Chemical sedimentary rocks are one of three major types of sedimentary rocks.
This mini-video explains how they are formed.
Imagine you took a tablespoon of salt and dissolved it in this small beaker of water. Then you added the water to a pan on the stove and cranked up the heat.
As time passed, in this case about 4 minutes speeded up,
the water would gradually evaporate and the salt concentration in the remaining liquid would get progressively higher
until salt began to precipitate out of solution.
You can see the salt crystals forming here and along the edges of the pan.
Eventually, when all the water is gone, the pan will be coated with the same salt that was originally dissolved in the water.
Essentially, the same thing happens in nature, just not as quickly.
This occurs when water in isolated seas or lakes evaporates leaving behind dissolved minerals.
For example, ancient Lake Bonneville covered much of Utah 10,000 years ago.
Most of the lake dried up leaving the flat lake bed covered in a layer of salt.
Because these chemical sedimentary rocks form by evaporation, they are known as evaporites
As sea water evaporates, the concentration of salts changes, and different rocks are formed.
Gypsum will precipitate first . . .
. . . followed by rock salt as salt concentrations increase in the shrinking waterbody.
Minerals are also dissolved in lesser concentrations in groundwater and freshwater lakes and rivers . . .
. . . and are also concentrated in hot waters associated with thermal features such as hot springs and geysers.
Precipitation in these cases occurs when there are small changes in water chemistry.
For example, water evaporates in caves to leave behind limestone deposits.
Each water drop loses carbon dioxide and deposits a film of the mineral calcite which eventually builds up to form features such as these stalactites.
Gases are also lost as warm groundwater comes to the surface and minerals precipitate to form travertine,
a type of chemical limestone, which forms these terraces in association with hot springs in Yellowstone National Park.
Elsewhere, silica-rich groundwater may replace limestone to form irregular chert nodules
or produce chert when it precipitates in buried trees to form petrified wood.
So, to summarize, chemical sedimentary rocks are typically formed as a result of evaporation or precipitation from mineral-rich waters.
Finally, how confident are you that you can successfully meet the learning objective for this lesson?