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Sedimentary Rocks Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us Sedimentary Rocks Grade Level: 9 – 12 Purpose: The purpose of this lesson is to introduce sedimentary rocks. Students will learn what sedimentary rocks are and how they form. It will teach them how to identify some common examples. Suggested Goals: Students will shake a flask of sand and water to see how particles settle. They will draw a picture showing where the various rocks form and they will identify some common examples. Targeted Objectives: All of Illinois is covered by sedimentary rocks. In most places, they reach a thickness that could be expressed in miles rather than feet. Ancient seas dropped untold numbers of particles for hundreds of millions of years creating the layers that bury the original igneous rocks in 2,000 to 17,000 feet of sedimentary layers. These valuable layers are used to construct our homes, build our highways, and some were even used to heat our homes. The Illinois Basin dominates the geology of southern Illinois. There the layers dip down until they are over three miles thick. The geology of Illinois cannot be told without discussing sedimentary rocks because they are what Illinois is made of. Students will learn to tell the difference between the major sedimentary rock varieties. Students will learn in what environments different sedimentary rocks form. Students will learn what sedimentary rocks are. Background: Most sedimentary rocks are formed when weathering crumbles the parent rock to such a small size that they can be carried by wind or water. Those particles suspended in water collide with one another countless times gradually becoming smaller and more rounded. When water is moving quickly due to flooding or due to a rapid change in elevation, larger particles can be carried by the streams and rivers but when the water slows down the particles begin to settle out. The particles dropped from water and wind are called sediment. As layers become thicker, the bottom particles get squeezed closer and closer together under the crushing weight. Many of the organisms that live in the oceans have shells and skeletons that are made of calcium carbonate. Their decay releases a natural cement into the water. The cement glues the particles of rock back together forming sedimentary rock. People who live in homes with hard water see the amazing affect of this calcium carbonate glue in their showerheads as the holes gradually become closed until the spray is reduced to a trickle. The variety of sedimentary rock often depends on the size of the particles that make up the rock. Conglomerate, sandstone, and shale form from different sized particles. Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us Conglomerate usually forms in places with fast moving water. Finding conglomerate made of large particles often suggests that when they were formed there were mountains nearby. Conglomerate means a variety and that is a fitting name because there are usually large and small particle intermingled within the rock. Sandstone forms where the flow is slower. Sand grains bounce down streams without much difficulty so even slow moving streams can carry vast amounts of sand. Shale is made from silt (dirt) in the water. The particles are so small that they can stay suspended in water for quite a while even after the river or stream has stopped flowing. The silt gradually settles to the bottom. Limestone and Dolomite do not form from particles washed from the land. They usually form on the ocean bottom far out to sea where no particles from the land ever reach. Limestone is made from an infinity of microscopic skeletons that rain down on the sea floor. In some cases the shells of larger organisms such as brachiopods are found, too. Limestone is almost totally calcium carbonate. Dolomite is similar to limestone except that it forms in the ocean in places that have magnesium dissolved in the water. It often forms from limestone as the calcium in calcite is partially replaced by magnesium. Materials and Preparation: Several 500 ml flasks (or larger) filled with 250 ml of sandbox (river) sand and then filled to 400 ml with water. Tightly seal each with a cork. A long jump pit is a good place to find the sand. Containers to catch any water that might spill from the flasks Typing paper Colored pencils Collections of limestone, sandstone, shale, and conglomerate. If possible, have enough collections so that there can be groups of two or three. Magnifiers Procedure: Pass out the flasks of sand created as described above and give each group a container for spills; in case there is an accident. Have the students shake the flasks quickly back and forth and then turn them while the water is still churning so that the cork end is down. Stop shaking. Have the students observe the sand and answer the following questions. Discussion: 1. Where are the larger particles and why? Near the bottom because they could not stay suspended in the water very long when the shaking stopped. Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us 2. Are there just large pieces at the bottom? No the large pieces are surrounded by hundreds of grains of sand. 3. As you look higher in the flask what is happening to the size of the sand grains? They are getting smaller. 4. Did all of the particles settle out right away? No the water remained cloudy and a thin layer of silt is being deposited on top. 5. When a river enters the ocean, the water slows down suddenly just like when you stopped shaking the flask. Which particles would you expect to drop closest to the mouth of the river? The largest ones. 6. If you were to sample the sand near a rivers mouth and then take another sample father out to sea, what would you expect to find? Farther out the sand particles would be smaller. 7. What would settle out even farther that the sand? Silt (mud) Preparation And Procedure: Draw the following diagram on the board and have your students copy it onto a piece of typing paper. Have them label and color it for a homework assignment. As you draw the diagram point out why each particle forms where it does. Gravels are close to shore because they are too large to stay suspended. Sometimes gravel does not occur if the stream flow is too slow. Sand is the next particle to settle out because it is the next largest but the sand grains become smaller and smaller the farther they are from shore. Mud is next to settle out because the tiny particles of silt can stay suspended for a very long time, like they did in the flask, after the water has stopped moving. The material farthest out forms from shells because no material from the land can make it far out to sea. This is where limestone forms. The only things that settle on the bottom far out to sea are those from the sea itself such as shells and dissolved minerals. Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us Discussion: 1. How would sedimentation be different in an environment where a very slow moving river enters the ocean? The first particles to settle out would be sand or mud. Where there was a fast current? The first particles would be larger. 2. None of the beaches in Florida have gravels. What is the major reason? Florida is very flat. The highest point is only 345 feet and most of the state is much lower. Florida streams are very slow moving so sand particles are the largest particles that they can carry. 3. What property of sedimentary rocks is used to identify most of them? The size of the particles 4. Which sedimentary rock is NOT identified by its particle size? Limestone, its identification is based on the chemical that it is made of which is calcium carbonate. Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us Activity Identification of Sedimentary Rocks Materials Collections of sedimentary rocks Magnifiers A Drawing from the previous activity for each group Weak hydrochloric acid or vinegar in acid bottles CAUTION…If you are diluting strong hydrochloric acid be sure to add the acid to water and NOT water to acid. Have your students wear safety goggles if they are using hydrochloric acid. Preparation and Procedure: Prepare several collections of rocks so that each contains a specimen of sandstone, limestone, shale, and conglomerate. Make enough so that you will have enough sets for groups of two or three students. Hint…Small baskets, boxes, or baggies work well and can be kept from year to year. Purchase dilute hydrochloric acid and place it in acid bottles with droppers. If you are mixing the acid from concentrate, be sure to add the acid slowly to water and not water to the acid. Wear gloves, long sleeves, and safety glasses while doing the mixing. If you are using vinegar, make sure before the lab that the limestone specimens you are using will bubble when a drop of vinegar is placed on them. Some do not! Have your students use the descriptions on their lab sheet to identify each specimen. Have the students use one of their groups drawings for this activity. Ask your students to place each variety of rock on the section of their group’s drawing where it would occur. A piece of conglomerate will be placed near the shoreline where it appears on the picture and so forth.
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