Salt: The Essence of Life
Multidisciplinary Curriculum for Students
Unit 6: Earth Science
Salt Formation
1 Class Period
(plus 5-10 minutes every day for a week)
Overview
This module highlights the different sources of salt, and emphasizes how salt is produced.
Background
Geologists believe that all salt deposits were formed from the oceans or from enclosed bodies of salt water. Evaporating water left beds of salt, which in many cases were buried by rock strata formed from other sedimentary deposits. Beds of salt range from a few feet to well over a 100 feet in thickness. Layers of salt deeply buried by rock strata may become mobilized by great pressure and flow upward to form salt domes. Salt domes are a source of salt for many salt mining operations.
About 25 million tons of dry salt are produced annually in the United States, principally by three methods: salt mining, solution mining, and solar evaporation.
In conventional underground salt mining a shaft is sunk into the deposit, where the salt is drilled, undercut and blasted with explosives, similar to coal mining. Trucks or shuttle cars carry the broken salt to a preparation station where it is crushed and screened into commercial grades. The salt is then hoisted to the surface for packaging and bulk shipment by truck, rail, ship or barge.
In solution mining, fresh water is injected through a well casing into the salt deposit. A second or concentric pipe removes the brine formed when water dissolves the salt. The brine is then evaporated in large vacuum pans where the salt crystallizes into small granules. The salt is dried and sold in packages, like table salt, or bulk for food processing and other industrial uses.
The third method is solar evaporation of sea water or natural brine. Large earthen ponds are flooded with a shallow layer of sea water or brine. The action of sun and wind successively concentrates the brine by evaporation. The brine is moved from pond to pond, and finally, the salt crystallizes on the floor of the last ponds in the series. The salt is then "harvested", washed and stockpiled for shipment.
Getting Ready
Time: 1 class period to set up experiment, and 5-10 minutes each day for about 1 week to check on and record results
Materials
Table salt, cup of water, and class set of shallow dishes (petri dishes or plastic lids), Facts About Salt brochure, Major Salt Deposits Map
Instructions
1. Place materials on lab tables for students to use in small groups.
2. Make copies of pages 3 - 4 in Facts About Salt, and copies of Major Salt Deposits Map
Activity
1. Provide students with Major Salt Deposits Map. Give them a couple of minutes to read over the map.
2. Ask them to identify three types of salt production sites in North AmericaMake copies or read, as a class, the information from Facts About Salt brochure pages 3 - 4 which explains the different methods of producing salt.
3. Look at the map and notice how salt production sites are situated geographically across North America. Try to identify spatial patterns of distribution. What general statements can you make about the location of different types of production sites? Salt is an inexpensive, bulk commodity. What effect does this have on the sale and distribution of salt?
4. Solar salt is produced by the evaporation of sea water or natural brines. Ask what type of climate and other physical conditions are necessary for solar salt production. Some solar salt plants are located near the ocean. Ask students what market advantage these solar salt plants may have.
5. To model the process of solar salt production do the following:
a. Mix 3 1/2 level teaspoons of salt with a 1/4 cup of water to make brine.
b. Place the brine in a shallow pan or petri dish. Make sure the water is less than 1/8" deep.
c. Place the container on a newspaper and on a sunny windowsill or radiator. Wait several days and then record their observations each day. To accelerate the process place a fan to blow over the container.
d. When the water has evaporated look at the bottom of the container.
e. Use a magnifying glass or microscope to take a closer look at the salt crystals.
f .Dray and describe their characteristics.
6. Discuss how the method of obtaining solar salt in this experiment is the same or different than that done in a large solar salt production facility.
7. Ask students why the oceans are a virtually limitless resource for solar salt production.
Assessment
Students can create daily drawings of the salt crystal solution in a notebook, or take daily photographs. A bulletin board display can be created from these illustrations. Students can also obtain more detailed information about salt processes from the Internet.
Conclusion
The lesson can be concluded by having students use the Major Salt Deposits Map to locate the salt deposit nearest their community. Discuss the geological history of the area and how it may have contributed to salt deposit formation.