Are you ready to learn all about rocks and the rock cycle in your classroom this year? If so, it’s important to know all about the three main types of rocks: igneous, sedimentary, and metamorphic. In this post, we will take a closer look at sedimentary rocks. Get ready to learn all about the formation, subtypes, properties, and examples of sedimentary rocks!
For Teachers: If you’re an educator planning to teach rocks, great visuals and activities are key to deepening understanding! Try my sedimentary rock printables, also found within my complete Rocks Unit – a complete set of all the printable worksheets, activity directions, and visuals you’ll need (plus you’ll support this blog with your purchase! ❤️)
How do sedimentary rocks form?
Sedimentary rocks form at or near the Earth’s surface. They form from deposits of sediments, minerals, and/or organic matter.
On the Earth, forces like water and shifting temperatures work endlessly to break rocks down. The tiny broken-down pieces of rocks are called sediments. Some of the minerals in rocks may also dissolve in water, the same way sugar dissolves in tea. We call the dissolved minerals solutes.
Sediments and solutes move and are deposited in new places through a process called erosion. As sediments build up, something special happens. Sediments compact together under their own weight or are cemented together by crystallizing minerals. This forms new rocks, known as sedimentary.
Properties of Sedimentary Rocks
There are many different sedimentary rocks, and they do not all look the same. Soft mudstones, rough sandstones, and even coal are all sedimentary! Their varied appearances are influenced by several properties including their mineral makeup and grain (sediment particle) size. Let’s learn more about some of the properties of sedimentary rocks…
Sedimentary Grain Sizes
You already know that many sedimentary rocks are made from compacted and cemented sediments. The size of those sediments determines the type of sedimentary rock. Mudstones are made from the tiniest clay particles that are so small you cannot see them. Sandstones are made from little grains of sand, which you can see with your naked eye if you look at them up close. Conglomerate and breccia are made from the largest sediments – like gravel and small pebbles.
Geologists call the size of the particles in a rock the “grain size”. Grain size is important because it impacts the porosity and permeability of a rock. Very porous rocks like sandstone and conglomerate make good aquifers that store water underground. Do you know where your drinking water comes from? If you drink well water, then your drinking water comes from an aquifer!
Sedimentary Composition
Sedimentary rocks contain compacted and cemented grains, crystals of mineral precipitates, or sometimes organic materials. However, each rock has its own special blend of minerals.
Some sedimentary chemical rocks (those made from recrystallized solutes) contain carbonates. Carbonates are minerals that react with acid – just like how baking soda reacts with vinegar. Geology students find out if a rock contains carbonates by placing a drop of acid on the rock and watching for bubbling or fizzing. Rocks that fizz or bubble in a reaction with acid contain carbonates.
Other sedimentary rocks contain organic carbon and consequently burn. Geologists test for organic matter by attempting to combust a rock. The organic matter in the rock will burn in a combustion reaction – allowing the geologist to see how much of the rock was organic.
Sedimentary Subtypes
There are several subtypes of sedimentary: clastic, chemical, organic, and biochemical. They are formed from different materials. Let’s learn a little more about each.
Sedimentary Clastic Rocks
Sedimentary clastic rocks, such as sandstone and siltstone, form from tiny solid particles of rock called sediments. Sediments form through a process called weathering, in which rocks break down over time into smaller pieces. These pieces move around thanks to forces like flowing water, wind, and gravity, eventually coming together to form new rocks.
But how do loose sediments become a solid rock? Two processes, compaction and cementation, are responsible. Compaction happens when sediments are buried and pressed into a rock by the weight above. Cementation happens when water evaporates, leaving behind minerals that act like cement, holding the sediments together. Clastic rocks vary in their grain sizes, from microscopic particles in mudstone to large gravel pieces in breccia and conglomerates.
Sedimentary Chemical Rocks
Have you ever stirred sugar into water until it disappears? Although you cannot see the sugar, it is still there, dissolved in the water. Like sugar, many minerals dissolve in water too! Most liquid water on Earth contains dissolved minerals.
Now perhaps you are wondering: how does water make rocks? Imagine an ancient ocean, with water rich in dissolved minerals. As time passes, the ocean begins to shrink, eventually drying up completely. When the water evaporates, the minerals from the water recrystallize to form sedimentary chemical rocks. This process, called precipitation, is the opposite of dissolving. Sedimentary chemical rocks are crystalline but often look fine-grained. Some are made of carbonate and react strongly to acid. Rock salt and limestone are two examples of sedimentary chemical rocks.
Sedimentary Organic Biochemical Rocks
Sedimentary organic and biochemical rocks are similar to sedimentary clastic rocks. However, they aren’t composed of sediments but rather form from buried plant and animal matter. Through compaction and cementation, the matter eventually becomes rock. Examples include coal, formed from compressed plant matter, and coquina, made of compacted and cemented shell fragments. These rocks can tell us fascinating stories about life on Earth in prehistoric times!
Examples of sedimentary rocks
Here are some examples of common sedimentary rocks. If you’re studying rock identification or creating a rock collection, you’ll want to make sure to examine the following rocks:
Clastic Rocks
- Mudstone
- Siltstone
- Sandstone
- Conglomerate
- Breccia
Chemical Rocks
- Limestone
- Rock salt
- Chert
- Ironstone
Biochemical and Organic Rocks
- Fossiliferous Limestone
- Coquina
- Diatomite
- Anthracite
- Coal
What’s Next?
For everyone: there’s more to learn about rocks! Take a closer look at igneous rocks. Or, continue reading more about the rock cycle or the difference between rocks and minerals.
For educators: check out my posts on how to set up a rock ID lab activity, directions for a fun rock ID review game, and a list of rock and mineral activity ideas that you can try in your classroom!
Study this topic with Wild Earth Lab!
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References and Further Reading
- Earle, S. (2019). Physical Geology. BCcampus Open Education. (Chapters 2-7). Available: https://opentextbc.ca/physicalgeology2ed/
- Johnson, C., Affolter, M.D., Inkenbrandt, P., & Mosher, C. (2017). An Introduction to Geology. Salt Lake Community College. (Chapters 3-6). Available: https://opengeology.org/textbook/
- Jones, C. E. (n.d.). Igneous Textures. University of Pittsburg. Department of Geology and Planetary Science. Available: https://sites.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures.html
- Plummer C.C., Carlson D.H., & Hammersley L. (2019). Physical Geology. McGraw-Hill Education. (Chapters 1-7).