Model the Rock Cycle with Crayons

Background Information for Teachers

The rock cycle describes the cycling of matter on and within Earth, which results in the three main types of rocks: sedimentary, metamorphic, and igneous (Figure 1).

• Sedimentary rocks are formed when small particles (like sand and dust) are compressed together in layers.
• Metamorphic rocks are formed when existing rocks are changed by heat and pressure.
• Igneous rocks are formed when other types of rocks melt, forming magma, and then re-cool into solid rocks.

Image CreditImage Credit: Wikimedia user Siim / Creative Commons Attribution Share-Alike 3.0 UnportedImage Credit: Wikimedia user Wilson44691 / Public domain
Figure 1. From left to right, examples of the three types of rock: a sedimentary rock, a metamorphic rock, and an igneous rock. Note that while distinct layers are clearly visible in the sedimentary and metamorphic rocks pictured here, that might not always be the case.

Each type of rock can turn into other types of rock through various processes within Earth or on Earth's surface (Figure 2). For example:

• Rocks can be broken into smaller pieces by weathering and erosion. These pieces accumulate to form layers (deposition), and are gradually compressed (lithification) to form sedimentary rocks.
• Rocks can be heated and compressed by the high temperatures and pressures under Earth's surface in a process called metamorphism. This deforms them and turns them into metamorphic rocks.
• Rocks can be melted by the high temperatures beneath Earth's surface, forming magma. When magma cools—either when it crystallizes below Earth's surface, or when it emerges through a volcano and is exposed to water or air—it forms igneous rocks.

Image Credit: Wikimedia commons user Actualist / Creative Commons Attribution Share-Alike 3.0 Unported

rock cycle diagram showing igneous rock, metamorphic rock, sedimentary rock, and magma with the following transitions: All three types of rocks can turn into magma by melting; Magma turns into igneous rock by crystallizing; Igneous and metamorphic rocks turn into sedimentary rock by erosion, deposition, and lithification; Igneous and sedimentary rocks turn into metamorphic rocks by metamorphosis

Figure 2. The rock cycle, showing how sedimentary, metamorphic, and igneous rocks can each turn into the other types. (Wikimedia Commons user Actualist, 2013. CC BY-SA 3.0).

In this lesson plan, your students will develop a model for the rock cycle using crayons. First they will use a sharp kitchen utensil (like a knife, potato peeler, or cheese grater) to make crayon shavings. These crayon shavings can be compressed in layers to form a sedimentary "rock". From there, students will explore how they can turn their sedimentary rock into other types of rocks, and whether they can change the other types of rock back. They will develop analogies for how processes that they use in the classroom (like pressing the crayon shavings together or heating them on a hot plate) relate to processes that occur within Earth, such as lithification (the process in which sediments compact under pressure) or heating from Earth's core.

Additional Background Links

• The Rock Cycle, Minerology4Kids
• Three Types of Rock, American Museum of Natural History
• Plate Tectonics, Geography4Kids

Prep Work (15 minutes)

Optional: You could use a blender to create bulk crayon bits/shavings in advance of the lesson; however, this may dull the blender blades and be difficult to clean. Make sure you keep each color of crayon shavings separate.

Engage (5 minutes)

Note: Students should be familiar with topics like weathering and erosion and plate tectonics before you begin this lesson.

1. Show your students examples of the three different types of rocks using this Rock Type Slideshow. Note that the slideshow shows "obvious" examples to make the distinction clear (e.g. sedimentary and metamorphic rocks with clearly distinct, different-colored layers); however, that is not necessarily the case for all rocks.

   Based on the pictures and what you know about Earth, do you have any ideas about how these different types of rocks are formed?

   Do you think one type of rock can turn into a different type of rock? How?

   Listen to students' ideas about how the rocks are formed, but do not give away any answers about the rock cycle yet.

2. Explain that in this lesson, we will use crayons to model the different types of rocks.

   How do you think we can use crayons to make these different types of "rocks"? Could you turn one type of crayon "rock" into another?

   Encourage students to share their ideas with their groups. Again, do not give any answers away yet.

Explore (45 minutes)

1. Break the students up into groups. Walk the groups through the process of making a sedimentary rock (Figure 3).
   1. Use a knife, vegetable peeler, or cheese grater to make crayon shavings from at least two different-colored crayons.
   2. Make at least 1 Tbsp. of each color. Keep them in separate bowls.
   3. Pile the shavings in layers on a piece of aluminum foil.
   4. Fold the piece of aluminum foil over on top of the layers.
   5. Press down hard on the aluminum foil for at least one minute, using a hard and heavy object like a pan. You can also put it on the floor and stand on it, but be careful not to rip the aluminum foil.
   6. Unfold the aluminum foil and carefully remove your sedimentary "rock." If it is crumbly and falls apart easily, you need to press on it harder.

