There are three main types of rocks: sedimentary, igneous, and metamorphic. Each type of rock has its own characteristics and properties, are part of the rock cycle, and can be found all over the world. Sedimentary rocks are those formed from sediment and cover nearly 80% of the Earth's land area, but they only make up about 5% of the Earth's crust. What does this mean? It means the rocks are usually found close to or on top of the Earth's surface where deposits of sediment tend to accumulate.
Sediment is the pieces of rock, sand, dead animals, plants, microorganisms and other matter that falls to the bottom of oceans and lakes. Other inorganic chemicals may also be included. Eventually, the sediment is compressed and then it is gradually transformed or changed and becomes sedimentary rock.
Sediment is also the materials in the air that are wind-blown or transported by water and deposited on the surface of the earth. Once settled and collected together, those sediments too will transform into sedimentary rocks. The study of sedimentary rocks is called sedimentology and the geologist who studies them is called a sedimentologist.
Layers and layers of sedimentary rock appear in cliffs which have been deposited over time. The cliffs can be seen near waterways or in the middle of deserts or other land areas.
The different layers were squeezed and compressed over time and become solid in a process called consolidation. Note the word solid is part of the word. Cementation then occurs when dissolved mineral components deposit in the spaces between the different layers of sediment. In short, the sediment becomes stuck together forming the rocks that eventually become layered. Of course, this does not happen overnight and may take millions of years.
There are many quarries of unconsolidated deposits where sand and pebbles are removed for use by the construction industry. The sand may be millions of years old. Consolidated sand, however, can be transformed into one type of very hard sedimentary rock called sandstone. All sedimentary rocks can be further changed by water, heat, or extreme pressure.
Sedimentary rocks can be classified by several kinds based on three different processes related to how the sediments are formed. The three processes include solids swept down from the land, then through transportation, wind and water move weathered and eroded pieces away from the rock. Second, wind and water put the sand grains down in a process called deposition as the bits and pieces are laid down and deposited in water, or pieces of shell or other materials sink to the bottom. Finally, chemicals in a solution may come from precipitation.
There are many, many kinds of sedimentary rocks. The three basic types include clastic, which are formed from mechanical weathering debris; chemical, dissolved materials from precipitation; and organic form from the accumulation of plant and animal debris.
Some of the most common sedimentary rocks include shale, sandstone, conglomerate, limestone, iron ore, dolomites, coal, halite, flint, chert, breccia, geode, gypsum, and many others.
It is the sedimentary rock that formed the Grand Canyon a very long time ago. The layers of sediment were once deposited in warm shallow seas and over millions of years, they compacted to form rock. The wind and rain eroded, and the result was the Grand Canyon.
Finally, as with all three major types of rocks, the various types go through many changes during the rock cycle and some sedimentary rocks may become igneous or metamorphic rocks.
Sedimentary Rocks Facts
Sedimentary Rock Examples
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Metamorphic Rocks Facts
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