Evolution of Earth’s Crust

Why?

Graph 1:

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1. Compare the various percentages of the elements in Graph 1. a. What are the two most abundant elements in the Earth’s crust?

b. What percent of Earth’s crust is made of each of the elements listed in 1a?

2. What does molten mean?

3. What is an igneous rock?

4. When does an igneous rock form?

5. The Facilitator may assign one group member to collect the following materials from the front counter:

6. Spread the Cheerios and M&Ms out on the piece of construction paper. Make sure you have the right number to start with. Let the teacher know if you need more of either material.

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7. Your mixture of Cheerios and M&M’s represents all of the molten materials in the mantle. a. Which food item will represent the silicate material in the mantle? Justify your answer.

1 piece of construction paper 45 Cheerios 3 baggies 56 M&Ms

Earth’s crust is made of various elements and compounds that make up different types of rock. Most rocks are formed from preexisting rocks through external and internal geologic processes. In this activity, we will explore how igneous rocks form and how igneous rocks can be classified based on the elements of which they are composed.

The abundance of elements that make up Earth’s crust is depicted in Graph 1. Beneath Earth’s crust is the mantle, which is made of melted rock (molten) materials including silicates (SiO2), Iron (Fe), and Magnesium (Mg). Each type of material has a unique melting point so the molten material will cool at different temperatures. When the molten material cools and crystallizes, it forms igneous rock.

Silicate molecules (SiO2) are usually lighter in color and are known to have a lower mass. Iron (Fe) and Magnesium (Mg) molecules are usually darker in color and have a higher mass.

Facilitator: __________________________________ Process Analyst: ______________________________Quality Control: ______________________________ Spokesperson: _______________________________Other: ______________________________________

b. Which food item will represent the iron and magnesium in the mantle? Justify your answer.

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8. The first material to melt is rich in Iron and Magnesium. As this mixture of magma gets closer to the surface of the Earth, it cools and crystallizes. Remove the sample of materials listed below and set it aside in Baggie #1.

2 SiO2 molecules15 Fe/Mg molecules

9. The materials in Baggie #1 will represent the rock called Basalt (Rock#1).

10. As the mantle material continues to melt, the next magma mixture has the following composition (push these ingredients to the right side of your construction paper):

39 SiO2 molecules26 Fe/Mg molecules

11. The leftover ingredients are left in the mantle (geologists call this “depleted mantle”). Push the leftovers to the left side of your construction paper, circle them, and label them as “depleted mantle”.

12. Look at the magma from step #10. Part of this magma will cool just beneath Earth’s surface and turn into a rock called Diorite (Rock #2). Remove the following ingredients and place them into Baggie #2:

21 SiO2 molecules19 Fe/Mg molecules

13. The rest of the sample of magma from step 10 reaches the surface of Earth. This type of magma is called lava. It will cool and crystallize on or above Earth’s surface. As it hardens, it is subject to erosion and chemical weathering. Some of the iron and magnesium breaks down while the silicates don’t break down as quickly. Remove 4 Fe/Mg molecules from the hardened lava mixture and place them on the top of the construction paper. Circle them and label them as “weathered Fe & Mg”.

14. If the remaining silica-rich material from the hardened lava is heated up again beneath the Earth’s surface, it will eventually make its way back to the surface, cool, harden and crystallize. We will call this rock Granite (Rock #3). Place this new rock material into Baggie #3.

15. Look at your Granite rock.a. How many SiO2 molecules do you have?

b. How many Fe/Mg molecules do you have?

16. Which rock formed first?

17. Which rock formed second?

When the molten material in the mantle gets closer to the heat of the Earth’s core, the temperature rises (this is usually accompanied with increasing pressure), and the molten material continues to melt and turn into magma. The minerals in the mantle have different melting points due to being made of different elements. Therefore, our first magma mixture does not have the same exact composition as our original mantle mixture.

18. Which rock formed third?

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19. Compare your three rock samples (which should be in the baggies). a. Which of your three rocks can be classified as a mafic igneous rock? Explain how you knew it

was a mafic rock.

b. Which of your three rocks can be classified as a felsic igneous rock? Explain how you knew it was a felsic rock.

c. Which of your three rocks can be classified as an intermediate igneous rock? Explain how you knew it was an intermediate rock.

d. Recall the original composition of mantle material (from Question #5). Which of your three rocks reflects the rock type closest in composition to the starting material in the mantle? Support your conclusion by explaining your reasoning.

e. Still thinking about the original composition of mantle material, which of your three rocks contains a more derived (most changed) rock composition? Support your conclusion by explaining your reasoning.

20. Take a moment to organize the information you have learned thus far about the three types of igneous rocks discussed in this activity. Complete the composition, color, density, and formation portion of Data Table 1. Leave the last row of your data table blank for the time being.

Data Table 1:

Felsic Intermediate Mafic

Composition

Color

Density

Order of Formation(1st, 2nd, or 3rd)

Classification of igneous rocks is based on mineral composition, crystal size, and texture. There are three classifications of igneous rocks based on mineral composition alone. Mafic rocks are igneous rocks that are rich in Iron and Magnesium. These rocks are typically dark in color and have a higher density. Felsic rocks are igneous rocks that have high silicate compositions. These rocks are lighter in color and have lower densities. Intermediate rocks are igneous rocks that have a more even composition of silicates, iron, and magnesium.

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Map 1:

21. What type of crust is made of mafic rock?

22. What type of crust is made of felsic rock?

23. Add “crustal type” to Data Table 1. Fill in the necessary information based on the information you gathered from Map 1.

Scientists have learned that the mantle material cools and crystallizes in predictable patterns similar to those that you just discovered above. Scientists are able to use this information to better understand the development and evolution of Earth’s crustal material. Map 1 shows the general locations of mafic, felsic and intermediate rock found on Earth. Use the information you have learned above to answer the questions about the map and Earth’s crust below.

Extension Questions

24. Identify all patterns you notice that involve the locations where each rock type is found and the other properties of each rock type. Be sure to clearly describe how the properties relate to each other and how these relationships could result in the locations of the rock types.

25. How do these patterns help us to compare and contrast oceanic crust and continental crust?

*Place all of the ingredients back into the “mantle” baggie and then place all the baggies into the large baggie. Return the large baggie to your teacher and wait for further instructions.*