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Earth & Space Science: Session 2

A Closer Look: Sedimentary Rocks

What is sedimentary rock?

Have you ever looked at an outcropping of rock and noticed that it’s made of bands of different colors and/or textures? Those layers that you’re seeing are the telltale sign that you’ve found sedimentary rock. Sedimentary rock covers most of the Earth’s surface. The layers in sedimentary rocks are formed from the weathered products of other rocks (igneous, metamorphic, or sedimentary rocks). These products are rock fragments called sediment. Sediment can range in size from the tiniest particles of clay to boulders. Sedimentary rock forms at or near the Earth's surface at relatively low temperatures and pressures, and usually in water.

How does sedimentary rock form?

sedimentary model
Model of the sedimentary process of deposition.

Deposition
Sediments that form sedimentary rocks can be transported by several mechanisms that include wind, ice, and, most commonly, moving water. As the energy of water currents decreases — as when a river or stream empties into a lake or ocean — the sediments settle out of the water due to gravity. The heavier and larger particles, such as gravel and sand, settle first. The lighter sediments, such as silt and clay, settle last. This laying down of sediments by natural processes is called deposition.

Compaction, Recrystallization, and Cementation
The deposited sediments gradually accumulate, forming layers. The weight of the overlying sediment compacts the sediment below. This compaction squeezes together the layers of sediment, forcing the grains together, making the pore spaces around each grain smaller, and squeezing out some of the water. The remaining water surrounding the sediments can contain dissolved minerals, which later recrystallize as new minerals in the pore spaces. The crystals interlock and connect the sediment grains, which essentially "glues" the sediment together. This forms solid rock. Further compaction and burial can cause additional recrystallization, making the rock even harder. Calcite and silica are common minerals that cement individual sediments together. Rocks formed in this way are called clastic sedimentary rocks because the rock is composed of “clasts” or individual sediments.

Chemical Processes
Sediments that have precipitated out of solution form chemical sedimentary rocks. Sedimentary rocks formed by the evaporation of sea or lake water are called evaporites. As water evaporates, the concentration of the salt solids (salt meaning soluble compounds, not just the familiar sodium chloride commonly known as table salt) increases and the amount of water decreases. Halite and gypsum are examples of evaporites.

Biological Processes
Biological processes can form sedimentary rocks in which the layers are formed from the remains of plants and animals. In warmer, shallower ocean water, layers of limestone can form from the accumulation of the calcium carbonate that composes the shells and skeletons of coral, shellfish, and other marine organisms. In colder, deeper waters, layers of sediment can form from the remains of microorganisms called foraminiferans and diatoms, which secrete shells of calcium carbonate or silica. Rocks formed in this way are called biochemical sedimentary rocks.

Biological processes also form sedimentary rocks from the accumulation and compaction of dead plant material, typically under wet, acidic conditions in regions with an abundance of growing vegetation. The vegetation proliferates so quickly that new layers of vegetation rapidly bury the dead and decaying plant material. When buried like this, the bacteria use up the oxygen that is available and cannot finish the decomposition of the vegetation. This matter may change to solid carbon in the form of coal, or be converted to hydrocarbons, the source of petroleum oil.

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