DRY (ROCK SALT) MINING

When you drive on an icy road, eat a potato chip or wear a pair of leather shoes, you may be using one of Michigan’s least known natural resources---salt. During the Paleozoic Era, beginning about 600 million years ago and ending about 230 million years ago, seawater invaded the Michigan Basin at least six times. As the seas receded and evaporated, rock and mineral deposits such as halite (rock salt), gypsum (calcium sulfate with water), liquid brines, petroleum, lime, clay, sandstone and coal were left behind.

During the early decades of the 20th century, Michigan led the nation in salt production. Michigan is a leading producer of many natural salines---underground waters rich in chlorides, calcium, magnesium, sodium and, in lesser amounts, potassium, bromine and iodine. Salt under Michigan has created fortunes, towns and manufacturing centers.  Michigan ranks first in the United States in the production of calcium chloride (salt) and in gypsum, fourth in cement and sand and gravel, and is a large producer of crushed stone for a variety of purposes. These minerals are found in the sedimentary rocks of the Michigan Basin or in the extensive glacial deposits. Salt is obtained from beds of rock salt (the Salina Formation is but one) over 1,100 ft below the surface in Detroit and from natural and artificial brines of dissolved salt that are pumped to the surface in Midland, Manistee, Muskegon, Wayne, and St. Clair Counties. The salt layers were laid down as evaporite deposits in the seas of the middle Paleozoic era---in the Mississippian, Devonian, and Silurian periods.

Long before settlers entered Michigan Territory, wild animals discovered numerous salt springs and used them as natural salt licks (see the map below).   Discoveries of musk oxen and mastodon bones in Michigan are often related to these salt seeps.

Indians took salt from the same springs and sometimes used it as an item of trade with neighboring tribes. Some of the earliest white settlements were begun at brine springs in the southeastern part of the state. (Brine is water saturated with common salt.)
    Salt was so important to early pioneers that during the winter of 1836-37 in Branch County a 20-pound venison ham could be traded for a fist-sized lump of salt. Used mainly as a preservative, salt was essential for survival on the Michigan frontier. Though Michigan possessed an abundance of salt springs, they were not commercially developed in the 1830s, and Michigan residents spent $300,000 annually on imported salt. Salt was so important that those in favor of internal improvements in the late 1830s used it as an example of an eastern commodity made more readily available by the building of Michigan’s first railroad line.
    Evaporite deposits (gypsum and rock salt, or halite) formed in the Michigan basin when waters flowed into the basin, and then evaporated, depositing the salts. This is referred to as the "Salt Cycle".

ROCK SALT
The rock salt of the Michigan basin is one of the major sources of salt in North America.  The map below shows where "table salt" or NaCl, is found (pattern of larger dots).   Note that the salts are found within the center of the Michigan Basin, which was surrounded by higher, reef deposits.

The map below shows the North American Silurian reef system. The dots represent some of the major Silurian reefs.  During the mid-Silurian, large pinnacle reefs surrounded the edges of the Michigan basin.  Such reefs are often referred to as "bioherms".  Each reef extended up, vertically, from the sea floor, some 200+ meters.  They commonly are up to a km in width.  Today, these reefs are important oil and gas "traps", and because the limestone in them is so hard and pure, the reefs themselves (when close to the surface) are important sources of limestone.  The large quarry on the south side of Chicago, the Thornton Quarry, is crossed by I-94; perhaps you have driven across it! 

While these reefs were forming, limestone and dolomite were precipitating in the deep waters of the basin, but near the "inlets" to the basin.  In the farthest and cleanest parts of the basin, however, halite salts and gypsum salts (anhydrite) were precipitating out in sedimentary beds (see below).

The image below shows salt beds exposed in a Michigan salt mine. The red markings are spray paint, put on the walls to identify the location in the mine.  From the top to the base of the image is about a meter.  Note that the salt occurs in distinct beds, or layers.  This layering is due to the fact that the salt was depoisted layer-by-layer in an evaporation basin.  The darker layers are still salt, but contain some admixtures of silt and clay (i.e., the water was muddier then).

