Chemical Sedimentary Rocks
Water traveling through rock dissolves minerals and transports them away. Chemical sedimentary rocks form from these minerals. Eventually dissolved minerals are redeposited, or reprecipitate, when the water evaporates or when the solution becomes over-saturated with minerals or when the water enters an area of low pressure, like a cavity in the rock. Some chemically precipitated rocks include stalactites and stalagmites you see in caves (also known as travertine). Chert is a precipitated rock from silica-saturated waters. Rock salt, known as table salt, is the result of water evaporating from salt-rich water.
How do Sediments Become Sedimentary Rocks?
To become a sedimentary rock, sediments must be lithified, or cemented into stone. As sediments pile atop other sediments, the earliest deposits become deeply buried. Weight of overlying sediment places pressure on those below. Sediment under pressure compacts. The tighter the sediment is squeezed, the less room there is for water or air between grains and grains to stick together. Something else has to happen to turn these sediments into rock.
Cementation is the word given to the sticking together of sediments by additional material between grains. Water flowing between grains contain silica, carbonate or other dissolved materials that precipitate out of solution and attach to grain surfaces. Eventually, enough mineral precipitates out that it cements grains into a whole.
Sedimentary Rocks are Clues to Past Environments
Sedimentary rocks can tell a geologist much about past environments. These rocks are keys to reading Earth's past. Connecting to the following activity will further develop this idea, Extension: Depositional Environments.
Why do Geologists Need to Identify Sedimentary Rock?
Sedimentary rock is where you find important things like oil, natural gas, coal, salt, potash, iron ore, and other important resources. Geologists also find fossils in sedimentary rock. In order for geologists to know where to find these things, they need to know what types of sedimentary rock to find them in.
- Explain to students what sedimentary rocks are and how they form. Be sure to introduce them to different types of sedimentary rocks, especially sandstone, limestone and shale.
- Divide students into groups of three and give each group a piece of limestone, sandstone, shale or conglomerate.
- Have students examine grain size for each rock. It may be helpful to use a magnifying glass. Have students test the rock to see whether it is a clastic sedimentary rock or a biologic (carbonate) sedimentary rock by putting a drop of weak acid on the rock and seeing whether or not it will fizz. While students are performing each experiment they need to be taking notes describing each outcome.
- Have students discuss and compare what type of rock they think they have based on their tests.
- Troubleshooting: If there is a sandstone, shale or conglomerate that fizzes, it is probably due to a calcium carbonate cement. This does not mean that the rock is a limestone, it just means that the cementing agent is made of calcium carbonate. You may want to test all of your rocks before giving them to students to avoid confusion.
What do geologists look for in sedimentary rock in order to identify them? What does grain size tell them? What does the acid test tell them? What are the two steps that sediments undergo to become sedimentary rock? Why do not all sediments become sedimentary rock? What are some important natural resources that we find in sedimentary rocks?
For more advanced students, you may wish to present them with information about depositional environments. For background information and an activity, go to Extension: Depositional Environments.