PIT OF THE EARTH
Earth's layers are like a chocolate covered cherry. To introduce the concept of Earth layers, students will diagram and label the candy and describe how each layer of candy relates to an Earth layer.
To visualize the properties of the different Earth layers
Students will be able to:
- List the four main layers of Earth
- Match the characteristic of each rock layer, with a layer of the cherry
- Diagram and label the layers of the Earth
Activity: 30 min.
- Chocolate covered cherries, one per student
- Student page, one per student
- Plastic knives
- Paper towels, sponges, etc., for cleanup
Earth is composed of five main divisions or layers:
atmosphere, crust, mantle, outer core, and inner core.
This lesson will discuss the interactions between the sub-layers and how they affect the crust.
The layer we are most familiar with is the crust. Surprisingly, it is the thinnest layer of the four, despite how massive it seems to us. It is approximately 5 to 40 kilometers thick. There are two types of crust: continental crust and oceanic crust. The continental crust is the layer on which we live. The thicker portion of the crust is the continental portion, and likewise, the thinnest part is oceanic crust. The crust is ridged and brittle, but seemingly solid. This layer corresponds with the chocolate cover of the candy.
The continental crust is much lighter, or less dense, than the oceanic crust. Because the oceanic crust is more dense, it submerges below the lighter continental crust when the two collide. (Both types of crust ride on top of the solid lithosphere, which is considered part of the crust. The lithosphere then rides on top of the weaker, partially molten asthenosphere, which is part of the mantle).
The mantle is the layer second most familiar to us. The mantle in portions is solid and rocky, but also fluid and molten towards its outer edge in places. Because it is partially solid and partially molten, it can be compared to a plastic, solid but flexible like a Pink Pearl Eraser. It is believed to contain convection currents that drive the movement of crustal plates (refer to Floating Continents). As heat rises to the surface, pieces of crust float around like rafts on a pool, taking thousands of years to move. Mantle thickness ranges from 40 to 2900 kilometers. In our chocolate covered cherry analogy, it is the cream or fluid surrounding the cherry under the chocolate.
Below the mantle is the outer core. Scientists believe that the outer core is composed of a molten, metallic layer 2270 kilometers thick. This layer is believed to contain large amounts of nickel. Scientists have been able to determine that it is a liquid by monitoring earthquake waves as they pass through the Earth's core. The waves dissipate as they pass through this layer in a way that indicates it is liquid and not solid. This is the cherry.
If a chocolate covered cherry had a pit, it would be the inner core, a solid iron-rich mass with a radius of 1216 kilometers. At center of the Earth, the pressure from gravity is thought to be so high, that any material in this location would have to be a solid. Scientists believe that the Earth's magnetic fields are related to the orientations of the outer and inner core.
The following activity allows students to slice through the layers of Earth discovering the different properties of those layers.
- Put chocolate covered cherries in the refrigerator and chill them. Do not freeze them. Starting with them cold will help to keep the workspaces cleaner.
- Make copies of a cross section of Earth layers, one for each student.
- Pass out photocopies of Earth layers, plastic knives, paper towels and the cold chocolate covered cherries. Explain to the students that they will get to eat the candy, but to wait until all the materials have been passed out.
- When all materials are disseminated, have the students spread a paper towel onto their tables and place the candy on the paper towel. Cut the cherry in half, as neatly as you can. (Dissecting a chocolate cherry is very messy business, the chocolate shell might simply break apart. Using knives with a serrated edge and sawing through the shell might prevent the total collapse of the candy's shell).
- Have them describe the layers of the cherry chocolate on their student pages, labeling each layer and writing one unique characteristic of their choice for each layer.
- Allow them to eat one half of their candy, but keep the other half on their paper towel. Talk about each layer of Earth as though we are microscopic and standing on the outside edge of the candy. The crust is brittle and is easy to break and is the outer most layer. Under the crust is the mantle; it is flexible and in some places fluid. Next is the outer core, very liquid but dense, just like the cherry.
- Describe the concept of the solid metal inner core and ask the students what part of the candy would be like the inner core. (Usually it is removed before the cherry becomes candy: it is the cherry pit).
- Have them diagram Earth's layers and label each layer correctly. At the bottom of the sheet have them describe each of the Earth's layers using one unique characteristic of their choice and explain in their own words how it correlates with each layer of the chocolate cherry.
- Finally, have them explain in their own words, how the Earth is NOT like a chocolate covered cherry and how they know this to be true. (The Earth is made out of molten and solid rock, it is too large to be eaten, dirt doesn't taste like chocolate, etc.).
- Cleanup and eat the other half of the candies.
If we pretend that we are standing on the surface of this candy, what would we be standing on? How does that chocolate shell correspond with the Earth, and what layer are we standing on? What is below the crust? What do we think it is like? What part of the candy did we say it was like? What about the outer core, where is that found? What about in the cherry candy? What do we think we know about the inner core? Why did we not see it in the candy today? How is the Earth like this chocolate covered cherry? How is it not like this candy? Why do you think we used this candy to illustrate what we think the inside of the Earth is like? Can we see the inside of the Earth? What part can we see (crust)? What else do we know about the crust? Is it completely solid? Is it broken like some of our chocolates?
This activity can also be done with a hard-boiled egg. The shell is the crust, cracked and broken. The egg white, solid but flexible is the mantle. The yolk can be correlated with the outer core, though it is not fluid, and there is no inner core correlation. This variation is less messy.
Included National Parks and other sites:
Utah Science Core:
2nd Grade Standard 6 Objective 1,2,3