Tar pit

thumb|Tar pit at [[La Brea, Trinidad and Tobago|Tierra La Brea, Trinidad]]

thumb|upright=1.3|An anticlinal trap is feeding the tar pit on the surface through the vertical fracture in the strata (indicated by the red arrow). Once the crude oil reaches the surface, evaporation takes place and lighter hydrocarbons are vaporized, leaving behind sticky asphalt.

Tar pits, sometimes referred to as asphalt pits or tar seeps, are large asphalt deposits. They form in the presence of petroleum, which is created when decayed organic matter is subjected to pressure underground. If this crude oil seeps upward via fractures, conduits, or porous sedimentary rock layers, it may pool up at the surface. The lighter components of the crude oil evaporate into the atmosphere, leaving behind a black, sticky asphalt. Anticlines are folds in stratigraphic layers in which each half of the fold dips away from the crest. Such structures are usually developed above thrust faults or in tectonic regions where the land is bending and folding. If the structure above the concave-down fold (arch) is a non-porous rock or aquitard, such as shale, it is considered an anticlinal trap.

Chemistry

Tar pits are pools of asphalt. However, at the beginning of their formation, they are not necessarily sticky and dense. The pools were composed of crude oil that originated below Earth's surface. Crude oil is a mixture of heteroatom compounds, hydrocarbons, metals, and inorganic compounds. Heteroatom compounds are organic molecules that contain elements that are not carbon or hydrogen, while hydrocarbons contain only carbon and hydrogen. Light hydrocarbons include the following alkanes: methane, ethane, propane, and butane. These molecules have very low molecular weights. Crude oils may also contain some inorganic impurities, such as CO₂, H₂S, N₂, and O₂. Crude oils will evaporate differently depending on their chemical composition. These oil deposits were formed during the Miocene Epoch when marine plankton organisms accumulated in an ocean basin. Over time, sediments buried the organisms 300 to 1000 meters below Earth's surface, subjecting them to high pressures. This process turned the organic matter into oil. The 6th Street Fault that cuts through the Salt Lake Oil Field is the conduit that feeds the La Brea Tar Pits. Springs of tar still ooze to the surface through fractures in the underlying stratigraphic layers of marine shale. A well-preserved horse skull was also found in the Binagadi asphalt lake. It is estimated to be 96–120 thousand years old. It is on display at the Natural-Historical Museum of Azerbaijan in Baku, Azerbaijan.

Pitch lake

The large, bowl-like depression filled with bitumen in Trinidad and Tobago. The lake has a maximum depth of 250 feet with an area of 100 acres, making it the largest deposit of solid bitumen on Earth. The lake is cold and dense near the shores, and it has a top layer that can be walked on. Underneath this skin, the asphalt is continuously churning. The lake gets gradually softer and hotter near the center where the bitumen begins to bubble. The gas being released in the middle of the lake is largely methane and an ample amount of carbon dioxide.

Pitch Lake formed thousands of years ago in the presence of tectonic activity. Movement along a fault created a fracture that tapped into an oil and gas reservoir deep within the crust. The oil and gas seeped upward to Earth's surface through the fracture over time, creating Pitch Lake. Due to this constant replenishment of the oil and gas, the lake has a slight current. The current is largely unnoticed because the top layer of Pitch Lake is mostly solid.  Once animals step into the tar, they become immobilized and begin sinking immediately if the asphalt is warm and sticky enough. Predators that see these helpless animals usually would advance into the tar pits with the hope of catching their next meal. As a result, prey are usually found beneath the predator during excavation projects. Fossils from saber-toothed cats and coyotes were also abundant. Hundreds of new species of bacteria were discovered that have the ability to thrive in environments with little to no water or air. They contain special enzymes that can break down hydrocarbons and other petroleum products. The biomass in Pitch Lake lake was reported up to 10⁷ cells per gram of asphalt. Many of these microbes survive on sulfur, iron, methane, or other hydrocarbons. The La Brea Tar Pits emit about 500 kg of methane per day. This event increased awareness of the potential dangers of methane pockets and hydrocarbon seepage in the area. Samples of Juniperus trees from the Last Glacial Period were recovered from the La Brea Tar Pits, and they revealed that the ratio of intercellular and atmospheric CO₂ was similar between glacial and modern trees. Since the amount of carbon dioxide during the Last Glacial Period was between 180 and 200 ppm (409.8 ppm today), there was less carbon available for photosynthesis. The Juniperus trees had to enhance CO₂ uptake to survive under limiting carbon conditions. It is likely that the trees’ stomatal conductance and chloroplast demand for CO₂ was higher during this period to increase their carbon consumption. She was between the ages 18–24 at death, and she was 4 feet and 8-10 inches tall.

For thousands of years, Native Americans used tar from the La Brea Tar Pits as an adhesive and binding agent.