A Secret Salt Mine

Each winter, cities and towns throughout the U.S. scatter nearly 20 million tons of salt on snowy roads and walkways. The salt helps melt ice-coated streets and sidewalks, making it safer for people to drive and walk. But where does all that salt come from? A lot of it is mined from a vast salt deposit beneath Lake Erie, one of the Great Lakes (see Salt Sources).

Each winter, cities and towns across the U.S. scatter salt on snowy roads and walkways. They use nearly 20 million tons of it! The salt helps melt slippery ice that covers streets and sidewalks. This makes it safer for people to drive and walk. But where does all that salt come from? A lot of it is mined from a huge salt deposit under Lake Erie, one of the Great Lakes (see Salt Sources).

The Morton Salt Mine lies near the town of Fairport Harbor, Ohio, just 48 kilometers (30 miles) east of Cleveland. Few people besides the mine’s workers have ever seen inside its shimmering white caverns and tunnels. In fact, many locals aren’t even aware the mine exists.

To reach the salt deposit, workers and engineers take a four-minute elevator ride, descending about 610 meters (2,000 feet) underground. There, miners use explosives to blast away at massive walls of solid salt. Huge machines drill, crush, and transport the salt up to the surface. Each move is carefully planned to ensure that the mine’s ceiling—and the rock and water above it—doesn’t come crashing down.

The Morton Salt Mine lies near the town of Fairport Harbor. It’s just 48 kilometers (30 miles) east of Cleveland, Ohio. Besides the mine’s workers, few people have ever seen inside its sparkling white caverns and tunnels. In fact, many locals don’t even know the mine exists.

Workers and engineers take a four-minute elevator ride to reach the salt deposit. The elevator lowers them about 610 meters (2,000 feet) underground. There, miners use explosives to blast away at massive walls of solid salt. Huge machines drill, crush, and carry the salt up to the surface. If the mine’s ceiling were to come crashing down, so would the rock and water above it. Each move is carefully planned to make sure that doesn’t happen.

The Great Lakes region didn’t always look as it does today. During the Paleozoic era—from about 544 million years ago to 245 million years ago—much of the land that is now North America was covered in a shallow, salty sea.

Because the sea was so shallow, huge rings of coral reefs sometimes rose above the water and formed massive, bowl-shaped basins. The air was hot and dry, causing the salt water caught within those basins to evaporate. As the liquid water turned to vapor, salt deposits were left behind on the seafloor. These deposits were largely composed of a compound called sodium chloride (NaCl). A compound is a substance made of two or more elements that are chemically combined. Sodium chloride also makes up ordinary table salt, which Morton extracts from other mines in North America.

The Great Lakes region didn’t always look like it does today. The Paleozoic era ran from about 600 million years ago to 230 million years ago. Back then, a shallow, salty sea covered much of the land that became North America.

Huge rings of coral reefs sometimes rose above the shallow water. They formed giant, bowl-shaped basins. The air was hot and dry. This caused the saltwater caught inside those basins to evaporate. The liquid water turned to vapor, and salt deposits were left behind on the sea floor. These deposits were mostly made of a compound called sodium chloride (NaCl). A compound is a substance made of two or more elements that are chemically combined. Sodium chloride also makes up everyday table salt. Morton gets that from a different mine.

The process of evaporation and the accumulation of salt deposits on the seafloor continued for millions and millions of years (see Salty Buildup). “Over time, a tremendously thick deposit of halite, or rock salt, formed,” says Randall Schaetzl, a professor of geography at Michigan State University. “It’s amazing how extensive these salt deposits are. They go on for miles.”

Water evaporated and salt was deposited for millions of years (see Salty Build-Up). “Over time, a tremendously thick deposit of halite, or rock salt, formed,” says Randall Schaetzl. He’s a professor of geography at Michigan State University. “It’s amazing how extensive these salt deposits are. They go on for miles.”

The Morton salt mine under Lake Erie is the deepest salt mine in North America. More than 1 million tons of salt per year comes from the mine, which has been in operation since the late 1950s. The mine spans nearly 13 square km (5 square mi)—about the size of 2,500 football fields.

The Morton Salt Mine under Lake Erie is the deepest salt mine in North America. More than 1 million tons of salt are removed from the mine each year. The huge mine has been open since the late 1950s. It covers almost 13 square km (5 square mi). That’s about the size of 2,500 football fields.

There’s no natural light inside the mine, says Ricky Rhodes, a photographer who was recently invited to capture images of the mine. “The darkness of the mine has really stuck with me,” he says. “I’ve never seen such a pure black in my life. If you turned off your headlamp, you could see nothing at all.” It’s also hot, dry, and full of salty dust. Fresh air must be pumped in, and workers are required to carry backup oxygen reserves.

To extract salt from the deposit, miners use a method called room and pillar mining. Using explosives and large machinery, they blast out large chambers from the solid rock salt. Removing too much salt could risk a mine collapse. So engineers make sure to leave behind gigantic pillars of salt to support the weight of the rock above before blasting out another chamber. This method leaves behind a collection of empty caverns, many of which are used to store broken mining equipment too large to remove from the mine. (To get machinery down into the mine in the first place, workers must bring it below ground piece by piece, where they then assemble it.)

