands deposits are found in Alberta, Canada and in Venezuela. The biggest in the United States are in eastern Utah. The total volume of oil the tar sands worldwide may exceed the reserves of oil in conventional deposits. The Alberta tar sands mining, by far the largest, extends over an area greater than England and has become one of the world's biggest industrial projects.
Because it is a solid or near-solid, bitumen must be mined directly using open-pit mining and then transferred to an extraction plant where it can be separated from the sand, clay and water. If the bitumen is too deep for direct mining, it can be extracted by injecting hot steam and/or solvents to to allow it to flow through a well to the surface.
There is a tremendous environmental cost of extracting petroleum from tar sands. Not only is this cost in the form of greenhouse gases, plastics, and other pollutants emitted by the consumption of the oil they produce, but it is also from the process of extracting the oil from the tar sands. Such extraction, particularly when open pit mining is used, destroys vast areas of land, mostly boreal forests and peat bogs in the case of Canada. It also consumes far greater amounts of water and energy compared to the extraction of oil from other sources. Moreover, it produces a vast amount of highly toxic waste, including contaminated sand and clays that can easily pollute water supplies over a wide area.