Image Credit: Ben Finio, Science Buddies / Science BuddiesImage Credit: Ben Finio, Science Buddies / Science Buddies

Image Credit: Ben Finio, Science Buddies / Science BuddiesImage Credit: Ben Finio, Science Buddies / Science Buddies

Image Credit: Ben Finio, Science Buddies / Science BuddiesImage Credit: Ben Finio, Science Buddies / Science Buddies
Figure 3. Process for making a sedimentary rock using crayons.

2. Now encourage each group to take their sedimentary rock and try to turn it into another type of rock (either metamorphic or igneous) using the tools and materials they have available. They should document the steps they take and the result on their worksheets.
   • Note: If you have enough time and crayons, each group can make a second sedimentary rock, and try to make one igneous rock and one metamorphic rock using their sedimentary rocks. If you only have enough time/materials for each group to do one, you can assign some groups to do metamorphic and some groups to do igneous.
3. As a reference for the teacher, here are the steps you can use to make a metamorphic or igneous rock from a sedimentary rock. Do not give this information away to the students yet! Let them try on their own, and you can give them hints if they have trouble. You can also refer to the Crayon Rock Cycle Video for a demonstration of each process.
   • To make a metamorphic rock, heat the sedimentary rock on a stove or hot plate until it begins to soften, but does not melt completely. Keep the crayon shavings on a piece of aluminum foil to make cleanup easier. Heating the shavings will cause the layers of the sedimentary rock to fuse together, and possibly swirl to make different patterns (Figure 4). This models the process of rocks being heated beneath Earth's surface.
   • To make an igneous rock, take either a sedimentary or metamorphic rock and melt it completely on the stove, forming "magma." Use a toothpick to stir the melted crayons so the colors mix completely. Cool the rock rapidly by pouring the melted crayons into a bowl of ice water (Figure 5). This models the process of rocks melting completely (forming magma) then cooling and solidifying.
   
   
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 4. Metamorphic crayon rock made by heating a sedimentary rock on the stove.
   
   
   Image Credit: Ben Finio, Science Buddies / Science Buddies
   Figure 5. Forming igneous rocks by pouring melted crayons into ice water, causing them to quickly solidify.
   
   
4. Now challenge each group to take the rock they just made and turn it into another type of rock. Turning it back into a sedimentary rock is an option—you can do this by breaking the igneous/metamorphic rock into small pieces and starting the process over. Students should document the process on their worksheet. Again, depending on the amount of time you have available, you may want to divide the different transformations between groups. There are six possible transformations:
   • Sedimentary to metamorphic
   • Sedimentary to igneous
   • Metamorphic to igneous
   • Metamorphic to sedimentary
   • Igneous to sedimentary
   • Igneous to metamorphic
5. As time allows, let each group continue to explore different transformations between types of rock. Note that as students continually break down and re-mix their rocks, the crayon colors will blend more and more. Eventually it may not be possible to see distinct colors in the sedimentary and metamorphic rocks, but this is OK!

Reflect (10 minutes)

1. On the Student Worksheet, students have a blank diagram with the words "sedimentary," "metamorphic," "igneous," and "magma (melted rock)." Ask each group to draw arrows on the diagram representing the processes they used to convert one type of rock to another, and label each arrow with a few words describing the process.
2. Show students the rock cycle diagram from the Background (included in the downloadable Rock Type Slideshow). As a class, discuss how the diagram is similar to the diagram in their worksheets. Make sure you cover all of the transformations done by the class, in case every group did not do every single transformation.

   How are the processes shown in the rock cycle diagram For example, how did we model the process of weathering and erosion? What about melting and crystallization?

   Students modeled this process by creating small pieces of crayon, layering them together, and then pressing on them. This represents how real rocks are broken into small pieces by erosion, forming layers that get compacted together. We modeled melting and crystallization by melting crayons and then cooling them with ice water to solidify them.

   How is your crayon model different from the real rock cycle? Specifically, how about the amount of time it took to make crayon rocks? How does this compare to the time it takes for real rocks to form?

   The transformations in the real rock cycle occur over thousands or even millions of years—much longer than a single human lifetime, let alone a single class period! Our model works much faster and allows us to visualize processes that would otherwise be far too slow to observe.

   At the start of the lesson, we wondered how one type of rock could change into another type. Do you know the answer now? Did your answer change?

Assess

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