Last year’s Deep Water Horizon well failure resulted in hundreds of thousands of gallons of oil being released into the Gulf of Mexico. Although oil was captured relatively close to the well head and some was dispersed through chemical and wave action, sufficient masses of oil remained relatively intact to form asphaltic ( commonly called tar) masses which floated to shore along with the oil.
Asphalt is a natural substance that forms when oils are exposed to evaporation and lose their volatile components. What remains is a sticky tar-like material that can collect as lakes (as in Venezuela) as tar pits (the La Brea tar pits of Los Angeles) and in many less well know locations throughout the world. These materials were known and used in Roman times, and long before, for waterproofing clay pots and for making sticky burning missiles that might be catapulted to enemy ships.
Although minor amounts of asphalt are still being mined, most is produced as a by-product of petroleum refining and sold to surface highways, as asphalt shingles on houses and to waterproof composite roofs. With the present rise in oil prices, the oil contained in the Alberta tar sands has become increasingly attractive as a domestic source of oil, although at a high cost in water consumption and potentially damaging pollution.
The longer asphalt is exposed to air the more of its contained lighter hydrocarbon components are evaporated with the result that it progresses from being fluid, to a viscous liquid, to a solid and to ultimately a hard solid that must be heated to soften it enough to use. It is this hard, black product that breaks like thick glass, that is used for commercial roofing asphalt.
Depending on its age, beach asphalt can have the consistency of goo to being rock-hard. Just as road asphalt becomes sticky on a very hot day, heat will also soften it. Gulf Coast beaches have dark materials on them that are derived from many sources and not all of them are related to escaped-oil-asphalt.
After a year of exposure the Horizon asphalt is now still flexible, you can typically break it with the hands and is being found in now-elongate and irregular masses that may be up to 1-foot long. Initially many of these were more pancake- like flat sheets, but wave action has usually broken these into smaller fragments. These very often have sand, bits of shell or stuck-on vegetation on their surfaces, although when broken they will show their solid black interiors.
Larger masses contain more of the volatile hydrocarbons. The motion of movement up and down a beach front tends to produce elongate rounded rods, provided the material is strong enough. There is some tendency to form these shapes, but most commonly the “tar balls” are irregular polygonal forms with rounded edges. Most will apparently ultimately form smooth, very-well-rounded flattened irregular masses. Fewer that are caught in tidal flow channels where the action is more like that of a conventional stream, will become rounded spherical forms.
Oil that was once droplets also evaporates and forms asphaltic grains or sand. Potentially these could re-aggregate if exposed to heat and a little pressure, but I have not observed this yet. These are observed as a black “sand” being deposited on the beaches at the high-tide mark.
Other asphalt found on the Gulf Coast beaches is from highway pavement that was broken up and transported by wave action associated with historic storms. This may be differentiated from oil-well-derived asphalt because the road asphalt will contain large amounts of rock-aggregate particles, whereas the oil-well-asphalt almost always have only sand, shell and vegetation fragments adhering to the outside of the masses.
Tar masses (Left half of photo), road asphalt (Right upper quarter) and iron concretion and shale (Right lower quarter) along with fragments of burned wood (not shown) are all found on Gulf Coast beaches.
Other dark materials that may appear on beaches include burned wood which I saw a lot of on the beach at Mobile, Alabama, dark mineral sands as on the South Carolina and Florida beaches as well as naturally occurring masses of clay and shale which form the bedrock on which the beaches are formed. Some, like the example shown in the photo, are ironstone concretions which may have been formed naturally or from iron derived from a nail or other man-made source. A single one-inch nail can form a concretion that is over an inch in diameter by cementing rock, sand and shell with iron oxides.
Asphalt is a collector, and holder, of heavy metal ions. Once chemically bonded to the organic materials, these will remain so long as the organic material is in solid form. They would be liberated only if the organic material’s bonds were broken by powerful solvents. Concerns have been expressed about the volatile organic compounds released while the asphalt is de-gassing. The majority of this takes place fairly quickly when at least some visible oil is present. As time passes smaller amounts of gas is released as the asphalt continues to harden.
While I do not think anyone would be comfortable on a beach where tons of sticky asphaltic material was still actively de-gassing, a few scattered pieces of beach asphalt are not likely to present any serious health risks from a trip to the beach. While I would not suggest that people eat tar balls, at its present stage in small quantities this beach asphalt presents less risks that passing a paving crew laying down a new stretch of pavement on the Interstate.
Each year we are exposed to millions of pounds of asphalt on the nations highways, on our roofs and under our feet. Widely scattered clumps of oil-well-derived asphalt on a beach are not likely to place the beach goer at personal risk. I would not build my house out of it, but casting artworks or carving Gulf Coast asphalt could be reasonably done provided that the work area was properly ventilated and the off-gassed products were not exposed to ignition sources.