Asphalt, also known globally as bitumen, is a highly viscous, black, and sticky form of petroleum composed of complex hydrocarbons. It is found in two distinct forms: naturally occurring deposits and as a refined product of crude oil. Understanding where asphalt is found involves tracing its origins from rare geological formations to its mass production as a residue of the modern petroleum industry.
Where Natural Asphalt Occurs
Natural asphalt forms over millions of years through the geological degradation and migration of crude oil. Deeply buried crude oil moves upward through fissures in the Earth’s crust toward the surface. When the oil reaches near-surface environments, lighter, volatile hydrocarbon components evaporate away, leaving behind the heavy, dense, and sticky residue known as bitumen.
The most famous examples are natural asphalt lakes, which are surface accumulations of this heavy residue, such as Pitch Lake in Trinidad. Another major source is found in oil sands deposits, like the massive Athabasca region in Alberta, Canada. Here, the bitumen saturates unconsolidated sandstone and clay, forming a dense oil that requires significant processing to extract.
The Industrial Source of Modern Asphalt
The vast majority of asphalt used is manufactured as a byproduct of oil refining, not mined from natural deposits. This industrial asphalt is sourced from selected crude oils that possess a suitable chemical composition. The refining process begins with the fractional distillation of crude oil, where the oil is heated to separate its various components. Lighter products such as gasoline, kerosene, and diesel fuel vaporize at lower temperatures and are collected higher up in the column.
Asphalt is the heaviest, non-boiling fraction of the crude oil, which is left behind as the residue at the bottom of the distillation column. To further process this heavy residue, it is often subjected to a second stage called vacuum distillation. Introducing a vacuum allows for the separation of the remaining components at lower temperatures, which prevents the molecules from thermally cracking or degrading.
Controlling the pressure and temperature during this vacuum distillation step determines the final physical properties, such as the viscosity and hardness, of the resulting asphalt binder. Different grades of asphalt cement are produced this way, allowing manufacturers to meet specific engineering requirements for various applications. This refining process ensures a consistent and reliable supply of asphalt, which is essential to the construction industry worldwide.
Common Commercial Uses
The general public encounters asphalt primarily as a finished construction material used on transportation surfaces. Its most widespread commercial use is as the binder in asphalt concrete, the composite material used to pave roads, highways, and parking lots. The asphalt cement is heated and mixed with aggregates—crushed stone, sand, and gravel—to create a smooth, durable, and load-bearing surface.
Beyond paving, the material’s water-resistant and adhesive properties make it valuable in various secondary applications. Asphalt is found in the production of roofing shingles, acting as a waterproofing agent and binder. It is also used in liquid form as a sealant and protective coating for foundations and underground structures, and in non-road applications like airport runways and industrial flooring.