sedimentary rock | Definition, Formation, Examples, & Facts | senshido.info
This activity on determining age of rocks and fossils is intended for 8th or 9th grade Explore this link for additional information on the topics covered in this lesson: 5) To use radiometric dating and the principles of determining relative age to Principle of superposition: Younger sedimentary rocks are deposited on top of. Through careful observation over the past few centuries, geologists have discovered that the accumulation of sediments and sedimentary rocks takes place. In order for sedimentary rock to form, the following processes must happen: To retake the quiz, reload the page and then select "no" when the "Resume Quiz".
Thus, we can use the texture of the resulting deposits to give us clues to the mode of transport and deposition. Sorting - The degree of uniformity of grain size. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density. If the particles have the same density, then the heavier particles will also be larger, so the sorting will take place on the basis of size.Laws of Relative Rock Dating
Sorting gives clues to the energy conditions of the transporting medium from which the sediment was deposited. Examples Beach deposits and wind blown deposits generally show good sorting because the energy of the transporting medium is usually constant.
Rounding - During the transportation process, grains may be reduced in size due to abrasion.
Random abrasion results in the eventual rounding off of the sharp corners and edges of grains. Thus, rounding of grains gives us clues to the amount of time a sediment has been in the transportation cycle. Rounding is classified on relative terms as well.
Sediment Maturity Sediment Maturity refers to the length of time that the sediment has been in the sedimentary cycle. Texturally mature sediment is sediment that is well rounded, as rounding increases with transport distance and time and well sorted as sorting gets better as larger clasts are left behind and smaller clasts are carried away.
Because the weathering processes continues during sediment transport, mineral grains that are unstable near the surface become less common as the distance of transport or time in the cycle increases.
Thus compositionally mature sediment is composed of only the most stable minerals. For example a poorly sediment containing glassy angular volcanic fragments, olivine crystals and plagioclase is texturally immature because the fragments are angular, indicating they have not been transported very far and the sediment is poorly sorted, indicating that little time has been involved in separating larger fragments from smaller fragments.
It is compositionally immature because it contains unstable glass along with minerals that are not very stable near the surface - olivine and plagioclase. On the other hand a well sorted beach sand consisting mainly of well rounded quartz grains is texturally mature because the grains are rounded, indicating a long time in the transportation cycle, and the sediment is well sorted, also indicative of the long time required to separate the coarser grained material and finer grained material from the sand.
The beach sand is compositionally mature because it is made up only of quartz which is very stable at the earth's surface. Types of Clastic Sedimentary Rocks We next look at various clastic sedimentary rocks that result from lithification of sediment. Conglomerates and Breccias Conglomerate and Breccia are rocks that contain an abundance of coarse grained clasts pebbles, cobbles, or boulders.
Geochronology | Earth science | senshido.info
In a conglomerate, the coarse grained clasts are well rounded, indicating that they spent considerable time in the transportation process and were ultimately deposited in a high energy environment capable of carrying the large clasts.
In a breccia, the coarse grained clasts are very angular, indicating the the clasts spent little time in the transportation cycle. Sandstones A Sandstone is made of sand-sized particles and forms in many different depositional settings.
Texture and composition permit historic interpretation of the transport and depositional cycle and sometimes allows determination of the source.
Quartz is, by far, the dominant mineral in sandstones. Still there are other varieties. An Arkose contains abundant feldspar. In a lithic sandstone, the grains are mostly small rock fragments.
Sandstones are one of the most common types of sedimentary rocks. Mudrocks Mudrocks are made of fine grained clasts silt and clay sized. A siltstone is one variety that consists of silt-sized fragments. A shale is composed of clay sized particles and is a rock that tends to break into thin flat fragments See figure 7. A mudstone is similar to a shale, but does not break into thin flat fragments. Organic-rich shales are the source of petroleum. Fine grained clastics are deposited in non-agitated water, calm water, where there is little energy to continue to transport the small grains.
Thus mudrocks form in deep water ocean basins and lakes. It can form as the result of deposition of grains from moving bodies of water or windfrom the melting of glacial iceand from the downslope slumping sliding of rock and soil masses in response to gravity, as well as by precipitation of the dissolved products of weathering under the conditions of low temperature and pressure that prevail at or near the surface of the Earth.
Sedimentary rocks are the lithified equivalents of sediments. They typically are produced by cementing, compacting, and otherwise solidifying preexisting unconsolidated sediments. Some varieties of sedimentary rock, however, are precipitated directly into their solid sedimentary form and exhibit no intervening existence as sediment.
Organic reefs and bedded evaporites are examples of such rocks. Because the processes of physical mechanical weathering and chemical weathering are significantly different, they generate markedly distinct products and two fundamentally different kinds of sediment and sedimentary rock: Sediments composed of weathered rock lithify to form sedimentary rock, which then becomes metamorphic rock under the pressure of Earth's crust.
When tectonic forces thrust sedimentary and metamorphic rocks into the hot mantle, they may melt and be ejected as magma, which cools to form igneous, or magmatic, rock. Created and produced by QA International. These clasts are transported by gravity, mudflows, running water, glaciers, and wind and eventually are deposited in various settings e.
Because the agents of transportation commonly sort out discrete particles by clast size, terrigenous clastic sedimentary rocks are further subdivided on the basis of average clast diameter.
Coarse pebbles, cobbles, and boulder-size gravels lithify to form conglomerate and breccia; sand becomes sandstone; and silt and clay form siltstone, claystone, mudrock, and shale. Chemical sedimentary rocks form by chemical and organic reprecipitation of the dissolved products of chemical weathering that are removed from the weathering site.
It gets even more fascinating to think about that we can actually see and walk on former ocean floor that looks amazing in many places on earth.
I have some pictures from such a place further down in the article. And then, when the ocean floor becomes rocks on land the weathering starts again. It is like an ongoing transportation of particles that never ends. Weathering I think you all know what weathering is but I include the definition anyway. Weathering occurs when a rock is fragmented by mechanical forces or are decomposed by chemical alteration.
It is only broken it down in smaller pieces. The end result is many small pieces from a single large one. Chemical weathering means that the rock goes through a transformation chemically into one or more new compounds.
Since water is a great solvent water is a major force in chemical weathering. But rocks are also weathered in other ways such as through dissolution, oxidation and hydrolysis that occurs in water.