Finding a sticky, black substance on the beach is a common experience. This residue, colloquially known as tar, is a form of petroleum product altered by the marine environment. These dark lumps are not naturally occurring material but are a clear sign of hydrocarbons entering the ocean. Understanding the source and formation of this material explains why it is encountered on shorelines across the globe.
What Exactly Is the Black Material?
The black material found on beaches is not true tar (a byproduct of coal or wood distillation), but is highly weathered crude oil or refined petroleum products. This residue forms when liquid oil loses its more volatile components after floating on the sea surface. The substance is primarily composed of the heavier, less-soluble compounds from oil, such as asphaltenes and resins.
The residue varies in physical form, often appearing as small, coin-sized pellets or larger, flattened masses called tar patties. Weathering creates a material that is often hard and crusty on the outside but may still contain a soft, sticky core. This final product is significantly denser and less toxic than the fresh liquid oil from which it originated, but it remains a persistent pollutant.
The Dual Origin of Ocean Oil
Tar balls wash ashore from two primary sources: natural seepage and human activity. Oil naturally leaks from fractures in the seafloor in areas with active petroleum basins, a continuous process known as geogenic seepage. These natural seeps are found in clusters worldwide, such as off the coast of Southern California and in the Gulf of Mexico, and can release tons of oil daily.
The second major source is anthropogenic, related to human activity, including operational and accidental releases. Chronic small spills from shipping, such as the discharge of oily bilge water and ballast tank residue, contribute a substantial volume of oil to the ocean every year. While major accidents like tanker spills are dramatic, chronic small-scale releases often contribute a greater total volume of oil over time. Oil from natural seeps and human spills look and behave similarly once in the ocean, though seep oil is generally more degraded before it reaches the surface.
From Oil Slick to Sticky Tar Ball
The transformation of liquid oil into a semi-solid tar ball is a complex process known as weathering, involving physical, chemical, and biological changes. Initially, the lighter, more volatile fractions of the oil (similar to gasoline or diesel) rapidly escape into the atmosphere through evaporation. Simultaneously, a small portion of the oil’s components dissolves directly into the surrounding seawater.
Wind and wave action cause the remaining oil to mix vigorously with water, forming a thick, sticky emulsion that often resembles chocolate mousse. As the lighter materials are lost, the remaining residue becomes denser and heavier, making it more prone to sinking or washing up on the beach. Microbes in the water slowly consume some of the hydrocarbons, but this degradation is a slow process, particularly for the heavier, resistant components left behind.
Handling Tar Balls: Safety and Removal
Direct contact with tar balls should be avoided, although brief contact poses a low toxicity risk to the average beachgoer. The main concern is minor skin irritation and the difficulty of removal. If you encounter an unusual number of tar balls or a large oil slick, report the finding to local environmental authorities or the U.S. Coast Guard.
For removal from skin, clothing, or pets, use simple, safe household products rather than harsh chemical solvents like gasoline or paint thinner. Natural oils are highly effective because they dissolve the petroleum compounds. Rubbing mineral oil, baby oil, or a common cooking oil onto the affected area will loosen the sticky residue. After the tar is dissolved, thoroughly wash the area with soap and water to remove the oil and any remaining residue. For clothing, after scraping off excess material, a pre-treatment with dish soap or rubbing alcohol followed by a regular wash cycle is recommended.