The map below shows the thickness of one of the salt deposits, the Salina Salt, in southern Michigan. The salt is thickest (over 400 feet thick!) where the basin was, presumably, deepest. One notable exception to this trend exists in NW Lower Michigan, where a 400+ foot thick layer extends to the SE of the barrier reefs that also exist there.  Note also that this salt is predominantly halite in the center of the basin, and has more anhydritic (gypsum-like) characteristics at its edges.

THICKNESS OF THE SALINA SALT IN THE MICHIGAN BASIN

From the Salina (late Silurian) salt beds we are assured of a salt supply of some trillion tons. Beds of pure rock salt 400 to 1600 feet thick, with additional thinner beds alternating with shales, dolomites, and gypsum, underly the Southern Peninsula. At Port Huron, St. Clair, and Detroit, hot water is pumped into the salt to form artificial brines which are then pumped up and used in the manufacture of salt and in the chemical industries. The last active salt mine in Michigan (now closed) was 1100 feet under Detroit in the clear Salina rock salt. One of the largest gas wells in the State is obtaining gas from the Salina formation; thus it is a potential producer of gas in other parts of the Michigan basin which it underlies.
    The rock salt is used for packing meat and fish, for the manufacture of soda ash, caustic soda, bleach, and other chemical preparations, refrigeration, and agricultural purposes. Salt brine is also pumped from the overlying Devonian rocks. The deeply buried Devonian rocks (above the Sylvania) have made mineral contributions that are the most recent and most spectacular. For many years salt brines have been obtained from the lowest of these rocks (the Detroit River formation) to supply the salt industry in Manistee and Ludington.

Mining Rock Salt

Executive summary on salt mining:  Salt is produced as brine and as rock salt. Initially, salt production centered around some brine wells in the Thumb area. An excess of wood products at the time (late 1800's) provided the raw materials required to "dewater" the brine. When the lumber industry fell off, so did the salt brine industry. Those old wells simply drew the brine up, and evaporated the water. Today, all brine operations inject steam or hot water into dry salt beds and extract the brine.
     Halite salt (NaCl) can be mined in two different ways: as a solution or in dry mining (see map below).  In solution mining, fresh water is injected through a pipe into deep shafts that end in the salt beds, and salty water (brine) is drawn upward and dried, to recrystallize the salt.  Or, salty brine found in shallow wells can simply be pumped to the surface and dried there, to make salt. In dry mining (below), the salt is mined in large underground caverns, much like one would mine coal or iron ore.  Dry mining is only practiced in the Detroit area.  Salt worth millions of dollars comes (or at least used to come) from underneath Detroit each year. But not a pinch of it goes on a boiled egg. Glistening and white under southwestern Detroit, are rock salt mines totalling over 100 tunneled miles. Hundreds of tons a day were once lifted a quarter of a mile to the surface.

The greatest rock salt production in the U. S. is obtained from the Michigan, Ohio, and Ontario. In 1958 the salt production from these areas amounted to 8.7 million tons or 35% of the combined total produced in the U. S. and Canada. The image below, from a salt mine, shows a vertical "wall of salt" about a little over a meter in height.

Three important factors lie behind the location and development of the Detroit salt mining area:
1) A healthy market requiring salt for the meat processing industry, chemical industry, water softening snow and ice removal from roads and highways.
2) Abundant salt at a reasonable depth and of sufficiently high quality to permit economical mining,
3) The availability of low cost water transportation on the Great Lakes which facilitates the movement of salt in Canada from eastern Saskatchewan Province to western Quebec Province, and in the U. S. from the "Dakotas" to western New York State.

A radical change in salt production occurred in 1906 when the Detroit Salt and Manufacturing Company started sinking a shaft for underground mining of rock salt. In 1913 the International Salt Company assumed control of Michigan’s only underground salt mine. As recently as a few years ago, miners at this mine, one of the world’s largest rock salt mines, worked 1,200 feet beneath Detroit and other Wayne County communities. The mine, which consists of 100 miles of tunnels, has never experienced a collapse or mine fatality. For years it produced tons of rock salt daily, most of which is used for ice control. Today it is closed.

