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Compaction in Geology: Definition & Examples

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Sedimentation, the final stage in the formation of sedimentary rocks, occurs in two steps. In this lesson, we will discuss the first of these steps, compaction, as well as look at some examples of rocks that are formed from this process.

Sedimentary Rocks

Though you can't see it, much of Earth's surface is made of rock. And by far the most common type of rock we find there is sedimentary rock. As the name implies, these rocks are composed of sediments that have been pressed and glued together. Sedimentary rocks cover a whopping two-thirds of the Earth's surface! They're also important story-tellers of Earth's history because they contain remnants of other organisms and rocks.

Sedimentary rocks take a LONG time to form, and there are four stages involved in this process. These include weathering, erosion, deposition, and sedimentation. While each step of the process is just as important as the others, this lesson is going to focus on sedimentation, the phase during which sediments are transformed into rock, and specifically, compaction. Ready to get started?

Two Steps of Sedimentation

During sedimentary rock formation, sediments are deposited horizontally. Each layer is added on top of the one below it
sedimentary rock layers

Sedimentation is the final step in the process of sedimentary rock creation. Before sedimentation can occur, sediments are produced from rocks through weathering and erosion, and are then deposited, or laid down, in horizontal layers. As new layers are added, and more sediment accumulates on top of old sediment the transformation into rock begins!

The first step is called compaction. The weight of the sediment above pushes down on the sediment below and compacts it, pushing the sediment particles tightly together. In between the sediment particles is water that gets pushed out as the particles get pushed together. I mean, where else is it going to go?

As this water is pushed out from in between the sediment particles, we begin the next step of sedimentation, called cementation. This water is special because it contains dissolved compounds that can fill the tiny spaces between the sediment particles and, combined with the water, glue those particles together just like cement!

Types of Sedimentary Rock

Bryce Canyon is an example of iron oxide in sedimentary rock. This dissolved compound creates a beautiful red/orange color in the rock
bryce canyon

The types of particles being glued together influence the type of sedimentary rock produced. For example, iron oxide 'cement' produces beautiful orange rocks like we see in Bryce Canyon National Park. Limestone is a type of sedimentary rock made of calcium carbonate from shells, corals, and fragments of other marine organisms. Much of the Rocky Mountains is composed of different types of sedimentary rocks, such as sandstone, dolomite, and shale. And you might be surprised to learn that coal is a sedimentary rock that forms from organic material such as dead plant and animal matter.

This limestone rock is made from the skeletons and fragments of marine organisms but also contains some larger marine particles and fossils
limestone sediments

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