4.13: Lithification of Sedimentary Rocks
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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 concept will explore how something like sand could become a rock.
Sedimentary Rock Formation
There are two main types of sedimentary rocks: clastic and chemical. Clastic rocks are made of sediments, which are sometimes called clasts. Creating rocks from sediments is called lithification. Chemicals precipitate from liquid to form chemical sedimentary rocks.
Over time, deposited sediments may harden into rock. First, the sediments are compacted. That is, they are squeezed together by the weight of sediments on top of them. Next, the sediments are cemented together. Minerals fill in the spaces between the loose sediment particles. These cementing minerals come from the water that moves through the sediments. Sedimentary rocks that form from sediments are called “clastic rocks.“ Clastic rocks are rock fragments that are compacted and cemented together (Figure below).
Conglomerates have larger clasts than sandstone, which have larger clasts than mudstone.
Chemical Sedimentary Rocks
Chemical sedimentary rocks form when crystals precipitate out from a liquid. The mineral halite, also called rock salt, forms this way. You can make halite! Leave a shallow dish of salt-water out in the Sun. As the water evaporates, salt crystals form in the dish. There are other chemical sedimentary rocks, like gypsum (Figure below).
Gypsum, with its rose-like structure, is a chemical sedimentary rock.
- Compaction and cementation lead to lithification of sedimentary rocks.
- Sediments are compacted by the weight of the rocks and sediments above them.
- Sediments are cemented by fluids that bind sediments together.
- Minerals precipitate to form sedimentary rocks.
- How does compaction lead to lithification?
- How does cementation lead to lithification?
- How do chemical sedimentary rocks form?
Use this resource to answer the questions that follow.
- What is the source of the water in this stream?
- What do you think a poorly sorted sediment looks like?
- What do you think a bimodal sediment looks like? Note that clasts is another name for a rock or rock fragment.
- Why would a sediment become well rounded?
- What is precipitating around the clasts?
- How do the rocks stick together to undergo lithification?
- How can rocks form at surface temperatures and pressures?