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
• bioclastic
• cementation
• chemical sedimentary rocks
• clastic
• compaction
• lithification
• organic

Introduction

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 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.

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.

Water erodes the land surface in Alaska’s Valley of Ten Thousand Smokes.

Streams carry huge amounts of sediment (Figure below). 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.

A river dumps sediments along its bed and on its banks.

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 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 (see Table below).

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 below.

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 a 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.

Table below shows some common types of sedimentary rocks.

Common Sedimentary Rocks

Picture	Rock Name	Type of Sedimentary Rock
	Conglomerate	Clastic (fragments of non-organic sediments)
	Breccia	Clastic
	Sandstone	Clastic
	Siltstone	Clastic
	Shale	Clastic
	Rock Salt	Chemical precipitate
	Gypsum	Chemical precipitate
	Dolostone	Chemical precipitate
	Limestone	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?

Further Reading / Supplemental Links

• A way to learn about the three rock types and some of the rocks within each type.

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?