The black goo under your tyres is not tar, and never really was
Roads are glued together with bitumen, a thick residue of petroleum so viscous it can seem solid while slowly flowing over long spans of time. People call it tar, and the word tarmac suggests the same, but tar comes from roasting coal or plants. Even the famous La Brea Tar Pits actually hold natural bitumen.
About 70% of the bitumen produced each year goes into roads, where it binds gravel and other aggregate into asphalt concrete. Most of the rest waterproofs roofs as felt and sealant. The vocabulary is a muddle: Americans call the binder asphalt, Britons call it bitumen, and in Canada bitumen means the vast deposits of very heavy crude, thinned with naphtha into dilbit for pipelines.
Tar once really did hold roads together. In the early and mid-20th century, making town gas from coal left plenty of coal tar, which was mixed into macadam surfaces and gave us tarmac. After natural gas replaced town gas in the 1970s, bitumen took over entirely. The two differ in origin too: tar comes from heating organic matter, largely from plants, while most bitumen formed when buried organic remains were slowly cooked deep underground without oxygen.
Natural deposits can be enormous. The Pitch Lake in southwest Trinidad, the world's largest natural bitumen deposit, is estimated to hold 10 million tons. Alberta holds most of the planet's reserves in oil sands spread over 142,000 square kilometres, larger than England, containing 166 billion barrels of established oil and giving Canada the world's third largest reserves. The Athabasca deposits are about 110 million years old, formed from algae in an ancient sea and pushed into sandy riverbeds as the Rocky Mountains rose.
Humans have used the stuff for a remarkably long time. In the Middle Paleolithic, people shaped it into tool handles or used it to fix stone blades to hafts, and Herodotus reported that bitumen was hauled to Babylon to help build its huge defensive wall. Chemically it is mostly carbon, about 80% by weight, with so many distinct molecules that fully cataloguing them is almost impossible.
Source: Bitumen
