Download Sedimentary rocks are formed when pieces of pre

Sedimentary rocks are formed when pieces of pre-existing rock or parts of once-living organisms accumulate in deposits on the Earth's
surface. There are three main types of sedimentary rocks. They are clastics, biological, and chemical sedimentary rocks. Each category of
sedimentary rock has distinct characteristics and depositional environments. In addition, the way the Earth's plates move, or plate tectonics, can
significantly affect sedimentation and the formation of sedimentary rocks.
Clastic, or terrigenous, sedimentary rocks are those rocks that are formed by the accumulation of pieces of pre-existing rocks. They are
often deposited in layers or bedding. The pieces of pre-existing rock, or clasts, can range in size from clay to boulders and the clasts are cemented
together with some sort of matrix. The fundamental basis for classifying terrigenous rocks is the clast size and based on this classification,
sedimentary rocks are classified as shale, siltstone, or sandstone. Shales are basically lithified mud and represent the smallest particle size. With
sandstone, the sand sized particles are dominant and are classified further by particles, matrix and cement. Siltstones are classified in between shales
and sandstones. The depositional environment would be any basin where the sediment that has eroded from pre-existing rock can accumulate. This
could include river systems, lakes, deserts, and glaciers, as well as beaches, deltas, estuaries, and tidal flats (Middleton, et al., 2003).
Biological (or organic) sedimentary rocks are those that form when the remains of living things accumulate, become compacted and
lithified (Boggs, 1995). Ancient plant deposits become coal, and deposits of animal shells may become limestone or coquina. The two dominant
minerals in carbonate rocks such as limestone are calcite and dolomite. The deposition of plant material to form coal usually happens in swamps or
closed fluvial basins (Middleton, et al, 2003). These rocks are precipitated within a basin of deposition.
Chemical sedimentary rocks are those that are formed by chemical precipitation. These rocks include carbonate rocks such as limestone
(both a biological and chemical sedimentary rock) and evaporites such as halite and gypsum (Boggs, 1995). Evaporites are formed when water with a
high mineral content (such as sodium chloride) evaporates in either a closed or open system and the minerals precipitate out of solution and are
deposited (Middleton, et al., 2003). Deposits of rock salt (halite) can be up to 1,000 meters thick. These rocks, as are the biological sedimentary
rocks are precipitated within a basin of deposition.
Plate tectonics not only affects sedimentation and the formation of sedimentary rocks, but it is used to describe how rocks move through
the rock cycle to be igneous, sedimentary, and metamorphic rocks. Tectonics affects sedimentation in general by either the subsidence of the crust or
the uplift of the land (Middleton, et al., 2003).
At convergent plate boundaries, plates are moving toward each other and one plate is subducted beneath the other. As the plate that is
being subducted moves beneath the other plate, it is taking with it the accumulated sediment and rocks. The subducted rocks and sediments are
heated to the point that they melt to become magma and eventually, igneous rock. In some areas where subduction is occurring, not all of the
subducted plate is actually subducted. These special cases refer to ophiolites. Sequences of rock from the subducted plate are actually shoved up
onto the over-riding plate. An additional way that sedimentation and sedimentary rocks are affected by plate tectonics at convergent boundaries is with
the intrusion of magma into the country rock of the overlying plate. The intrusion causes sedimentary rock to be exposed to heat and pressure and
may change the sedimentary rock into metamorphic rock (Boggs, 1995).
At divergent boundaries such as ocean spreading ridges, sedimentation is also affected. In theory, sediments are laid down in horizontal
beds and at any place in that bedding layer, the sediments are the same age. At divergent plate boundaries, because the plates are moving away
from each other, new plate material is coming up at the boundary. Therefore, the youngest sediments would be found closest to the plate boundary
(Boggs, 1995).
Boggs, Sam Jr., 1995. Principles of Sedimentology and Stratigraphy, Second Edition. Prentice Hall: Englewood Cliffs.
Middleton, G., et al. 2003. Encyclopedia of Sediments and Sedimentary Rocks. Springer