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Sedimentary Rocks

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Sedimentary Rocks

Sedimentary rocks are very complex rocks that give us information on our past. Distinguishable characteristics such as color, size, and rounding of the sediments tell scientists about the environment that was once there long ago. How are the sediments formed to preserve this archive of our past? The system of how a rock is formed from sediment is complex and can occur without even knowing. The first step is the sediment. Sediments are tiny particles that are shaved off of rocks, soil, and other solid objects by a fluid source. These sources can include wind, water, and ice. In order to break down this process, the types of sediments must be introduced.
Clastic Sediment
Clastic sediment is formed from rock and mineral debris as a result of weathering and erosion. The clasts come in a variety of sizes and this is how they are classified. The biggest of the clasts is gravel. Gravel is classified as sediment with pea-sized or larger particles. When gravel becomes compacted and forms a rock, it is known as a conglomerate. The next type of clast is sand. Sand sediments are smaller than gravel, usually the size of a pinhead. When sand hardens, it forms a rock known as sandstone. Getting even smaller is silt. Silt is around the size of a grain of salt. Silt can form into siltstone once compacted and hardened. Finally, there is clay. Clay can be the size of flour particles, or even smaller. Mudstone or shale is the result of rock formation. Another type of sediment is known as volcaniclastic sediment. This sediment is different from all the others because its origin is from a volcano. When a volcano erupts it emits various sizes of hot rock. Some argue to say that these are igneous rocks. That may be true up until they come out of the volcano.

Chemical Sediment
Chemical sediments are the result of dissolved chemicals in water that find a way to a lake or a sea and precipitate from them. Plants or animals living in water can change the balance of chemicals in the water. Limestone is formed from an excessive amount of carbon dioxide dissolved in the water. This causes calcium carbonate to precipitate which in turn gives you limestone. Evaporation can also cause chemical sediments. When the evaporation rate becomes more than the input of new water the water level may become shallow which allows salts dissolved in the water to precipitate as solids.

Biogenic sediment
Remains of plants and animals will give you a result of biogenic sediments. Shells, bones, wood, and leaves are examples of material left behind. Limestone and coal can be formed from this type of sediment. Yes, limestone can be either chemical or biogenic. The calcium from skeletons from marine life can be hardened and this gives you the limestone.

Now that we know the types of sediments we can talk about how they are formed. As sediments are continuously eroded and weathered away from previous rocks, the sediments become buried. It then undergoes a process known as lithification. Lithification is a group of processes that transforms sediments into sedimentary rock. During the first part of lithification the sediments become buried layer after layer and start to compact. Cementation can occur when water evaporates leaves chemicals that precipitate, cementing the particles together. Recrystallization can also occur. Limestone is a common example of this. Calcium crystals in reefs change from aragonite to calcite. This makes the rock more stable and in turn makes crystals that were separate, together. The types of sediments will also give you the different types of sedimentary rock. As discussed earlier, clasts will give you different types of rocks: conglomerate, sandstone, siltstone, and shale. Chemical rocks often are a result from evaporation. Biogenic rock is formed from the lithification of animal and plant matter.
Characteristics of rocks
Not all the rocks can look the same otherwise we would only have one type of sedimentary rock on our planet. How do scientists tell the rock types apart from each other? The location of the sedimentary rock plays a big role in how we classify the rocks. You would more likely find a conglomerate rock on a mountain than you would on a beach. This is due to the fineness of the sediments found at each location. Gravel is more common on a mountain than on a beach, where you will more than likely find sand.…...

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