F is for Fault
To investigate why earthquakes occur in the outer layer of earth’s crust.
spongy foam balls (Nerf ®)
balls of clay
wooden craft sticks
chalk or dry-erase board
paper and pencils
As students learned in “F is for Fault”, earthquakes occur along faults, or cracks in the earth’s rocky crust, or lithosphere. What do they think affects where faults occur in the lithosphere?
Give each group of students a foam ball, a ball of clay, and a solid plastic craft stick. Explain that these three materials represent dif- ferent rock types in the earth’s crust. One type of rock is found near the top of the crust, another is found near the middle of the crust, and the third is found toward the bottom of the crust (near the mantle).
Draw the following cross-sectional view of earth on the board.
NATIONAL SCIENCE EDUCATION STANDARD:
Earth and Space Science, Grade 5–8, Structure of the Earth System: “The solid earth is layered with a lithosphere; hot, convecting mantle and dense, metallic core. Lithospheric plates... constantly move. Major geological events, such as earthquakes,...result from these plate motions.”
Have students research the Ring of Fire, the area around the edge of the Pacific Ocean where many earthquakes and volcanoes occur. How do the locations of these earthquakes and volcanoes cor- respond to the locations of the tectonic plates in that area? Why are areas prone to earthquakes often the same as those prone to volcanic activity? Have students create maps of the Ring of Fire and label the oceans and countries that surround it.
Students can begin their research at the PBS Web site for the television series Savage Earth (www.thirteen.org/savageearth/). Have them click on The Ring of Fire sidebar under “Hell’s Crust: Our Everchanging Planet” to see a map of the ring and read more about it.
Ring of Fire
Remind students that the earth is hottest at its core, and gets cooler as you move toward the surface.
Review with students the fact that materials in the earth’s crust are subjected to stress, including bending and squeezing forces. Have students bend and squeeze the three “rock” materials and record how each responds to the stress. (The foam ball collapses when squeezed but resumes its original shape when released. The clay ball changes shape when squeezed but doesn’t return to its original shape when released. The plastic stick cracks when bent.)
Now, introduce and define the terms elastic, inelastic, and brittle. Have them categorize each of their rock types as belonging to one of these groups. (The foam ball is elastic; the clay ball is inelastic; the plastic stick is brittle.) Explain that some rocks in the earth’s crust are elastic, some are inelastic, and others are brittle.
Now explain that warmer rocks are more elastic while colder rocks are more brittle. Challenge them to identify which of their rock materials is found near the surface of earth’s crust. (The brittle rock.)
Have students write a paragraph summarizing what they learned in this activity. Then have them infer why earthquakes only occur in the top 10 km of earth’s crust. (The rock is brittle there. When subjected to stress, it breaks instead of bending. This results in earthquakes.)