5.4: Sediments and Sedimentary Rocks

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Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts
VIVA, the Virginia Library Consortium

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Sediments are turned into sedimentary rocks only when they are buried by other sediments to depths in excess of several hundreds of meters. Sediments are derived from the weathering, or physical and chemical breakdown of pre-existing rock exposed at (or very near) the surface. Extrusive igneous rocks enter into the weathering cycle as soon as they harden however, intrusive igneous rocks form at depths of several hundreds of meters to several tens of kilometers. Most metamorphic rocks are also formed at great depths. These rocks cannot begin the weathering cycle until they are exposed at the surface by various processes of plate tectonic mountain building. Steven Earle. From: https://opentextbc.ca/physicalgeology2ed/part/chapter-6-sediments-and-sedimentary-rocks/ is licensed under: Creative Commons Attribution 4.0 International License — Figure \(\PageIndex{1}\): The Rock Cycle (Creative Commons Attribution 4.0 International License; Steven Earle, https://opentextbc.ca/physicalgeology2ed/.)

Sedimentary rock formation is a cycle unto itself and is discussed in more detail elsewhere in this text. The flow through that cycle appears below.

The steps in the Sedimentary Rock Cycle. By: Callan Bentley — Figure \(\PageIndex{2}\): The steps in the Sedimentary Rock Cycle. (Callan Bentley (2020))

The cycle begins with the exposure of pre-existing rock to the “agents” of weathering and erosion. Weathering involves exposure of pre-existing rock to Earth’s surface weather conditions. These “agents” include wind, water and ice — plus gravity. Erosion is the removal of weathered rock material from its original location. The effects of weathering on pre-existing rock can be both physical and chemical.

Physical Weathering

Physical weathering is simply the process of physical disintegration: the breaking of solid rock into smaller pieces. The “agents” assist in this process which results in the exposure of an ever increasing number of surfaces to additional weathering.

Chemical Weathering

Chemically weathered surface (red-brown) of an igneous diabase (mafic, shallow intrusive) from Virginia. The chipped surface (black) shows the original color of the unweathered diabase. Credit: Callan Bentley — Figure \(\PageIndex{4}\): Chemically weathered surface (red-brown) of an igneous diabase (mafic, shallow intrusive) from Virginia. The chipped surface (black) shows the original color of the unweathered diabase. (Callan Bentley)

Chemical weathering produces changes in the chemistry of minerals in the pre-existing rock. These changes may result in new mineral formation through the addition or subtraction of elements from the original mineral structure or, in the further disintegration of the pre-existing minerals by chemical decomposition. The dominant agent involved in both physical and chemical weathering is water. Water acts as an agent of change, both introducing elements that can interfere with the chemistry of the exposed minerals and also acting as a flux, physically removing weathered rock material and ions lost through decomposition.

Weathering produces particles of sediment (of various sizes) plus ions in water. The sediment is transported to some setting where it gets deposited. This depositional location may be very close to the origin of the sediment (such as a rock fall off a cliff face) or many hundreds (or even several thousand) of kilometers away (for example the beach along the continental shoreline). Following deposition, the sediment is buried, compacted, and cemented together to form solid rock. We call these processes “diagenesis,” and collectively they serve to lithify the sediment into a rock.

The ions in solution (weathered chemical elements) can be:

used in cementing particles sediment.
extracted from the water to support life, both biochemical processes (in plants or animals) and in the formation of skeletal material, such as shells.
precipitated from the water through supersaturation triggered by evaporation or by changing physical conditions of the surrounding water.

The importance of these processes to Historical Geology are discussed in much more detail elsewhere in this text. The simple classification of sedimentary rock is presented below.