Sedimentary rocks...
Formation of Sedimentary Rocks
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
These are the different processes in order:
sedimentation → compaction → cementation
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
These are the different processes in order:
sedimentation → compaction → cementation
What are they like?
Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are:
The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed.
Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are:
- chalk
- limestone
- sandstone
- shale
The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed.
FUN FACT: This skeleton of a Jurassic dinosaur from China could be the oldest known bird, scientists report.
Do you know how fossils are formed? watch this video to find out...
characteristics of sedimentary rocks
There are 3 different types of sedimentary rocks:
Clastic Sedimentary Rocks
The characteristics are summarised in the picture below:
- Clastic
- Chemical
- Organic
Clastic Sedimentary Rocks
- Have layers like a stack of pages or books
- The grains do not interlock because they did not crystallise and grow together in the rock
- May contain fossils, such as shells
The characteristics are summarised in the picture below:
Clastic sedimentary rocks are classified mainly according to the size and nature of the rock grains and chemical composition of the cement. You can see some in the photos below:
Chemical Sedimentary Rocks
- Most chemical sedimentary rocks have crystals in them
- Soft, this helps to distinguish them from harder igneous rocks
- Rare to see any layering in chemical sedimentary rocks
- Occasionally contains fossils
Organic Sedimentary Rocks
- Can be layered, depending on the way in which fossils and sediments are deposited
- Usually soft, although a few such as chert (also called flint) can be fairly hard
Chapter 8.3: Sedimentary rocks quizlet
questions
- How do different rocks look different from each other?
- Can you name any famous Rock structures?
- Only one of these rocks is a sedimentary rock. Which one?
b- Slate
c- Granite
4. Where are the oldest layers of rock usually found in a cliff made from sedimentary rock?
a- At the top
b- In the middle
c- At the bottom
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