Sedimentary Rocks

INTRODUCTION

In this experiment, students will learn about sedimentary rocks, how they are formed, and how they are classified. They will then make pieces of sandstone and conglomerate rocks, and learn to compare and contrast the different types of rock.

MATERIALS

Classification

• Office supplies such as: erasers, pencils, paper clips, pens, rubber bands, paper, post-its, markers, alligator clips (binder clips), tape, ruler, measuring tape, highlighters, etc.
• Sedimentary rocks such as: shale, sandstone, conglomerate, and other sedimentary rocks

Making a Piece of Sandstone

• Dry Sand
• Cementing Solution (2 parts water to 1 part Epsom salt)
• Paper cups

Making a Piece of Conglomerate Rock

• Shoebox
• Garbage bag
• Paper cups
• Sandwich baggies
• Dry Cement
• Dry Sand
• Water
• Rocks

Making a Piece of Limestone

• Shoebox
• Garbage bag
• Paper cups
• Dry plaster
• Water
• Pieces of shells

Once Rocks Are Dry (2-3 days later)

• Magnifying glass

WHAT TO DO

Classification

1. Take bag full of office supplies, all mixed together.
2. Dump contents in front of students and ask how they might classify, or organize, the different supplies.
3. Let the students sort the supplies. Suggest sorting by size, shape, color, use, etc.
4. Explain that geologists have to sort rocks in much the same way. There are several different ways to classify, and geologists, like the students, must decide which way is best to sort first.
5. Show students the sandstone, shale, conglomerate, and other rocks. Explain that these are part of a classification called sedimentary rocks. Sedimentary rocks are most often formed in the presence of water. Ask students what kinds of water might be used. Suggest answers like rivers, lakes, and oceans.

Making a Piece of Sandstone

1. Fill a small paper cup halfway with sand.
2. Slowly add the cementing solution until the sand is wet all the way through.
3. Put the sandstone in a warm place until the top is dry (overnight).
4. Remove sandstone from cup. It will still be wet, but should be dry enough to hold its shape. Leave the sandstone on a paper towel for 2-3 days, or until it is completely dry.

Making a Piece of Conglomerate Rock

1. Line the shoebox with plastic.
2. Add one cup dry cement, one cup dry sand, and one cup cold water. Mix thoroughly with a stick.
3. Add many rocks to the mixture, again mixing thoroughly.
4. Pour into small cups lined with sandwich baggies, one for each child.
5. Place cups in a warm area where they will not be disturbed for two or three days, until the mixture is dry.

Making a Piece of Limestone

1. Line the shoebox with plastic.
2. Add plaster and water. Mix thoroughly with a stick.
3. Add shells and mix together with plaster. This is because limestone is the most common place to find fossils.
4. Pour into small cups, one for each child.
5. Place cups in a warm area where they will not be disturbed for two or three days, until the mixture is dry.

Once Rocks Are Dry (2-3 days later)

1. Students should remove their conglomerate rock from the cup. Remove the plastic from the rock.
2. Students should then remove their limestone from the cup.
3. Have students compare and contrast their hardened sandstone, limestone, and conglomerate rock with a magnifying glass.

QUESTIONS

1. What are rocks made up of? (Minerals)
2. How do we classify different types of rock? Sedimentary rock? (By grain size, what they’re made up of, etc.)
3. What materials make sedimentary rocks? (Silt, clay, sand, gravel, plant materials)
4. What size breaks might we use for each category? (The size breaks used to distinguish different sedimentary rocks are:
   • Shale: fine grain (1/16 mm diameter) sedimentary rock formed by cementing silt and clay. Has laminations and can be split into pieces.
   • Sandstone: a medium-grain rock with grain sizes from 1/16 mm to 2 mm in diameter. This is formed by cementing sand grains together.
   • Conglomerate: A course-grain sedimentary rock with grains coarser than 2 mm. This is formed by cementing rounded gravel.
   • Gravel: rounded rock particles > 2 mm in diameter.
   • Coal: A sedimentary rock formed from the consolidation of plant material.
   • Limestone: precipitated from aqueous environments. Example: limestone is made up of mostly calcium carbonate. It is often precipitated from warm shallow seawaters.
   • Till: A mixture of all possible grain sizes. This mixture is caused by glaciation and is present beneath the topsoil of Ohio.)
   • Topsoil: loose sediment laying above the other forms of sedimentary rocks.
5. Where does the glue (cement) come from in nature? (Solutions of dissolved minerals like calcium carbonate.)
6. What might cause folds (crooked-looking layers) in sedimentary rock? (Earthquakes, tectonic plates shifting, and so forth.)
7. Why do you think there are not layers in limestone? (Limestone is formed by chemical reactions, so it looks like it is all one large piece.)

SUMMARY

• Rocks are composed of tiny bits of minerals. They are formed naturally over millions of years. Sometimes there are natural occurrences such as a river running or wind blowing over rock, which cause the rock to break down. This creates sediment, or small pieces of rock. The sediment may then be cemented together with minerals to become sedimentary rock. This occurs only under huge amounts of pressure. Other sedimentary rocks are different in that they may be formed by chemicals bonding together. In our sandstone experiment, the Epsom salt (a type of mineral) took the place of the mineral deposits found in water that bond the sediment together.
• The different types of sedimentary rock include shale, sandstone, conglomerate, coal, and limestone. Shale is made up of cemented silt and clay, otherwise known as dirt and mud. Sandstone is made of sand and mineral deposits found in the water. Sandstone is often formed by an extinct beach many of the minerals come from the water, and the sand from the old beach. Conglomerate rock is made of cemented round gravel. Coal is from plant material. These sedimentary rocks are formed under great amounts of heat and pressure. Limestone is made of chemicals like calcium carbonate, often precipitated from warm shallow seawater. Limestone is not formed like other sedimentary rocks in that it has no "glue," since it is all chemicals bonded together. These chemicals are calcium and carbonate, which come from things like old shells. Because limestone is formed by chemicals bonding, geologists do not see layers of limestone it all looks like one big piece!
• Generally, newer sedimentary rock is found above older sedimentary rock this is called the Law of Superposition. However, sometimes this is not the case due to folds in rock. These are bends or wavelike features found in rocks, which may be caused by the ground rising, especially during earthquakes.

EXTENSIONS

1. Students may research different uses for sand and sandstone.
2. Measurement of Porosity of Sand
   1. Take a graduated cylinder and measure a known volume of dry sand (50mL).
   2. Place this dry sand in a cup.
   3. Measure a known volume of water (100 mL). Slowly add the water to the sand. Where is it going?
   4. Continue to add the water to the sand until the sand is completely saturated with the water. Do not overfill the sand with water. How much water was used to fill the air spaces between the sand particles? This describes the pore volume of the sand. Groundwater is found in sedimentary deposits like what was just made. As much as 40-50% of the total volume may be filled with water. The porosity is calculated by placing the pore volume over the total volume.

SOURCES

• "Physical Geology." Plummer, McGeary and Carlson, McGraw-Hill, 1999.
• "Rocks and Minerals: Make your own sandstone." Albert, Toni and Ling, George, 1994, Carson-Dellosa Publ, p. 40.
• "Stories in Stone Formation of Sedimentary Rocks." Cuff. Kevin, 1995, Lawrence Hall of Science, University of California Berkeley, p. 65.
• GEOEDGROUP- The Ohio State University

© S. Olesik, WOW Project, Ohio State University, 2000.