- Geographical Layers
- Formation of Domes
- Composition of Salt Domes
- Gas and Oil and Sulfur
- Storage in Domes
Over time, particles of rock and debris settle on the surface of the earth. The composition of this debris changes over time, forming layers or strata on the Earth’s crust. These rock layers can be related to time. This relation, called the Geologic Time Scale, is subdivided into Eons, Eras, Periods, Epochs, and even more specific divisions.
Formation of Domes
Millions of years ago, the earth was covered by salty oceans and inland seas. When water receded and evaporated, fields of salt remained. The abundance of salt domes in Louisiana and other southern states are a result of the fluctuations of the Gulf of Mexico over thousands of years. These salt fields were covered year after year by rocks and sediment. Salt, being a crystal, is lighter and more buoyant than the heavy rocks above it. Due to this, salt will move upwards, breaking through weakest geographic layers above them. Salt behaves somewhat fluidly underground, pooling together in to huge ‘domes’ which push upwards. As they near the surface, these domes push up the rocks above them creating hills. The “five islands” (Jefferson Island, Avery Island, Weeks Island, Cote Blanche and Belle Isle) in South Louisiana were all created by salt domes. Red River flows over an underground salt dome near the Texas-Oklahoma border, causing it’s high salt content. Domes can be one to five miles across and reach as deep as 10,000 feet.
Composition of Domes
Salt domes are mostly composed of NaCl, known commonly as Halite, sodium chloride, or table salt. Other compounds can also be found within the domes. Anhydrous Calcium Sulfate (CaSO4) is one of these. The word ‘anhydrous’ means without water or un-hydrated. Gypsum, another compound found in the domes, is the same compound as Calcium Sulfate, but this compound is hydrated. Gypsum has two associated water molecules; it is referred to as Calcium Sulfate Dihydrate. Particularly in Louisiana salt domes, the Gypsum and Calcium Sulfate in the ‘cap-rock’ of the dome are converted to elemental sulfur by bacteria.
Gas and Oil and Sulfur
Salt domes are prime places to drill for oil and natural gas. The salt which forms the dome is nearly impenetrable, being composed of compacted crystals. As the apex of the dome pushes upwards it bends the rock strata above and around it, creating empty pockets. These pockets often collect oil and natural gas. Gulf Coast Domes are particularly well known for the natural resources which can be mined near them. The sulfur ‘cap-rock’ of many Louisiana domes also provides a valuable resource.
Read more about Salt Domes here.
Picture from geology.com
Domes can be hollowed easily using water to dissolve the salt crystals. These excavated domes are occasionally used for underground storage. The emptied domes are ideal places to store oil, natural gas, and helium. Occasionally, radioactive or toxic wastes are stored underground in these former domes. The debate continues on the environmental and societal impacts of storing gas or toxic waste in this way. What do you think empty domes should be used for?
To hear more, check out some of these opinions on the topic: