Sedimentary rocks are formed on or near the Earth’s surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth. The most important geological processes that lead to the creation of sedimentary rocks are erosion, weathering, dissolution, precipitation, and lithification.

Erosion and weathering include the effects of wind and rain, which slowly break down large rocks into smaller ones. Erosion and weathering transform boulders and even mountains into sediments, such as sand or mud. Dissolution is a form of weathering—chemical weathering. With this process, water that is slightly acidic slowly wears away stone. These three processes create the raw materials for new, sedimentary rocks.

Precipitation and lithification are processes that build new rocks or minerals. Precipitation is the formation of rocks and minerals from chemicals that precipitate from water. For example, as a lake dries up over many thousands of years, it leaves behind mineral deposits; this is what happened in California’s Death Valley. Finally, lithification is the process by which clay, sand, and other sediments on the bottom of the ocean or other bodies of water are slowly compacted into rocks from the weight of overlying sediments.

Sedimentary rocks can be organized into two categories. The first is detrital rock, which comes from the erosion and accumulation of rock fragments, sediment, or other materials—categorized in total as detritus, or debris. The other is chemical rock, produced from the dissolution and precipitation of minerals.

Detritus can be either organic or inorganic. Organic detrital rocks form when parts of plants and animals decay in the ground, leaving behind biological material that is compressed and becomes rock. Coal is a sedimentary rock formed over millions of years from compressed plants. Inorganic detrital rocks, on the other hand, are formed from broken up pieces of other rocks, not from living things. These rocks are often called clastic sedimentary rocks. One of the best-known clastic sedimentary rocks is sandstone. Sandstone is formed from layers of sandy sediment that is compacted and lithified.

Chemical sedimentary rocks can be found in many places, from the ocean to deserts to caves. For instance, most limestone forms at the bottom of the ocean from the precipitation of calcium carbonate and the remains of marine animals with shells. If limestone is found on land, it can be assumed that the area used to be under water. Cave formations are also sedimentary rocks, but they are produced very differently. Stalagmites and stalactites form when water passes through bedrock and picks up calcium and carbonate ions. When the chemical-rich water makes its way into a cave, the water evaporates and leaves behind calcium carbonate on the ceiling, forming a stalactite, or on the floor of the cave, creating a stalagmite.

 

Sedimentary Rocks

An example of a sedimentary rock, which is, by definition, composed of many, smaller rocks.

carbonate
adjective, noun

mineral created by the action of carbon dioxide on a base.

clastic sediment
Noun

rock composed of fragments of older rocks that have been transported from their place of origin.

detrital rock
Noun

sedimentary rock produced from small pieces of other rocks

dissolution
Noun

termination or destruction by breaking down, disrupting, or dispersing

Noun

act in which earth is worn away, often by water, wind, or ice.

geomorphology
Noun

study of geographic features on the landscape and the forces that create them.

halite
Noun

natural mineral form of salt (sodium chloride.) Also called rock salt.

limestone
Noun

type of sedimentary rock mostly made of calcium carbonate from shells and skeletons of marine organisms.

lithify
Verb

to change into stone or rock.

Noun

all forms in which water falls to Earth from the atmosphere.

Noun

solid material transported and deposited by water, ice, and wind.

shale
Noun

type of sedimentary rock.

stalactite
Noun

rock formed by mineral-rich water dripping from the roof of a cave. The water drips, but the mineral remains like an icicle.

stalagmite
Noun

mineral deposit formed on a cave floor, usually by water dripping from above.

Noun

the breaking down or dissolving of the Earth's surface rocks and minerals.