Do you ever wonder where the salt you sprinkle on your dinner or the salt they spread on the roads in the winter comes from? How about the origin of the agricultural fertilizer potash? They both some from geological deposits which are derived from the evaporation of large volumes of seawater; table salt is a mineral known as halite (NaCI) and potash is another type of salt known as sylvite (KCI).
Millions of years ago (about 380 million or so) large seas over what is now the Canadian Prairies, began to evaporate. As they did, the halite dissolved in the water (that'’ what makes it taste salty) began to precipitate (or form into crystals) and settle to the bottom of the sea. As the sea evaporated further, other salts, such as sylvite, began to precipitate. Eventually, the thick layer of salts was covered by other rocks such as limestone and sandstone. Today the salts can be found at a depth of about 1000 m below the surface. Similar deposits can be found in New Brunswick, Germany, and New Mexico.
About 200 million years later, thick deposits of halite were precipitated along what is now the U.S. Gulf of Mexico coast (Texas, Louisiana, and Mississippi). These deposits were buried much deeper than those in Saskatchewan (1500 – 6000 m). Because salt is less dense than most other rocks, the salt has ‘floated’ to the surface in some places, much like oil in water. These huge ‘bubbles’ of salt are known as salt domes or diapirs (Figure 1). More than 500 have been mapped in the gulf coast region. Some are as much as 10 km in diameter and up to 6000 m deep.
Salt does not behave at all like other rocks; it has the ability to ‘flow’, not like water, but like a piece of warm toffee. If you take a piece of toffee, put it in the fridge, and then try to pull it, it will break. However, if you leave the toffee at room temperature and pull it, it doesn’t break, but it stretches or ‘flows’. Salt does not flow as fast as toffee; salt domes may only move upward as little as a few centimeters per century. The ability to flow, or more correctly ‘creep’, allows salt to form into domes. It also allows it to ‘creep’ into openings created by mining.
Underground salt and potash mines are made up of a series of rooms and pillars (Figure 2-1). A pillar is a large piece of salt left in the mine to support the roof and weight of the overlying rock, much like the walls in your house support the roof. After the rooms have been mined out, the pillars must support the load once carried by the entire salt deposit. This causes them to creep or grow shorter and fatter with time. Given enough time (perhaps several centuries), the pillars will shorten and flatten enough to close the room altogether (Figure 2-2, 2-3).
At the University of Waterloo, we have been studying the creep of salt for about 7 years at a research laboratory known at WATSALT. Here, specimens of salt are placed in high pressure test cells and subjected to pressures similar to those caused by the weight of overlying rock in the mine from which they came (Figure 3). We monitor the amount and rate at which the salts creep under different pressures (Figure 4).
Once the behavior of a certain salt is understood in the laboratory, we can use that information to predict how it will behave in the mine. This information is useful to mine designers who want to remove as much salt as possible from the mine, but still leave large enough pillars to support the overlying rock.
To date, we have been involved in many interesting salt research projects. We have tested potash and salt from Saskatchewan and New Brunswick for potash mining companies who have used our results to design the size and shapes of the rooms and pillars in their mines. Our most interesting research project was for a court case in the United States. A mining company closed their salt mine in Louisiana citing unsafe conditions (ie. The pillars were creeping too fast). Data from the WATSALT laboratory incorporated into a computer model showed that creep rates in the pillars (rates of shortening and fattening) were safe and that the mine need not have been closed.
So next time you are sprinkling salt on your dinner or fertilizing your lawn, remember what interesting minerals salts are and how differently they behave to other rocks. And if you are ever near the Department of Earth Sciences, drop by and ask to see the creep at Waterloo!