What is a salt dome?

Salt domes are massive underground salt deposits. Mushroom-shaped and thousands of feet thick, they form where shallows seas once stood. They built up over tens of thousands of years as saltwater flooded these former marine basins, then evaporated.

With time, the salt was buried by thick layers of sediment. The sediment became rock which put pressure on the salt, causing it to become plastic and puttylike. Like lava in a lava lamp, the salt pushes up toward the ground, shoving aside denser rock and creating long, finger-like columns known as diapirs. As the salt displaces rock strata, pockets are formed that collect petroleum. These have been responsible for an important portion of US domestic oil production.

About 500 salt domes exist in the U.S., all located near or in the Gulf of Mexico, where an ancient sea stood until the Jurassic age. It left behind a layer of salt called the Louann Salt that stretches from East Texas to the Florida Panhandle and as far north as southern Arkansas.

In addition to trapping petroleum, salt domes have been eyed for another energy-related use; some geologists believe they could be safe, stable places to store nuclear waste. The idea is to excavate salt deep inside the dome, create a natural vault where radioactive material could be stored. Some petroleum-related waste is already stored in similar chambers. It is argued that salt’s plasticity could help make these spaces more secure than similar ones excavated from denser rock, because if radioactive material caused a fissure its wall, it could possibly close the rupture on its while other types of rock could not.

Proponents of storing waste in salt domes argue that they can withstand geologic activity and be impervious to water. But drilling near salt domes may cause them to become unstable and salt caverns used to store petroleum have leaked into surrounding sand.