By Lowell and Kaye Christie, F47246
Whether it’s found in ocean water or deep underground, salt seems to be everywhere.
Several years ago we made our first plane flight into San Francisco, California, arriving in the early evening. As we descended toward the airport, we could see an exceptionally large painted patchwork of rich reds, pale greens, and various shades in between. Getting closer, we saw it was actually colored water, with the many hues separated by earthen levies.
Only later did we learn we had flown over the Don Edwards San Francisco Bay National Wildlife Refuge, and what we saw was a 9,000-acre salt pond system developed by Cargill Salt, one of the world’s largest makers of salt products. A manufacturing facility combined with a wildlife refuge? Perhaps it’s not as strange as it sounds.
These salt ponds have been actively harvested for their mineral content for more than a century, and each year more than a million native and migratory shorebirds and waterfowl have used the waters as a feeding ground. In 1974 the area of the salt ponds became the first urban national wildlife refuge, with Cargill Salt retaining perpetual operating rights. Fortunately, the birds don’t seem to mind.
Visits to that refuge gave us our first glimpse into the world of salt, and we soon discovered there is a lot more to this familiar condiment than meets the eye (or the tongue). The San Francisco salt ponds display the oldest method of acquiring salt: solar evaporation, or letting the sun help extract salt from seawater.
Water from San Francisco Bay (which is slightly less salty than ocean water) is transferred into holding ponds, and as the liquid evaporates, it leaves behind water with a higher salt content. Algae living in the ponds change their color with changes in the salinity. Lower amounts of salt produce shades of green. As the salinity increases, an algae called Dunaliella salina shifts the color to a pinkish red. Tiny brine shrimp cause some of the pools to display an orange cast.
Eventually the brine becomes so concentrated that the salt crystals can’t remain in suspension, and thus drop to the bottom of the pond, forming a hard layer that can be up to 12 inches thick. To extract the salt, the remaining liquid is removed and machines similar to snowplows break up the hard material. This raw salt is 99.5 percent pure sodium chloride.
There are additional steps in the process, which takes approximately five years, and the end result is a 99.9 percent pure product. Each year Cargill Salt produces 650,000 tons of natural sea salt from this site for industry, agriculture, and the home market. And it also provides an important protected area for wildlife.
Not all of the more than 200 million tons of salt produced in the world each year is extracted from seawater. Two other methods, solution mining and deep-shaft mining, are used to access deposits of salt underground. But at one time, this salt also was suspended in water.
Ancient salt lakes, or bays cut off from the ocean, evaporated, leaving behind their salt content. This salt became buried under eroded mountains, often deep below the surface. Underground pressure compressed the salt into solid blocks. The salt itself is physically weak, and under moderate pressure may flow almost like glacial ice, sometimes protruding upward through other rocks to become balloon-shaped salt domes, which are widespread in the south-central United States. These underground deposits are retrieved either by solution mining or deep-shaft mining.
Solution mining requires drilling a well down to the level of the salt formation and injecting water to dissolve the salt. Then the salt solution, or brine, is pumped out of the salt chamber, placed in sealed containers called vacuum pans, and boiled to remove the liquid. The remaining salt is then dried and refined. This is the method used to produce most table salt.
Deep-shaft mining is similar to mining for other minerals, often using what is called a room and pillar system. A shaft is dug down through the ground surface and into the salt layer, reaching the level that will become the floor of the salt chamber. The salt is then removed by blasting and digging sideways, but always leaving upright pillars of earth in place to support the roof of the chamber.
The oldest working salt mine in the United States is located on Avery Island, Louisiana, where commercial salt making started in 1791. Today miners work 1,300 to 1,600 feet below the surface, but there is no danger of running out of salt. Avery Island is a salt dome, and a typical salt dome is more than a mile in diameter and 30,000 feet to 40,000 feet in depth.
When you start checking, you will find salt mines in the most unexpected places, and the size of their operations is amazing. There’s one under the city of Cleveland, Ohio. More than 9,000 acres have been mined nearly 2,000 feet below the surface. Rooms excavated under Cleveland are typically 45 feet wide, with the ceilings 18 feet to 20 feet high. Elevators at the entry shafts can carry 30 people and take more than four minutes to drop to the mine floor.
A 1,400-acre working salt mine is located a quarter-mile directly below the city of Detroit, Michigan, and has been in operation since the beginning of the 20th century. Originally, mules were used to transport the salt around inside the mine. They were lowered on a one-way trip down the mine shafts on ropes and worked the rest of their lives underground.
In later years, trucks and other machinery were lowered into the mine the same way, but this larger equipment had to be completely disassembled to fit down the shafts and then rebuilt in underground workshops. Today there are more than 100 miles of roads inside the Detroit salt mine.
If you want to tour an actual mine, you’ll have to travel to Hutchinson, Kansas, and visit the Kansas Underground Salt Museum. The Hutchinson mine taps into the largest salt formation in the United States, as much as 400 feet thick and extending into the states of Oklahoma, Texas, and New Mexico. Visitors descend 650 feet in an elevator to reach the tram tour.
In addition to the museum, this is still a working mine, and many of the excavated sections are now rented out for other purposes. Salt mines have a constant temperature and humidity, making them perfect repositories for long-term storage of sensitive materials. For example, the Hutchinson mine provides storage for the original copies of many of Hollywood’s more valuable feature films.
One thing’s for sure “” we’ll never run out of salt. The United States alone contains an estimated 55 trillion metric tons of it, and the oceans contain enough salt to cover the continents to a depth of 500 feet.