(RM) Sedimentary Rocks Lesson
Sedimentary Rock
Sedimentary rocks are formed from particles (sediment) deposited by wind and water. Sediment is the small, solid pieces of material that come from rocks or living things. Sediment is formed from the weathering and erosion of Earth materials. These pieces are transported by wind, water and even ice and deposited in layers elsewhere.
Erosion
Heat, cold, rain, waves and ice grinding against the surface all make up Earth's destructive forces. These forces break up and wear away all the rocks on Earth's surface. Erosion occurs when running water or wind loosen and carry away the fragments of rock.
Deposition
Deposition is the geological process by which sediments, soil, and rocks are added to a landform or land mass. Wind, ice, and water, as well as sediment flowing via gravity, transport previously eroded sediment. As this sediment slows down it will begin building up layers of sediment. Sediment can also include shells, bones, leaves, stems and other remains of living things. As remains of living things are added to sediment and left there over time, the sediment may slowly harden and change into fossils trapped in the rock.
Compaction
In the summer months, countless numbers of children dig in the sand, on beach shores, to build sandcastles. Children learn very quickly that filling a pail or mold with sand is not sufficient for building. The sand will fill the pail loosely; falling apart easily once the pail or mold is spilled. In order to make the sand hold together and maintain the shape of the mold, the sand must be pressed down with more layers added on top, and water mixed in to hold the grains together.
The phase known as compaction functions much the same way. Over millions of years, thick layers of sediments build up and are pressed down heavier and heavier upon the layers beneath. The process that presses sediments together is compaction.
Cementation
The process in which dissolved minerals crystallize and glue particles of sediment together is cementation. As the sediment is being compacted the minerals slowly dissolve in water and seep into the tiniest of spaces between particles of sediment. Over millions of years compaction and cementation transform the loose sediment into sedimentary rock.
Sedimentary Rock Types
Sedimentary rocks are classified into one of three groups:
- clastic
- organic
- chemical
Clastic Rocks
Weathering processes break down rocks into fragments of pebble, sand, or clay particles by exposure to wind, ice, and water. These fragments are squeezed together to form clastic sedimentary rock. Clastic sedimentary rock is most commonly made up of quartz. Clastic and nonelastic sedimentary rocks are the only members of the rock family that contain fossils as well as indicators of the climate (ripple marks, mud cracks and raindrops) that was present when the rock was formed.
Clastic sedimentary rocks are named according to the grain size of the sediment particles.
Conglomerate = coarse (64 mm to >256 mm), rounded grains
Breccia = coarse (2mm to 64 mm), angular grains
Sandstone = grains ranging in size from 2mm to 1/16 mm
Shale = grains are small and the rock is smooth and splits easily
Organic Rocks (Bioclastic)
Organic Rocks form from the accumulation and lithification of plant or animal debris. Bio = life and clastic = fragments. These rocks include coal and limestones when formed from the accumulation of shell, coral, algai and fecal debris.
The organic material may be disseminated throughout the rock giving it a uniform dark color, and/or may be present as discrete occurrences of tar, bitumen, asphalt, petroleum, coal or carbonaceous material. Organic-rich sedimentary rocks may act as source rocks which generate hydrocarbons that accumulate in other sedimentary "reservoir" rocks.
Chemical Rocks
As the name implies, chemical sedimentary rocks form due to chemical reactions. Chemical rock forms when minerals that are dissolved in a solution having positively and negatively charged atoms forming ionic and covalent bonds as they crystallize. Halite and gypsum are examples of chemical rocks.
Putting sedimentary rock to use
Sandstone was used in the building of one of the most famous houses in the United States; the White House in Washington, D.C. Sandstone and limestone can be used in building and for decorative purposes. Cement is made from limestone too.
Sedimentary rocks are deposited in layers, usually underwater, with the oldest layers on the bottom and the youngest layers on the top.
Sedimentary rocks are the only type of rocks that contain fossils. Fossils give us a great deal of information. They tell us about the environment when the sediments were deposited. For example, if we find coral fossils in the rock, it was most likely deposited in sea water because corals live in marine environments.
If we find dinosaur fossils we look at the type of dinosaur fossils as well as plant pollens and other paleontological clues. This helps us to determine that the rock was deposited on or near land because most dinosaurs were terrestrial animals that walked on land.
The beauty of sedimentary rocks is that their strata are full of clues to what the past world was like. Those clues might be fossils or sedimentary structures such as marks left by water currents, mud cracks or more subtle features seen under the microscope or in the lab.
From these clues we know that most sedimentary rocks are of marine origin, usually forming in shallow seas. But some sedimentary rocks formed on land: clastic rocks made on the bottoms of large freshwater lakes or as accumulations of desert sand, organic rocks in peat bogs or lake beds, and evaporites in playas. These are called continental or terrigenous (land-formed) sedimentary rocks.
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