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Sedimentary Rocks
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Printed: September 23, 2012
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C ONCEPT
Concept 1. Sedimentary Rocks
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Sedimentary Rocks
Lesson Objectives
• Describe how sedimentary rocks form.
• Describe the properties of some common sedimentary rocks.
• Relate some common uses of sedimentary rocks.
Vocabulary
biochemical sedimentary rocks
Rocks that form from materials created by living organisms removing ions from water and falling to the bottom to
become sediments.
bioclastic
Sedimentary rock that forms from pieces of living organisms.
cementation
When fluids deposit ions to create a cement that hardens loose sediments.
chemical sedimentary rocks
Rocks that form from the hardening of chemical precipitates.
clastic
Fragments or clasts of preexisting rock; a sedimentary rock made of clasts.
compaction
When sediments are squeezed together by the weight of sediments and rocks on top of them.
lithification
The creation of rock from sediments.
organic
Something from living organisms.
Introduction
The White House (shown in the Figure 1.1) 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 answered, “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 for 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.
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FIGURE 1.1
The White House of the USA is made of a
sedimentary rock called sandstone.
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 1.2). 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.
Streams carry huge amounts of sediment (Figure 1.3). The more energy the water has, the larger the particle it can
carry. A rushing river on a steep slope might be able to carry boulders. As this stream slows down, it no longer has
the energy to carry large sediments and will drop them. A slower moving stream will only carry smaller particles.
Sediments are deposited on beaches and deserts, at the bottom of oceans, 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 1.4. Two important steps are
needed for sediments to lithify.
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Concept 1. Sedimentary Rocks
FIGURE 1.2
Water erodes the land surface in Alaska’s
Valley of Ten Thousand Smokes
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 (see Table 1.1).
TABLE 1.1: Sedimentary rock sizes and features.
Rock
Conglomerate
Breccia
Sandstone
Siltstone
Shale
Sediment Size
Large
Large
Sand-sized
Silt-sized, smaller than sand
Clay-sized, smallest
Other Features
Rounded
Angular
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FIGURE 1.3
A river dumps sediments along its bed
and on its banks.
FIGURE 1.4
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 first, and another
layer is deposited on top of it. So each layer is younger than the layer beneath it. When the sediments harden, the
layers are preserved. Sedimentary rocks formed by the crystallization of chemical precipitates are called chemical
sedimentary rocks. As discussed in the “Earth’s Minerals” chapter, dissolved ions in fluids precipitate out of the
fluid and settle out, just like the halite in Figure 1.5.
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 a biochemical sediment, which may then become compacted and cemented into solid rock
(Figure 1.6).
Table 1.2 shows some common types of sedimentary rocks.
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Concept 1. Sedimentary Rocks
FIGURE 1.5
The evaporite, halite, on a cobble from the Dead Sea, Israel.
FIGURE 1.6
Fossils in a biochemical rock, limestone,
in the Carmel Formation in Utah.
TABLE 1.2: Common Sedimentary Rocks
Picture
Rock Name
Conglomerate
Type of Sedimentary Rock
Clastic (fragments of non-organic
sediments)
Breccia
Clastic
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TABLE 1.2: (continued)
Picture
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Rock Name
Sandstone
Type of Sedimentary Rock
Clastic
Siltstone
Clastic
Shale
Clastic
Rock Salt
Chemical precipitate
Rock Gypsum
Chemical precipitate
Dolostone
Chemical precipitate
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Concept 1. Sedimentary Rocks
TABLE 1.2: (continued)
Picture
Rock Name
Limestone
Type of Sedimentary Rock
Bioclastic (sediments from organic
materials, or plant or animal remains)
Coal
Organic
Uses of Sedimentary Rocks
Sedimentary rocks are used as building stones, although they are not as hard as igneous or metamorphic rocks.
Sedimentary rocks are used in construction. Sand and gravel are used to make concrete; they are also used in
asphalt. Many economically valuable resources come from sedimentary rocks. Iron ore and aluminum are two
examples.
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 when living creatures using ions in water to create shells, bones, or soft
tissue 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?
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Further Reading / Supplemental Links
• A way to learn about the three rock types and some of the rocks within each type: http://geology.com/rocks/
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?
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