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What Is Asphalt Rock?

By C.B. Fox
Updated May 23, 2024
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Asphalt rock is formed when layers of petroleum harden in the pores of other rock. Most was formed naturally, millions of years ago, though it is possible for engineers to create asphalt rock artificially. Asphalt rock is made of hardened asphalt, which is also known as tar, the substance formed when petroleum dries out. This type of rock is has been used for roads since ancient times.

Though there are a number of minerals that may be present in asphalt rock, petroleum is the main component. Over time, the heavier components of petroleum settle while the lighter components evaporate away. The material that is left behind ranges in consistency from a thick liquid to a crumbly sand or stone. This heavier substance, when it is a liquid, is known as asphalt. The solid pieces of the petroleum can form into asphalt rock.

Petroleum becomes 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 the asphalt is lodged in. Extracting asphalt from other types of rock can be an expensive and time consuming process.

Asphalt is not a component of all deposits of limestone or sandstone. Miners drill small test holes in known deposits of these types of rock in order to find out whether asphalt rock is present. Most of the asphalt rock that occurs naturally 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 rock, which was used historically but which is not often used in 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 may be added to the rock in order to help it bind better. Crumbled pieces of stone may be used for a road, with liquid asphalt added in order to bind the pieces together.

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