Sedimentary Rocks Properties: Geological Processes to Create Sedimentary Rocks

In this paper sedimentary rocks properties including color and text, and then geological processes to create sedimentary rocks will be discussed briefly. Sedimentary rocks are one of three main types of rocks, along with igneous and metamorphic.

They are formed on or near the Earth’s surface from the compression of ocean sediments or other processes. 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.

Sedimentary Rocks Color

The color of a sedimentary rock is often mostly determined by iron. Iron is an element with two major oxides: iron (II) oxide and iron (III) oxide. In fact, iron (II) oxide (FeO) only forms under low oxygen (anoxic) circumstances and gives the rock a grey or greenish color.

Iron (III) oxide (Fe₂O₃) in a richer oxygen environment is often found in the form of the mineral hematite and gives the rock a reddish to brownish color. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange color.

Sedimentary Rocks Organic Material

The presence of organic material can color a rock black or grey. Organic material is formed from dead organisms, mostly plants. Normally, such material eventually decays by oxidation or bacterial activity. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material.

This can, for example, occur at the bottom of deep seas and lakes. There is little water mixing in such environments; as a result, oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay. Dark rocks, rich in organic material, are therefore often shales.

Sedimentary Rocks Properties: Geological Processes to Create Sedimentary Rocks

Sedimentary Rocks Texture

The size, form and orientation of clasts in a sediment is called its texture. Clasts are the original pieces of rock. The texture is a small-scale property of a rock, but determines many of its large-scale properties,

such as the density, porosity or permeability. The 3D orientation of the clasts is called the fabric of the rock. The size and form of clasts can be used to determine the velocity and direction of current in the sedimentary environment that moved the clasts from their origin. Fine, calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water.

Well Sorted or Poorly Sorted Rock

Grain size of a rock is usually expressed with the Wentworth scale, though alternative scales are sometimes used. The grain size can be expressed as a diameter or a volume, and is always an average value, since a rock is composed of clasts with different sizes.

The statistical distribution of grain sizes is different for different rock types and is described in a property called the sorting of the rock. When all clasts are more or less of the same size, the rock is called well-sorted, and when there is a large spread in grain size, the rock is called poorly sorted.

Geological Processes to Create Sedimentary Rocks

The most important geological processes that lead to the creation of sedimentary rocks are erosion, weathering, dissolution, precipitation, and lithification.

Erosion and Weathering

Processes 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

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

Lithification and precipitation 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.

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Sources

en.wikipedia.org/wiki/Sedimentary_rock