The question of whether tar pits are hot is often fueled by popular culture. A “tar pit” is a common term for a natural asphalt or bitumen seep, which is a pool of heavy, sticky petroleum residue. Natural tar pits are not hot; they are typically at the same temperature as the surrounding environment, which is generally cool or ambient. These geological features are not volcanically heated or boiling cauldrons.
The Temperature of Tar Pits
The asphalt in a natural seep maintains a temperature that closely mirrors the air and ground around it. This means that in a typical environment, the material is at ambient temperature, often feeling cool to the touch. The dark color of the asphalt can lead to increased solar absorption on a sunny day, making the surface layer noticeably warmer than the air. However, even when heated by the sun, the temperature of the asphalt remains far below boiling or scorching hot.
The physical state of the asphalt is directly dependent on these ambient temperatures. When the weather is colder, the bitumen becomes stiff, hard, and extremely sticky, making it less fluid and more difficult to move through. Conversely, when the ambient temperature rises, the asphalt softens, becoming more pliable and exhibiting a lower viscosity. This change in consistency is why the warmer seasons historically presented the greatest entrapment hazard for animals.
Geological Formation and Composition
These natural asphalt pools originate from deep subterranean oil reservoirs and are formed through a passive geological process that does not involve heat. Crude oil, which is lighter than the surrounding rock, migrates upward through fissures, faults, and porous rock layers in the Earth’s crust. This upward movement is driven by pressure and buoyancy, not by high temperature.
When the crude oil reaches the surface and is exposed to the atmosphere, a natural process of evaporation begins. The lighter, more volatile components of the oil, such as gasoline, kerosene, and methane, off-gas into the air. What remains is a heavy, thick, and highly viscous residue composed of long-chain hydrocarbons, which is the natural asphalt or bitumen that forms the pit.
Addressing the Common Misconception
The widespread belief that tar pits are hot likely stems from a confusion between natural geological seeps and industrial materials. The asphalt used for paving roads or roofing materials is processed and applied at very high temperatures, often exceeding 150 degrees Celsius, to make it fluid enough to work with. This industrially heated substance is dramatically different from the cool, naturally occurring asphalt found in places like the La Brea Tar Pits.
Another source of confusion may be the visible bubbling that sometimes occurs in the pits. This bubbling is not a sign of boiling or an internal heat source. Instead, it is caused by the slow, continuous release of methane and other light hydrocarbon gases that were dissolved in the crude oil. As these trapped gases escape, they create the visual effect of a slowly churning or bubbling pool.
Temperature’s Role in Fossil Preservation
The cool, ambient temperature of the asphalt is an important factor in the exceptional preservation of the fossils found within the pits. Preservation of organic matter is heavily influenced by the rate of decomposition, which is significantly slowed down in a cool environment. The low temperatures inhibit the biological activity of the bacteria and fungi that typically break down soft tissues and bone collagen.
Once an organism is trapped, its remains are rapidly saturated and sealed by the viscous asphalt, which also acts to exclude oxygen and water. This anoxic, water-resistant, and cool environment creates conditions that are highly unfavorable for decay-causing microbes. Although soft tissues rarely survive, the bones, teeth, and exoskeletons of trapped animals are preserved in remarkable detail for tens of thousands of years.