New York State High School Regents Exam Prep|
What are Sedimentary Rocks?
Sedimentary rocks are the result of the weathering process on earth. Most sedimentary rocks are composed of fragments or particles which strongly resemble sediments. Most are formed by the compaction and cementing of sediments. These sediments are deposited in the oceans and usually have parallel layers. These layers are very helpful in identifying sedimentary rocks. Some other characteristics that sedimentary rocks have are rounded particle, fossils and a dull (earthy) appearance. The chart on page 7 of your reference tables will help you understand more about sedimentary rocks. You should use this chart everytime that a questions involves a sedimentary rock.
Groups of Sedimentary Rocks
Clastic (fragmental) sedimentary rocks- are made up of different sized particles or sediments that have been compressed and/or cemented together. These rocks are classified by the size of their sediments or grains. Shale has the smallest particle (clay) and conglomerate is a mixture of all the different particle sizes. Look at the chart on page 7 in your reference tables to find the specific sizes.
The diagram below shows how sediments turn into sedimentary rock after they have gone through compaction and cementation. Based on the size of the particles shown in the diagram this sample is a conglomerate.
Chemical sedimentary rocks- are formed from the evaporation of seawater or chemical activity. They are also called evaporties. The particles or minerals that form these rocks precipitate (settle) to the bottom of the ocean. Rock salt is the most common chemical sedimentary rock.
Organic sedimentary rocks- are formed from the accumulation of plant and animal remains. This group of rocks is usually rich in fossils. Some examples of this group of rocks are, coal, which is made from plant remains and fossil limestone, which is made from cemented shell fragments and skeletal remains.
New York State High School Regents Exam Prep Center: Earth Science