Tuesday, April 28, 2015

The Detroit Area Salt Mine

Bulk salt waiting to be loaded for shipment

Twelve hundred feet below the surface of the state of Michigan lies the largest salt deposit in the world--seventy-one trillion cubic tons of salt deposits. Over four hundred million years ago, horizontal salt beds formed as the result of ancient oceans evaporating in what geologists have named The Michigan Basin--a circular pattern of sedimentary strata that began to sink over time.


 
This depression of Precambrian rock is 16,000' deep at its center and tapers to 4,000' at its edges. The basin extends throughout most of Lower Michigan. As the basin began to sink about a billion years ago, salt water repeatedly back-filled the depression and evaporated leaving the salt deposits behind.

This occurred during the Cambrian Period of the earth's development before the age of the dinosaurs. The only life on the planet were hard-shelled aquatic trilobites. These ancient salt beds were buried by the intrusion of heavier igneous rock from the earth's mantle--mainly basalt, and glacial activity from four ice ages.

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Rock salt was discovered beneath Detroit in 1895. Eleven years later, work began on the first tunnel shaft--which was was completed in 1910--at the cost of many lives and the bankruptcy of the mine's original owners. In the early days of mine operation, mules were lowered in harnesses into the mine to live out their lives as beasts of burden. By 1914--due to the use of electric energy and advancements in mining technology--the mine was producing 8,000 tons of salt a month for the leather and food processing industries.



In 1922, a second, larger mine shaft was begun and finished in three years. The first shaft was now used to haul men and small materials. The new shaft was used to lower machinery used in the mine. Most equipment was massive and had to be disassembled on the surface--piece by piece--and reassembled in the machine shop below.

The mine has changed hands many times in its over 100 years of existence. International Salt closed the mine in 1983 because of falling prices, but its present operator--Detroit Salt Company--reopened the mine in 1998. Today, the only products the Detroit mine produces are deicing rock salt for roadways and bagged rock salt for consumer use. From the 1920s until the 1980s, guided public tours were allowed by the mine's management. Since the new owners took over, only rare private tours are given.

Salt Pillar
The room and pillar method of extraction is used to mine salt. The rooms vary in width from 30' to 60'--with a height of between 17' to 40'. For safety reasons, a minimum of 30% of any cavity must be pillared. During the day and afternoon shifts, miners undercut a solid wall surface at floor level with an industrial-sized chain saw device that bites out a channel ten or more feet deep. This first cut leaves a smooth floor for picking up the salt after blasting. Deep holes are drilled at strategic places along the face of the wall and loaded with explosives that are set off electronically after the work shift.

The next morning, heavy equipment loads the large salt pieces and takes them to massive crushers where they are loaded onto conveyor belts and hauled to the surface in buckets capable of lifting 100 tons. Once above ground, the salt is screened and sorted for size. Some of the salt is conveyed to individual storage bins to await packaging. The rest is loaded into railroad cars, semi-trucks, or river barges and sold as bulk salt.


Here are some factoids about the Detroit salt mine:
  • the tunnel's shafts are deeper than the height of the Empire State Building
  • the mine's temperature is a constant 56-60 degrees
  • the mine covers an area of over 1,500 acres
  • the mine head is in Southwest Detroit and the mine extends beneath the eastern portions of Dearborn, much of Melvindale, and the northern reaches of Allen Park
  • there are one hundred miles of roads cut through the salt beds
  • the underground streets are 60' wide to handle the heavy loading equipment
  • 100,000 cubic feet of fresh air is pumped into the mine per minute
  • no living thing exists in the mine except the miners
  • the mine shaft opening is at 12841 Sanders Street, Detroit, Michigan 48217.
In 1940, Detroit was the first major city in America to use rock salt for snow removal. The increased salt level buildup in the soil along Michigan roadsides has caused native roadside vegetation--like cattails--to be replaced with salt water tolerant plants--like sea grass. Over time, seeds from these invasive plants were inadvertently spread by transport trucks from ocean coastal areas to the Midwest. Now these plants have a foothold in the Michigan soil.