Michigan's Salt Beds: Growing an Evaporite
Model how thick rock-salt layers form when water evaporates — the origin of the salt mined beneath Detroit.
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Standards
HS-ESS2-5 — Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.
MS-ESS2-1 — Develop a model to describe the cycling of Earth's materials and the flow of energy that drives this process.
Why Does This Matter?
Hundreds of meters below Detroit lies a thick bed of rock salt that formed more than 400 million years ago, when a shallow sea covering the Michigan Basin repeatedly evaporated. Salt mined from beneath the city is used to de-ice roads during Detroit winters. Today you will model the same process on a lab bench.
Objective
Evaporate a saturated salt solution under different conditions and compare the crystals that form, then relate your results to the formation of thick underground salt layers.
Materials
- Table salt (plain or kosher salt works best)
- Warm tap water (about 40°C) and a 400 mL beaker
- Spoon or stirring rod
- 3 shallow dishes or saucers, plus marker for labels
- Plastic wrap and a toothpick (to poke a vent hole)
- Hand lens or low-power microscope
- Ruler or millimeter grid paper
- A warm sunny spot or heat lamp, and a normal room-temperature spot
Safety: Wear goggles. Warm water only — do not use boiling water. Do not taste any lab material. Keep heat lamps away from plastic and paper.
Procedure
- Add salt to 200 mL of warm water a spoonful at a time, stirring, until no more dissolves and a little settles at the bottom. This is a saturated solution. Let it settle and carefully pour off the clear liquid.
- Pour 30 mL of the clear solution into each of three labeled dishes: Fast (under a lamp or in direct sun), Medium (room temperature, uncovered), and Slow (covered with plastic wrap that has one small vent hole).
- Record the start time. Check each dish at least once a day and note when the first crystals appear and when the dish is completely dry.
- When dry, measure the longest edge of the largest crystal in each dish and count the crystals in a 1 cm × 1 cm square using the hand lens.
- Sketch the shape of a typical crystal from each dish.
Data Table
| Dish | Time to first crystals | Time to fully dry | Largest crystal (mm) | Crystals per cm² |
|---|---|---|---|---|
| Fast | ||||
| Medium | ||||
| Slow (covered) |
Analysis Questions
- How did the evaporation rate affect the size and number of crystals? Suggest an explanation.
- Why does salt come out of the water as crystals when the water evaporates?
- Rock salt is classified as an evaporite. Is it igneous, sedimentary, or metamorphic? What does a thick salt layer tell geologists about the climate and geography when it formed?
- Seawater contains about 30 kg of sodium chloride per cubic meter, and solid salt has a density of about 2,170 kg/m³. Estimate how deep a column of seawater would have to evaporate to leave a 1-meter-thick layer of salt. What does your answer suggest about how long thick salt beds take to form?