Salt domes
Salt domes are geological formations consisting of massive rock salt that rise toward the Earth's surface from depths of up to 6,000 meters, often taking on cylindrical or mushroom-like shapes. Typically ranging from 1 to 3 kilometers in diameter, some can reach up to 12 kilometers. These formations are usually capped by layers of less soluble minerals, such as limestone, gypsum, and anhydrite. Salt domes are primarily formed in shallow, hypersaline marine environments, where the salt deposits can reach thicknesses from one meter to several hundred meters.
As salt beds are buried under sediment, they can deform and rise through overlying rock layers, creating distinct dome shapes. This process can lead to the formation of valuable natural resources; for instance, the caprock may contain commercial quantities of sulfur that can be extracted using the Frasch process. Additionally, salt domes are significant for hydrocarbon storage, as their impermeable nature traps oil and gas migrating through upturned sedimentary beds. They also provide potential for underground storage of materials, including nuclear waste, due to their self-healing properties when fractured. Despite accounting for only 5 percent of the world’s salt reserves, salt domes have the capacity to meet global demand for thousands of years.
On this Page
Salt domes
Salt domes are a major source of the world’s salt. Their caprocks are major sources of gypsum and sulfur. Upturned sediments on the flanks of salt domes form stratigraphic traps for oil and natural gas.
Definition
Salt domes consist of roughly cylindrical to mushroom-shaped plugs of massive rock salt extending toward the earth’s surface from depths as great as 6,000 meters. These salt pillars typically range in diameter from 1 to 3 kilometers; however, some cores reach 12 kilometers in diameter. The plug is usually topped by a limestone, gypsum, and anhydrite caprock.
Overview
Salt beds, ranging in thickness from a meter to a few hundred meters, are deposited in shallow, hypersaline, marine environments such as basins of restricted circulation in regions where evaporation exceeds precipitation. The salt is commonly pure white and is associated with gypsum, anhydrite, and shales. Some deeply buried salt beds form mobile salt columns that rise toward the surface. Salt domes occur in the Colorado-Utah area, the Gulf Coast of the United States and Mexico, Spain, France, Romania, Iran, Arabia, and India.
Salt domes are emplaced when beds of salt deform plastically under the pressure of overlying rocks and rise through overlying layered sediments. The rising salt forces the overlying rocks into domes and punches through them to leave rock layers upturned along its flanks. The depth of the salt core beneath the surface varies widely. Deep domes may be more than 1,750 meters beneath the surface, but others may expose salt at the surface. As salt reaches near the surface, it encounters groundwater, which dissolves the rising salt. A caprock of less soluble minerals, mostly anhydrite, forms on top of the rising plug. Often, anaerobic bacteria in groundwater break down the anhydrite of the caprock, forming calcite and native sulfur in the process.
Commercial quantities of sulfur are dispersed within the caprock of a few domes. Sulfur is extracted from the caprock by the Frasch process. Water heated to 150° Celsius is discharged into the caprock to melt the sulfur, and hot air is used to drive it to the surface. The molten sulfur is then piped to storage, where it solidifies.
At shallow domes, anhydrite, gypsum, and limestone in the caprock may be quarried for road metal or building materials. Salt is recovered by underground mining techniques. Salt domes account for only 5 percent of the world’s reserves of salt, but alone they could supply the world’s demand for thirty thousand years. Upturned sedimentary beds around the flanks of the domes provide traps for oil and gas that migrate updip and are impounded against the impermeable salt. The limestone of the caprock also forms a petroleumreservoir in some salt domes.
Cavities may be excavated within salt domes either by standard mining techniques or by pumping fresh water into the dome to form a solution cavity. The resulting cavity may then be used to store oil or gas. Because salt is impermeable and self-healing when fractured, it has been used as a storage site for nuclear waste disposal.
Bibliography
Baikpour, S., et al. "Geological and Geophysical Study of Salt Diapirs for Hazardous Waste Disposal." International Journal of Environmental Science & Technology, vol. 13, no. 8, pp. 1951–72.
Hayward, Philip. "Enduring Perceptions: Placenaming and the Perception of Louisiana's Salt Dome Islands." Island Studies Journal, vol. 11, no. 2, 2016, pp. 417–30.
Heidari, Mahdi, et al. "A Simplified Stress Analysis of Rising Salt Domes." Basin Research, vol. 29, no. 3, 2017, pp. 363–76.
Mason, Betsy. "New Seafloor Map Reveals How Strange the Gulf of Mexico Is." National Geographic, 26 May 2017, news.nationalgeographic.com/2017/05/new-seafloor-map-gulf-of-mexico/. Accessed 24 Oct. 2017.
"Salt Domes and the U.S. Strategic Petroleum Reserve." ScienceDaily, 16 May 2017, www.sciencedaily.com/releases/2017/05/170516115311.htm. Accessed 24 Oct. 2017.