What steps led to this rock formation?
What do you see? The rock is a sandstone, so first there were rocks that weathered and eroded. The cross-bedding indicates that the sand was deposited in a dune. The sand was then buried deeply enough that it turned into rock. This lesson will explore how something like sand could become a rock.
Sedimentary Rock Formation
Accumulated sediments harden into rock by lithification, as illustrated in the Figure below. Two important steps are needed for sediments to lithify.
- Sediments are squeezed together by the weight of overlying sediments on top of them. This is called compaction. Cemented, non-organic sediments become clastic rocks. If organic material is included, they are bioclastic rocks.
- Fluids fill in the spaces between the loose particles of sediment and crystallize to create a rock by cementation.
The sediment size in clastic sedimentary rocks varies greatly (see Table in Sedimentary Rocks III: Classification).
This cliff is made of sandstone. Sands were deposited and then lithified.
bioclastic: Sedimentary rock that forms from pieces of living organisms.
cementation: When fluids deposit ions to create a cement that hardens loose sediments.
clastic: Contains fragments or clasts of preexisting rock; sedimentary rock made of clasts.
compaction: When sediments are squeezed together by the weight of sediments and rocks on top of them.
lithification: The creation of rock from sediments.
- Sedimentary rocks are made of fragments of older rocks or pieces of organisms.
- Compaction and cementation lead to lithification of sedimentary rocks.
- Compaction is the squeezing of sediments by the weight of the rocks and sediments above them. Cementation is when cement from fluids bind sediments together.
Use this resource to answer the questions that follow.
1. Explain how sedimentary rocks form.
2. What is a conglomerate?
3. Explain how limestone forms.
4. Why are sedimentary rocks important?
1. How does compaction lead to lithification?
2. How does cementation lead to lithification?
3. What is the difference between clastic and bioclastic sedimentary rocks?