Lesson Objectives

  • Describe how sedimentary rocks form.
  • Describe the properties of some common sedimentary rocks.
  • Relate some common uses of sedimentary rocks.
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The White House of the USA is made of a sedimentary rock called sandstone.
The White House (shown in the Figure above) is the official home and workplace of the President of the United States of America. Why do you think the White House is white? If you said, "Because it is made of white rock," you would be only partially correct. Construction for the White House began in 1792. Its outside walls are made of the sedimentary rock, sandstone. This sandstone is very porous, and is easily penetrated by rainwater. Water damage was common in the early days of construction of the building. To stop the water damage, workers covered the sandstone in a mixture of salt, rice, and glue, which help to give the White House its distinctive white color.

Sediments

Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include:
  • fragments of other rocks that often have been worn down into small pieces, such as sand, silt, or clay.
  • organic materials, or the remains of once-living organisms.
  • chemical precipitates, which are materials that get left behind after the water evaporates from a solution.
Rocks at the surface undergo mechanical and chemical weathering. These physical and chemical processes break rock into smaller pieces. Physical weathering simply breaks the rocks apart. Chemical weathering dissolves the less stable minerals. These original elements of the minerals end up in solution and new minerals may form. Sediments are removed and transported by water, wind, ice, or gravity in a process called erosion (Figure below). Much more information about weathering can be found in the Weathering and Formation of Soil chapter. Erosion is described in detail in the Erosion and Deposition chapter.
River eroding volcanic ash flow in Alaska's Valley of Ten Thousand Smokes.
River eroding volcanic ash flow in Alaska's Valley of Ten Thousand Smokes.

Water erodes the land surface in Alaska
Streams carry huge amounts of sediment (see Figure below). The more energy the water has, the larger the size particle it can carry. So a rushing river on a steep slope might carry boulders. As this stream slows down, it no longer has the energy to carry large sediments and so will drop them. A slower moving stream will only carry smaller particles.
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A river dumps sediments along its bed and on its banks.
Sediments are deposited on beaches and in deserts, at the bottom of the ocean and in lakes, ponds, rivers, marshes, and swamps. Avalanches drop large piles of sediment. Glaciers leave large piles of sediments too. Wind can only transport sand and smaller particles. The type of sediment that is deposited will determine the type of sedimentary rock that can form. Different colors of sedimentary rock are determined by the environment where they are deposited. Red rocks form where oxygen is present. Darker sediments form when the environment is oxygen poor.

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.
  1. 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.
  2. 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.
Sedimentary rock sizes and features.|| Rock || Sediment Size || Other Features ||
Conglomerate
Large
Rounded
Breccia
Large
Angular
Sandstone
Sand-sized

Siltstone
Silt-sized, smaller than sand

Shale
Clay-sized, smallest

A cliff made of sandstone is an example of lithification.
A cliff made of sandstone is an example of lithification.

This cliff is made of sandstone. Sands were deposited and then lithified.
When sediments settle out of calmer water, they form horizontal layers. One layer is deposited and then the next on top of it. So each layer is younger than the layer under it. When the sediments harden, the layers are preserved. Sedimentary rocks formed by the crystallization of chemical precipitates are called chemical sedimentary rocks. Dissolved ions in fluids precipitate out of the fluid and settle out, just like halite, as discussed in the Earth's Minerals chapter (Figure below).
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The evaporite, halite, on a cobble from the Dead Sea, Israel.
Biochemical sedimentary rocks form in the ocean or a salt lake. Living creatures remove ions, such as calcium, magnesium, and potassium from the water to make shells or soft tissue. When the organism dies, it sinks to the ocean floor to become biochemical sediment, which may then become compacted and cemented into solid rock (Figure below).
Fossils in a biochemical rock, limestone, in the Carmel Formation in Utah.
Fossils in a biochemical rock, limestone, in the Carmel Formation in Utah.

Fossils in a biochemical rock, limestone, in the Carmel Formation in Utah.
Table below shows some common types of sedimentary rocks.
Common Sedimentary Rocks|| Picture || Rock Name || Type of Sedimentary Rock ||
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Conglomerate
Clastic (fragments of non-organic sediments)
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Breccia
Clastic
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Sandstone
Clastic
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Siltstone
Clastic
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Shale
Clastic
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Rock Salt
Chemical precipitate
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Rock Gypsum
Chemical precipitate
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Dolostone
Chemical precipitate
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Limestone
Bioclastic (sediments from organic materials, or plant or animal remains)
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Coal
Organic

Lesson Summary

  • Weathering and erosion produce sediments. Sediments are transported by water, wind, ice, or gravity.
  • After sediments are deposited, they undergo compaction and/or cementation to become sedimentary rocks.
  • Biochemical sedimentary rocks form as living creatures use ions in water to create shells, bones, or soft tissue and then die and fall to the bottom as sediments.

Review Questions

  1. What are three categories of things that might be part of the sediments in sedimentary rock?
  2. If you see a sedimentary rock outcrop with layers of red sandstone on top of layers of tan sandstone, what do you know about the ages of the two layers?
  3. Why do sedimentary rocks sometimes have layers of different colors?
  4. Describe the two processes necessary for sediments to lithify into sedimentary rock.
  5. How are bioclastic rocks different from clastic rocks? Give an example of a bioclastic rock.
  6. What type of sedimentary rock is coal?
  7. In what environment do you think chemical sedimentary rocks are most likely to form?

Vocabulary

outcrop An exposure of rock at the surface. organic Something from living organisms, made of organic material. lithification The creation of rock from sediments. compaction When sediments are squeezed together by the weight of sediments and rocks on top of them. chemical sedimentary rocks Rocks that form from the hardening of chemical precipitates. cementation When fluids deposit ions to create a cement that hardens loose sediments. bioclastic Sedimentary rock that forms from pieces of living organisms. biochemical sedimentary rocks Rocks that form from materials created by living organisms removing ions from water and falling to the bottom to become sediments.

Points to Consider

  • Is a rock always made of minerals? Do the requirements for something to be a mineral need to be met for something to be a rock?
  • Which type of rocks do you think yield the most information about Earth’s past?
  • Could a younger layer of sedimentary rock ever be found under an older layer? How do you think this could happen?
  • Could a sedimentary rock form only by compaction from intense pressure?