Mining
The geology of the Great Lakes salt mining area consists of sedimentary deposits of shale, limestone, sandstone, dolomite, gypsum, anhydrite, and rock salt (halite). These formations within the mining area are relatively continuous and generally undisturbed by faults or other forms of sharp ground movements.  The Salina Group actually consists of a number of individual salt beds separated by layers of shale, dolomite, and anhydrite. The salt beds themselves contain bands of anhydrite which vary in thickness from 1/6 in. or less to several inches. The anhydrite (calcium sulphate) content will vary from a trace to approximately 2% by weight of the mined rock salt.
    The top of the Salina Group of the Eastern Salt Basin varies in its vertical distance below the surface. For example, under Michigan the formation varies from a minimum of approximately 800 ft to a maximum depth of approximately 6800 ft, while the top of the formation under Ohio, near the West Virginia boundary, reaches a maximum depth of approximately 6000 ft. This variation in depth is in part attributed to a gradual downwarp or sinking of the earth's crust into basins during formation of the salt beds. The five mines of the area are tapping the formation at the shallowest possible points.

The room and pillar method of mining is employed in all salt mines in Michigan, Ohio, and Ontario. The rooms vary in width from 30 to 60 ft and in height from approximately 17 to 40 ft. Pillar size is adjusted so that the extraction or recovery will attain a maximum of 70%. Actual mining is confined completely to the salt bed. Therefore, usually a minimum of 1 ft of salt is left on the floor, while 4 to 6 feet of salt is left to form the roof. Generally speaking, the absence of gas, water, excessive dust, and the presence of sound roof and ribs as well as comfortable temperatures of 55° to 75° F, permit working conditions in a salt mine to be excellent.

A comparison of the maps above and below will show that the main salt dry mining sites in Michigan are located in Detroit, while solution mining takes place along Lake Michigan and along the St. Clair River.  The biggest salt dry-mining operation in Michigan today is in Wayne County. The Detroit Salt Company produced the first salt there in 1895. Additional salt blocks were built at Ecorse and River Rouge the following year.   Solution mines are often called "brine mines". 

Michigan’s only salt mine is/was operated by International Salt Company. The miners here have mined out an area 22 feet high for about 300 acres. Officials drive about in automobiles on the solid salt underground "streets," which are 60 feet wide. In the mine, the temperature is between 56 and 60 degrees in all season. The Company pumps 100,0000 cubic feet of fresh air a minute down one shaft. Unlike a coal mine, the operation is free of the hazards of water and gas. The room-and-pillar system is used to take out the salt. Rooms from 50 to 60 feet wide are driven out and blocks of salt of the same width are left for roof support. Workmen "undercut" the walls of salt with a machine that bites out a channel at the floor. Then holes are drilled in the wall from floor to roof. At night, the holes are loaded with dynamite and the blasting operation sends the wall of salt crumbling down. Electric shovels dip the salt into trailers the next morning. Trucks powered by electricity take it to a giant crusher underground. Reduced in size, the product is taken by conveyer to an underground screening plant where the various sizes are separated. Again by conveyer, the salt is taken to the larger shaft where it is lifted in nine-ton buckets.
    The biggest development in the industry in recent years is increased use of the product for highway purposes.

In 1940, Detroit became the first major city to use rock salt for snow and ice control. Other cities followed. The Detroit mine now supplies Michigan and several other states, and the amount used for highways is approaching the volumes for chemical purposes. Highway uses include the tons mixed with road bases to make for greater density and lower the freezing point to reduce cracking. The images below show what road salt looks like when it is crushed into a form that is "road ready".  Note that parts of the road salt are dark, reflecting the thin "muddy" seams in the salt (see the images higher up in this page).

Virtually every city, county, and municipality has a storehouse of Michigan salt, to be used on roads in winter.  This one is a small storage shed of salt for the Meridian Mall, Okemos.

    The estimates of salt deposits in Michigan are astronomical. In the Detroit area alone, it is believed that there are over 71 trillion tons of unmined salt. Geological studies estimate that 55 counties of the Lower Peninsula cover 30,000 trillion tons of salt. But like much of Michigan’s mineral wealth, only a fraction of this salt can be economically recovered.

Some of the images and text on this page were taken from various issues of Michigan History magazine and from C.M. Davis’ Readings in the Geography of Michigan (1964).