Diane Cavalier, age 11, of Detroit, M1Ch.,‑for her question;
How did the salt get into the salt mines?
America produces most of the world’s salt and most of it is mined in Michigan just north of Detroit. These vast beds of salt are pocketed in porous rocks. More salt is mined in Texas and Louisiana where our thickest deposits of salt occur. These supplies are taken from the vast salt domes along the Gulf of Mexico.
We have a good idea how beds of salt get into the earth. But only lately are geologists beginning; to understand the dome shape se often taken by a salt deposit. The salt was originally collected from the land by running water. It was dissolved from soil and rocks and dumped into the sea. There it stayed and increased for very little escaped to the air by evaporation.
Through the long history of the earth, the seas have flooded and retreated from the land time and again. When the waters retreated, a salt lake or arm of the sea was often cut off from the ocean. In time, this seawater might dry up, leaving its load of salts in a dry layer of dregs. This is happening around the Dead Sea and the Great Salt Lake of Utah in the present time.
The layer of salty chemicals is soon covered with dust and debris. Floods and streams cover it with silt and mud. The earth’s crust may hump to form mountains. The rocky layers may be upset by volcanoes and shaken by earthquakes. Most of these adventures are bound to happen through millions of years. In time, the original layer of salt may become buried under miles of rocky crust.
The layer of salt feels the pressure and heat from the rocks about it. The particles of salt tend to cling together rind form plastic rock salt. Because it is salt, it is lighter than the most of the rocks around it, It gains bouyancy,
Slowly the deeply buried layer of rock salt begins to seep upstairs, It forms a new layer of rock salt nearer the surface. When the edges cannot extend, it piles up in the center. We believe this is why so many deposits of salt are dome shaped. The history of a saltdome goes back millions of years. It develops because salt is a light mineral and under heat and pressure becomes a plastic rock salt. The rocksalt seeps up through heavier rocks to form a concentrated layer nearer the surface.
The making of a salt dome stops when all the deep original layer has moved upstairs or when a balance between the two layers of salt is reached. The dome may reach the surface. In the desert of Iran, the domes have glaciers of salt down their sides. In Rumania, the surface of the salt domes is washed smooth by running water,
The layer of rock salt may be several miles deep. In Poland there is a salt mine reaching down almost two miles. It is a warren of tunnels and g allerys and some 40 miles of railway haul the salt to the surface. The thick salt beds of the Gulf States are said to reach down five miles. All this salt was originally stolen from the land and returned in concentrated deposits by the sea. There is plenty to supply our needs far, far into the future.