Marjorie K. Palmer, age 12, of Ephrata, Washington; for her question:
How did salt form in the earth?
The salty minerals are always ready to dissolve in water. When rains fall and streams rush over the ground, the soft chemical salts join the watery parade and sweep along down to the seas. This has been going on since the first rains fell upon the new made earth. Most of the salt dumped into the sea remains there, but through the ages some has been returned to the dry land and buried in vast layers deep below the surface.
The ore in a salt mine is called halite and most of it is the chemical sodium chloride alias table salt. Halite is a rather soft mineral about twice as heavy as water. It is very brittle and it tends to crack apart in cube shapes. In pure form, halite is clear as glass but it is almost always colored with impurities.
Halite deposits may be dirty grey or muddy brown. They many be red, yellow or deep violet. Almost always the basic salt is mixed with gypsum, calcium and other minerals found in the salty sea. In a California desert there is a crusty layer of salt on the surface ground. In New York State, Michigan and other places the deposits are buried more than half a mile below the surface.
Some of the buried deposits are half a mile thick and in North America there is a deeply buried layer of halite covering 100,000 square miles. Halite is a soft mineral and pressure from rocks above and around it tend to shove it out of shspe. Along the Gulf Coast there are round salt domes which geologists think have been pushed from a salty layer one to three miles below.
All these buried mountains of salt have been toted from the land to the sea and back again to the land. Since the earth began the rains have been salty chemicals in the land and dumping them
Time after time the seas have invaded the land and retreated. Salty lakes and lagoons were left to evaporate and, when the water dried up, the salty chemicals were left behind in sedimentary layers of halite rocks. The salt mines of Michigan, we are told, were forr3ed when a shallow sea stood aver some 100,000 square miles of our central and eastern states. The water evaporated some 300 million years ago, leaving its chemicals behind. Here, as in other places, the salty sediments were later covered by layers of newer rocks.
Where the halite is near the surface it is a simple matter to dig mine shafts and tots the ore to the surface to be purified. Where the deposits are deep or difficult to reach, the mineral is extracted with drills and pumps. The salt dissolves readily when streaming water is pum pumped down into the halite deposits. The briny mixture is brought up to the surface and treated to extract the pure salt and other minerals.