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Making Sedimentary Rocks!

Summary:
Students make a model of sedimentary rock layers to understand how rocks form layers and represent ancient environments. Materials:

For each group:

  • Sand (1-2 cups)
  • Gravel (1-2 cups)
  • Soil with the sticks and leaves sifted out or very fine sand/silt (1-2 cup)
  • Crushed white chalk (1 cup)
  • Empty half-gallon milk carton with the top opened up
  • Optional: Seashells or shell fragments, small fish bones
  • Plaster of Paris (about 4 cups mixed)
  • Water
  • 2 large disposable cups
  • A disposable spoon or fork
  • Fine sand paper
  • Clean up supplies (towels, tarp, newspaper, plastic drop cloths, etc.)
  • Optional: Hand lenses, plastic knives, (metal picks and rock hammers if your school allows)

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Source:
Windows to the Universe staff member Lisa Gardiner based this activity on one by teacher/naturalist Edith Sisson (author of Nature with Children of All Ages).
Grade level:
5-9
Time:
50 minutes or several class periods
Student Learning Outcomes:
  • Students will understand that sedimentary rocks form in layers over time.
  • Students will understand that different types of sedimentary rock layers represent different environments.
  • Students will understand that changes in the types of sedimentary rock layers mean that there were changes in environments, often because of climate change/sea level change.
Lesson format:
Hands on activity or demonstration with participation

National Standards Addressed:

DIRECTIONS:

  1. Introduction
    1. Review what a sedimentary rock is. Review common types of sedimentary rocks (sandstone, conglomerate, shale and limestone).
    2. Have students stack papers on their desk. Ask them which paper got there first (A: the one on the bottom). Sedimentary rocks form in the same way, in layers, with the older ones at the bottom.
    3. Tell class that during this project they will simulate (or model) what happens over hundreds of thousands to millions of years as sedimentary rocks are formed in layers in different environments.
    4. Discuss what a model is. (Examples of models: model airplane, dolls, dinosaur model, video games)
  2. Divide students into groups of about 4.
  3. For each of the environments in the table below (river, beach, shallow and deep ocean):
    1. Have students describe from their experience what the environment is like. What sorts of things do they think they would see there?
    2. After describing an environment, have student groups choose which of the materials they would include in their milk carton to represent that environment (these items are listed in the second and third columns of the table).
    3. Have students fill one of their cups about 2/3 full of the appropriate sediment and associated fossils.
    4. Mix plaster with water according to manufacturers directions. Have each student group fill the remainder of their cup with plaster and stir. Explain that this is much faster than rocks are actually made. The plaster acts like the cement that holds real sedimentary rocks together.
    5. Have each group put sediment mixed with plaster into their milk carton and pat it down to form a flat layer.
    6. Start the next environment in the table by the same process. Make sure that student groups do not mix different layers or shake their milk carton. Mix plaster in small batches (one for each environment) to avoid it drying too quickly. For the limestone layer, mix plaster a little more watery than usual because chalk will absorb water. The plaster of the first layer does not need to be dry before adding the next. If it is really soupy, sprinkle a little dry plaster on the top before adding the next layer.
  4. After plaster has dried (about 20 minutes), take the layers of sedimentary rock out of the milk carton. (You may need to rip the milk carton off!)
  5. Have student groups rub it lightly with very fine sand paper and draw what the layers of "rock" look like in their notebook (noting colors, textures, and other features in the margins of their picture). Show them images of real rock layers from places like the Grand Canyon, southern Utah, or something closer to home.

ASSESSMENT:

  • If your class has already covered types of sedimentary rocks, ask students to identify the types of sedimentary rocks present in their model, even though they are not real.
  • Ask students to recall which types of environments each rock type represents. If the environment in this one spot changed over time from a river to a beach to a shallow ocean to a deep ocean what must have happened? Sea level rise!
  • Extension: Have students be paleontologists and dig for fossils in the layers of rock. Where would you expect to find the most clamshell fossils? Fish fossils? Use picks, chisels and small hammers to find them.

EXTENSIONS:

Have students be paleontologists and dig for fossils in the layers of rock. Where would you expect to find the most clamshell fossils? Fish fossils? Use picks, chisels and small hammers to find them.

BACKGROUND INFORMATION:

Sea level changes can be caused when either the land level sinks (called subsidence) or when the water level rises, or when both processes are happening together. Water level can rise because glaciers melt, adding water to the oceans, or when plate tectonic movements shallow the ocean basins displacing water onto the edges of continents. It is a natural process that has gone on since there have been oceans on Earth!

This activity works best when students have already reviewed types of sedimentary rocks (conglomerate, sandstone, shale, and limestone). Note that the same rock types can form in several different environments. This is a good topic of discussion, especially if students recognize that the soil is potting soil found on land. Shale that forms in swampy floodplain areas can look very much like shale that is from the ocean floor or even shale from a lake bottom. Fossils are a good way to tell the difference. Similarly, sand dunes formed in the desert are made out of sandstone just like the beach sand (and not all beaches are made of sand). One must be a detective to figure out what past environments were like!

For a shorter demonstration version of this activity, omit the plaster and milk cartons and tell students the story of changing environments as you add layers of sediment and "fossils" to a rectangular fish tank (or any container that you can see through). They are able to see the layers right away, although the connection to sedimentary rocks might be more of a challenge.

Table of Environments:

Environment Type of Sediment Biological remains you might find there Rock type produced
River Pebbles
fish, plants
conglomerate rock
Beach
Sandy shells, seaweed sandstone
Bottom of the shallow ocean Silt/mud shells fish shale
Bottom of the deep ocean Crushed white chalk few shells, fish limestone

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