There’s no natural light inside the mine, says photographer Ricky Rhodes. He was asked to take pictures of the mine not long ago. “The darkness of the mine has really stuck with me,” he says. “I’ve never seen such a pure black in my life. If you turned off your headlamp, you could see nothing at all.” It’s also hot, dry, and full of salty dust. Fresh air must be pumped in, and workers have to carry backup oxygen supplies.

Miners use a method called room and pillar mining to get salt from the deposit. They use explosives and large machinery to blast out large chambers from the solid rock salt. If they remove too much salt, the mine could collapse. So engineers make sure to leave behind gigantic pillars of salt. The pillars support the weight of the rock above. Then miners can blast out another chamber. This method leaves behind empty caverns. Broken mining equipment is stored in many of these caverns. The equipment is too large to remove from the mine. (How do workers get machinery down into the mine in the first place? They bring it below ground piece by piece. Then they put it together.)

Once the salt from a chamber is collected, it’s crushed into smaller pieces and transported above ground for processing at a plant in Fairport Harbor. All the salt from the Morton Salt Mine is used for road salt. While table salt is chemically identical to road salt, it requires further processing to remove impurities. Road salt, however, doesn’t need to be as pure.

The salt is collected from a chamber. Then it’s crushed into smaller pieces and carried above ground. It gets processed at a plant in Fairport Harbor. All of the salt from the Morton Salt Mine is used for road salt. Table salt and road salt are chemically the same. But table salt needs more processing to remove impurities. Road salt is just bagged and shipped.

Road salt prevents ice from forming on streets by lowering water’s freezing point. The freezing point is the temperature at which a substance changes from a liquid to a solid. Water normally turns to ice at 0°C (32°F). When salt is added to water, though, temperatures need to dip much lower for it to freeze. Water that is 5 percent salt, for example, freezes at -3°C (27°F). Water that is 15 percent salt freezes at -11°C (12°F).

Road salt keeps ice from forming on streets. That’s because it lowers water’s freezing point. The freezing point is the temperature at which a substance changes from a liquid to a solid. Water normally turns to ice at 0°C (32°F), but that changes when salt is added to water. Then temperatures need to dip much lower for it to freeze. For example, water that is 5 percent salt freezes at -3°C (27°F). Water that is 15 percent salt freezes at -11°C (12°F). 

Salt also helps to melt snow and ice already on the road. As long as temperatures aren’t too low, salt grains dissolve snow and ice. The resulting brine, or salty water, will spread and help melt more frozen water. This brine eventually becomes diluted. Added meltwater reduces the concentration of salt, and more salt is needed to stop more ice from forming.

Road salt saves lives by preventing slippery driving conditions. But it might also have negative effects on the environment, says Mark Green, a hydrologist (water scientist) at Plymouth State University in New Hampshire. He’s found that the use of road salt can increase the salinity, or saltiness, of streams and other water sources near roads. This can negatively affect fish and amphibians in the water, as well as people who drink it. Soil near major roads can also become salty over time, which can damage plants and crops.

What if snow and ice are already on the road? Salt helps to melt them. If temperatures aren’t too low, salt grains dissolve snow and ice. This creates brine, or salty water. Brine will spread and help melt more frozen water. Little by little, this brine becomes diluted. That means more meltwater is added, but the amount of salt in it stays the same. So more salt must be added to stop more ice from forming.

Road salt saves lives, because it prevents slippery driving conditions. But it might also have negative effects on the environment, says Mark Green. He’s a hydrologist (water scientist) at Plymouth State University in New Hampshire. He’s found that road salt can affect streams and other water sources near roads. It increases their salinity, or saltiness. This can cause problems for fish and amphibians in the water, plus people who drink it. Soil near major roads can also become salty over time. That can damage plants and crops.

Road salt also takes a toll on infrastructure, such as bridges, in areas that receive a lot of snow. That’s because salt is corrosive. Under certain conditions, it causes damage to things it touches, like iron (Fe) in the steel used to build bridges. Salt speeds up oxidation, the process in which iron rusts when exposed to oxygen and water. Fixing salt-damaged roads and bridges costs billions of dollars each year.

Despite the hidden costs of using road salt in the winter, it remains an effective solution to keeping roads safer. Until researchers develop a better way to deal with this annual winter issue, miners under Lake Erie will likely continue to dig up salt for years to come.

Road salt causes other problems in places that get a lot of snow. It weakens big infrastructure, such as bridges. That’s because salt is corrosive. Under certain conditions, it damages things it touches, like iron (Fe) in steel bridges. Salt speeds up oxidation. This process makes iron rust when it meets oxygen and water. Fixing corroded bridges costs billions of dollars each year.

Use of road salt in winter has its hidden costs, but it does a good job of keeping roads safer. Maybe someday researchers will find a better way to deal with this yearly problem. Until then, miners under Lake Erie will likely continue their work—digging up salt for years to come.