Asphalt rock forms in some limestone deposits.
Asphalt rock is formed when layers of oil harden in the pores of another rock. Most were formed naturally millions of years ago, although it is possible for engineers to create asphalt rock artificially. Asphalt rock is made from hardened asphalt, also known as tar, the substance formed when oil dries. This type of rock has been used on roads since ancient times.
Road construction is the most common use for asphalt rock.
Although there are a number of minerals that can be present in asphalt rock, petroleum is the main component. Over time, the heavier components of the oil settle while the lighter components evaporate. The material left behind varies in consistency from a thick liquid to a crumbly sand or stone. This heavier substance, when in liquid form, is known as asphalt. The solid chunks of oil can turn into asphalt rock.
At most, asphalt rock contains about 15% asphalt, while the rest is made up of the stone in which it is housed.
Oil turns into asphalt rock as it hardens within the pores of other rocks, especially limestone and sandstone. Only about 5% to 15% of the total composition of asphalt rocks is asphalt, the remainder being the stone in which the asphalt is housed. Extracting asphalt from other types of rock can be an expensive and time-consuming process.
Asphalt is not a component of all limestone or sandstone deposits. Miners drill small test holes in known deposits of these rock types to find out if asphalt rock is present. Most naturally occurring asphalt rock was formed in the Pennsylvania era, about 300 million years ago, or in the Cretaceous period, between 65 and 145 million years ago.
The most common use for asphalt rock is road construction, and humans have used asphalt on roads for hundreds of years. Natural asphalt rock, which has been used historically but is not often used on modern roads, contains a hard stone that forms a tough surface for the road and a material that holds the crushed stone pieces together. Additional liquid asphalt can be added to the rock to help it bond better. Crumbled pieces of stone can be used for a road, with liquid asphalt added to bind the pieces together.