Asphalt is a type of rock made from hardened petroleum that forms in the pores of other rocks. It is mainly used for road construction and was formed naturally millions of years ago. Extracting asphalt from other rocks can be expensive and time-consuming. Natural asphalt contains a hard stone that forms a strong surface for roads and a material that binds the crushed pieces of stone together. Additional liquid asphalt can be added to improve bonding.
Asphalt forms when layers of petroleum harden in the pores of other rocks. Most formed naturally, millions of years ago, although it is possible for engineers to create asphalt rock artificially. Asphalt is made from hardened asphalt, also known as tar, the substance that forms when oil dries. This type of rock has been used for roads since ancient times.
Although there are a number of minerals that may be present in asphalt rock, petroleum is the major component. Over time, the heavier components of oil settle while the lighter components evaporate. The material that is left behind varies in consistency from a thick liquid to a crumbly sand or stone. This heavier substance, when liquid, is known as asphalt. Solid chunks of oil can turn into asphalt rock.
Petroleum becomes asphalt rock as it hardens within the pores of other rocks, especially limestone and sandstone. Only about 5% – 15% of the total composition of asphalt rocks is asphalt, the rest is stone in which the asphalt is deposited. 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 of the naturally occurring asphaltic rock was formed in the Pennsylvanian age, 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 in roads for many hundreds of years. Natural asphalt, which was used historically but is not often used on modern roads, contains both a hard stone that forms a strong surface for the road and a material that binds the crushed pieces of stone 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.
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