Science Lesson: Between a Rock and Hard Place

Abstract
In this activity students will explore the relationship between the density of two plates and the possible boundary types. Previously students have explored the location of volcanoes and earthquakes on a global scale. They have also observed models of mantle convection.

Purpose – Exploration of the interaction between continental and oceanic plates.

Objectives
Students will be able to:
1. Describe in a brief paragraph how the density of the differing plates affects plate boundary interactions.
2. Construct a graphic which relates the model to plate boundaries and convection currents in the mantle.

National Science Education Standard:
CONTENT STANDARD A – Science as Inquiry
Formulate and Revise Scientific Explanation and Models Using Logic and Evidence

CONTENT STANDARD D – Earth and Space Science
Energy In The Earth System
• The outward transfer of earth's internal heat drives convection circulation in the mantle that propels the plates comprising earth's surface across the face of the globe.

The Origin And Evolution Of The Earth System
• Interactions among the solid earth, the oceans, the atmosphere, and organisms have resulted in the ongoing evolution of the earth system. We can observe some changes such as earthquakes and volcanic eruptions on a human time scale, but many processes such as mountain building and plate movements take place over hundreds of millions of years.

Teacher Background
The Earth’s tectonics plates “float” in the asthenosphere, the uppermost portion of the mantle. The different types of terrestrial plates and the asthenosphere have different densities. These densities affect plate boundary interactions. Oceanic plates, containing basalt, are quite dense while continental plates, containing granite, are much less dense. Because oceanic plates are denser than continental, they subduct when they collide with continental crust. Collision between two continental plates results in uplifting due to the equal densities. Collision between two oceanic crust plates results in subduction of the older, denser plate. This collision accounts for both mountain and volcanic mountain formation. Divergent boundaries create new crustal material as they push plates apart. Transform boundaries cause earthquakes as they slide past each other. (Please note earthquakes will be dealt with in more detail later on in the unit.)

Related and Resource Websites
http://visearth.ucsd.edu/VisE_teach/ (From Lessons/Isostasy Lesson - density)

Image Courtesy of Remote Sensing Tutorial - "Geologic Time"

Activity

Prior to Class
Set up a tray of water at least 6 inches deep. Collect a sponge, a foam block and a wooden block that are roughly 10 cm by 13 cm. (It is important that they are the same size.)

Day One
1. Review the findings from the previous days. Where were the earthquakes and volcanoes located?

2. Have students answer the question they were left with the day before: “Do you think there are patterns in the way the plates move and the results?” Allow a few minutes for students to discuss their thoughts before proceeding.

3. Tell students, “The patterns you found previously show the boundaries of the plates that Alfred Wegener discussed in his theory of Continental Drift. Earthquakes occur at all plate boundaries; however, other geologic events do not. That is why you saw volcanoes in some of the same places as earthquakes but not all. In order to better understand these geologic events, we need to understand the plates themselves. What do you already know about these plates? How many types are there? What is under them? [Pause for students’ answers-hopefully students will mention continental and oceanic plates floating on top of the mantle. As students talk about the mantle introduce the term asthenosphere, which is the term given to the top of the mantle and the lithosphere.

4. Draw the students’ attention to the sponge, foam and wooden blocks. Ask them which they think is the least dense? Which is the densest?

5. Put the blocks into the tray of water and have students observe. Ask students, “Based upon your observations and knowledge of the Earth, what do you think these three items represent? [foam = continental crust, sponge = oceanic crust, wood = asthenosphere].

6. Tell students that back at their labs, they will explore what happens during a variety of oceanic and continental collisions. What is going on at their labs? They should record their findings in their lab notebooks.

Lab Stations:

7. At their stations students will model all the various permutations of different plate boundaries interactions. Before they start students should construct a table which includes the types of plates, what they are using to represent each plate and the outcome. Within the table students should both describe verbally and via a diagram the action at the plate. Encourage students to diagram what they think is happening within the mantle. Have students include a blank column so that they can add the geologic term for the process they are observing at a later date.

Closure
When students are finished have them regroup as a class to wrap-up the day’s lesson. Have the students answer the following questions in the discussion: What happens when you push the sponge and the foam together? [Sponge subducts] What happens when you push the sponges together? [One sponge should subduct under the other] What happens when you push the 2 foam blocks together? [They should crash together.] What other motions might occur on the Earth that these models do not show well? [Divergent boundaries and transform boundaries].*Note, this is a good time to introduce the vocabulary since the students are describing the actions and boundary types
.

Embedded Assessment
Can students describe the difference in densities? Can students identify what each material represents? Can students describe action of plates and mantle at a plate boundary in a graphic?

Homework
Write a 2-3 sentence conclusion in your science notebook. Conclusions should tell what you learned and be thought provoking.

Embedded Assessment