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PLATE TECTONICS AND SEDIMENTARY ROCK
Plate Tectonics and the Formation of Sedimentary Rock
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PLATE TECTONICS AND SEDIMENTARY ROCK
Abstract
Plate tectonics play an impressive role in the formation of sedimentary rocks. The
movement of tectonic plates provides a landscape for the deposition of sediment. Furthermore,
plate movement which results in the formation of igneous and metamorphic rock provides
precursory material for sedimentary rock. This material results from weathering and is then
compacted into a sedimentary layer. The process of lithification completes the formation of
sedimentary rock into its clastic, chemical, or biogenic subtypes. The three subtypes of
sedimentary rock are identified by differences in appearance and texture.
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PLATE TECTONICS AND SEDIMENTARY ROCK
Sedimentary rocks comprise nearly 75% of all rocks on the Earth’s surface. Sediment is
generated from existing igneous and metamorphic rocks that have been weathered, eroded, and
broken down. Upon the deposition of sediment on the earth’s surface, it undergoes lithification
to form new rock. At a convergent plate boundary where plates are moving toward one another,
one plate moves beneath the other plate. As a result of this process, any sediment or rocks on the
surface are taken with the place and heated to become igneous rock. Plate tectonics can also
influence sedimentation if at a convergent boundary, where magma is introduced into the rock of
the superior plate. In this instance, the sedimentary rock is transformed into metamorphic rock
by heat and pressure. At a divergent plate boundary, where plates are moving away from one
another, new bedding would be found closest to the plate boundary, while older bedding would
be found at positions more distant from the boundary (Prothero & Schwab, 2004).
Sedimentary rock formation begins with the weathering process. During this processes,
the existing rock is exposed to the elements, such as wind and water. As a result of tectonic plate
movement, new landscapes are formed and weathered fragments are deposited on these surfaces
(Murck, Skinner, & Mackenzie, 2010). Over a period of time, the exposure of these rocks to the
elements results in the formation of sediments. Sediments are also created from the breakdown
of dead or dying organisms. Following the process of weathering, sediments are transported in
wind, glaciers, and bodies of water to other places across the earth. The formation of a
sedimentary layer is induced by gravity, surface topography, and other external forces such as
fluid flow. Over the course of many years, mineral, chemical, and biogenic sedimentary deposits
accumulate to form bedding (Prothero & Schwab, 2004).
The process of lithification begins with the collection of sediment on a surface.
Lithification results from multiple processes (Murck, et al., 2010). The high pressures that result
from the initial sedimentary layer being buried by either the deposition of additional sediment or
by tectonic plate movement lead to compaction. Compaction reduces the spaces within a layer
of sediment. Lithification is also achieved through cementation and recrystallization. Through
the process of cementation, any water contained within the sediment is expelled and is cause to
migrate up. As this water leaves the sediment and is cooled, ions dissolved within the water
form mineral precipitates. These mineral precipitates then function to bind the grains of
sediment together. Recrystallization results from the high pressures exerted on the sedimentary
layer. Through this process, any unstable minerals rearrange into more stable forms. At the end
of the lithification process, the sediment becomes hardened, solid, sedimentary rock (Prothero &
Schwab, 2004).
There are three main categories of sedimentary rocks (Murck, et al., 2010). Clastic
sedimentary rocks are composed of clasts, or broken fragments of other rocks. Clasts, which
work as the building blocks for sedimentary rock can be boulders, cobbles, pebbles, sand, silt, or
clay. Clastic sediment is derived from rocks and mineral debris as a result of weathering and
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PLATE TECTONICS AND SEDIMENTARY ROCK
erosion. Volcanistic sediment is a subcategory of clastic sediment, and is derived from volcanic
eruptions. The four classes of clastic sedimentary rock are conglomerate, sandstone, siltstone
and mudstone, which are derived from gravel, sand, silt, and mud, respectively. Given that
clastic sediments typically collect in low-lying areas, the formation of clastic sedimentary rock is
immensely influenced by tectonic plate movement (Prothero & Schwab, 2004).
Chemical sedimentary rocks are composed of mineral crystals formed by chemical
processes. Chemical sediment results from the precipitation of dissolved matter from sea or lake
water. Therefore, chemical sedimentary rocks are a result of the lithification of chemical
sediments. Most chemical sedimentary rocks are composed of only one mineral, and are called
monomineralic (Prothero & Schwab, 2004). Typically calcite, dolomite, gypsum, and halite are
the primary minerals that comprise a chemical sedimentary rock. Biogenic sedimentary rocks are
formed from the remains of living things, such as skeletons and plants. Limestones are an
example of biogenic sedimentary rocks.
The subtypes of sedimentary rock can be distinguished based on appearance (Prothero &
Schwab, 2004). Clastic sedimentary rock tends to have a grainy appearance. Chemical
sedimentary rocks tend to have a banded appearance. These rocks may also have a salty taste or
texture because sea water is involved in their formation. Chemical sedimentary rocks appear
crystalline in structure and may also contain quartz. Lastly, biogenic rocks formed from plant
remnants are typically dark colored rocks. Furthermore, biogenic rocks may appear brittle,
chalky, or soft. The variety of material properties and textures that result from these different
types of sedimentary rock provide resources for human use such as building material, fuel, and
fertilizers.
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PLATE TECTONICS AND SEDIMENTARY ROCK
References
Murck, B., Skinner, B., & Mackenzie, D. (2010). Visualizing Geology (2nd ed., pp. 212-243).
Hoboken, NJ: Wiley.
Prothero, D. R., & Schwab, F. (Eds.). (2004). Sedimentary Geology: An Introduction to
Sedimentary Rocks and Stratigraphy (2nd ed.). New York, NY: W. H. Freeman and
Company.
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