sedimentary rocks reading - team 7b science€¦ ·...
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Sedimentary RocksUnit 10
Some rocks form from rock particles Most sedimentary rocks are made from other rock particles called sediments. Sediments are any material that settles out of water or air to form a sedimentary rock. Examples of sediments include rocks, minerals, and the remains of dead plants and animals (Figure 1). A sedimentary rock is a rock made out of sediments. Sedimentary rocks mostly form in layers. You can see this in Bigure 2
Fig. 1:Examples of sedimentary rocks made out of other rocks. (Left) Conglomerate and (right) breccia.
Fig. 2:Sedimentary rocks often form in layers. These can be easily seen in canyons and cliffs. Below is a picture from Canyonlands National Park in Utah.
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Most sedimentary rocks are formed in a 4ive-‐step process In order for a sediments to become a sedimentary rock, they must go through a process. This process helps the particles become glued together to form a sedimentary rock. There are Bive steps to this process. Figure 3 illustrates this process.
Step One: WeatheringWeathering is the process that breaks down rocks and minerals. This creates sediments needed to form a sedimentary rock.
Step Two: ErosionErosion is the process that transports the sediments formed during weathering. Erosion is caused by the water and wind. These carry the particles away. Erosion does not break down rocks into particles. However, erosion can cause weathering.
Step Three: DepositingDeposition is when the sediments are deposited out of the water or wind carrying the sediments. When deposition occurs, bigger and heavier particles settle Birst. These particles will settle into layers.
Step Four: CompactingCompaction is the process that presses the layers together. This causes the spaces between the sediments to become much smaller and disappear. The layers are pushed together by the weight of the layers above.
Step Five: CementingCementation is the process that glues the sediments into a rock. This is caused by minerals crystallizing and gluing the sediments together. This creates a strong sedimentary rock. If this step doesn’t happen, the sedimentary rock is brittle and is easy to break apart.
Fig. 3: This diagram shows steps 2-‐5 of the Give-‐step process. (A) Sediments are eroded (transported) and deposited. (B) Sediments are compacted together. (C) Sediments are cemented together and form a sedimentary rock.
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Sedimentary rocks are classi4ied by the sediment they are made of With so many sedimentary rocks, scientists classify the rocks into groups based on what sediments make up the rock. There are three main groups of sedimentary rock: clastic, organic, and chemical.
Sedimentary rock formed from other rocks is clasticClastic rock is the most common type of sedimentary rock. It is made of broken pieces of other rocks. Common examples include:•Shale made of small grains of clay.•Sandstone (Figure 4) made of grains of sand.•Conglomerate made of rounded rock grains.•Breccia made of jagged rock grains.
Sedimentary rock formed from the remains of plants and animals is organic Organic rock contains a high amount of organic materials. Organic material comes from once living things, like plants and animals. Examples include shells and leaves. The two most common organic rocks are coal and limestone. Coal (Figure 5)is an organic rock made of the remains of plants: wood, bark, leaves, stems, etc. Coal forms through the Bive step process. Dead plant matter is layered, compacted, and glued together. The coal we have today is made of the remains of organisms from millions of years ago! Limestone is an organic rock made up of carbonate minerals, such as calcite. These minerals come from the shells and skeletons of animals. Limestone forms just like coal: the skeletons and shells of dead animals form layers, get compacted, and cemented together. A very common use of limestone is chalk! This type of limestone is called fossilitic limestone (Figure 6, bottom) because it is made of once-‐living things.
Sedimentary rock formed from dissolved minerals is chemical Chemical rocks form when minerals dissolved in a solution crystallize together. This is considered a rock, and not a mineral, because it is not chemically consistent and lacks a crystal structure.
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Fig. 5: Example of an organic rock showing the imprints of the plants that helped make it.
Fig. 4: Picture of banded sandstone. It is made of particles of sand. This makes it a clastic sedimentary rock.
Chemical rocks form in a different process than the Bive-‐step process. To form a chemical rock, minerals must Birst dissolve in water. This is done by water that has carbon dioxide dissolved in it. This is normally called “acid rain.” The second step is when the water evaporates. When this happens, the minerals begin to recrystallize-‐or form again. Many rocks form through this process. The most common chemical rocks are gypsum, rock salt (Figure 7), and limestone. This type of limestone is called oolitic limestone. Oolitic limestone (Figure 6, top) is made of little circles of calcite called oolites.
Sedimentary rocks have many uses Sedimentary rocks are used a lot in construction. For example, limestone and sandstone are used in buildings because they are easy to cut. Rock salt is used to help melt snow and ice on roads. Coal is used as a source of energy and as a way to punish bad children.
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Fig. 6: These are two pictures of limestone. The top picture is oolitic limestone. Notice how it is made of small circles of calcite called oolites. The bottom picture is fossilitic limestone. Notice how it is made of shells.
Fig. 7: This an example of a chemical rock: rock salt. It was formed when halite (salt) was dissolved bywater and later re-‐crystallized to form rocksalt. It is commonly used to de-‐ice roads and melt snow.