My Science Box

Standards
Grade 6
Plate Tectonics and Earth's Structure

1. Plate tectonics accounts for important features of Earth's surface and major geologic events. As a basis for understanding this concept:
a. Students know evidence of plate tectonics is derived from the fit of the continents; the location of earthquakes, volcanoes, and midocean ridges; and the distribution of fossils, rock types, and ancient climatic zones.

Going Further
There are a large number of ways to reinforce these same concepts back in your classroom.

• Use the Virtual Earthquake software in the Earthquake Fingerprints lesson.
• Whose Fault is it Anyway? is a great kinesthetic way to model epicenter finding developed by Eric Muller from the Exploratorium Teachers’ Institute. Students hold hands and propagate a p and s wave through a human chain. The difference in arrival times can be used to figure out who started the earthquake.
• Finally, the Center for Science Education at the University of California Space Sciences Laboratory has a fantastic compilation of hands-on inquiry activity for the classroom on earthquakes. In addition to the standard stuff on reading seismograms for location and magnitude information, this series of lessons covers everything from using earthquake data to infer things about

Teacher Background
When an earthquake strikes, several seismic waves radiate outward from the origin of the earthquake. These seismic waves may be thought of as ripples through the Earth’s crust that are similar to the ripples in a pond after a pebble has been tossed into the water. The origin of the earthquake is known as the focus or hypocenter of the earthquake. The epicenter is the point on the Earth’s surface directly above the hypocenter.

Summary
The Lawrence Hall of Science in the hills above UC Berkeley offers fantastic hands-on workshops and exhibits related to earthquakes and plate tectonics. The middle school program, “Earthquakes: Whose Fault Is It?” provides an excellent introduction to seismology. The program begins with a large puzzle of the Earth’s tectonic plates to introduce the idea of plate tectonics and begin a discussion of the location and movement of the tectonic plates. Students then investigate earthquakes and learn to read real and simulated seismograms. Finally, students use seismic recordings to locate the epicenter of an earthquake. Afterward the workshop, the permanent outdoor exhibit, “Forces that Shape the Bay” provides a free-form venue to explore plate tectonics through hands-on exhibits. The other exhibits and planetarium are also worthwhile.

Sources
All the models - chocolate pillow basalt and cookie chert – were introduced to me by Eric Muller of the Exploratorium Teachers’ Institute. For detailed information about the chocolate pillow basalt demonstration, see his write up “Chocolate Lava” on his website.

Lesson Plan

1. Before going on this trip, review any of the science concepts that students should have fresh in their minds: rock cycle, basic geology, stratigraphy, geologic time, seafloor spreading, and plate tectonics
2. Go to the Point Bonita Lighthouse in the Marin Headlands.
3. Investigate the pillow basalts:
• Begin by asking students to draw or photograph the pillow basalts.
• Model the formation of pillow basalt with ice water and Magic Shell. Fill a cup with ice water and then pour Magic Shell into the water – the cold water makes the outside of the chocolate syrup harden and the chocolate soon piles up in pillow shaped piles.
• Discuss the model with the students, pointing out how the syrup represents magma welling up on the ocean floor.
• Ask students where magma might well up from the ocean floor: volcanic islands and mid-ocean ridges.
4. Travel to Rodeo Beach.
5. Investigate the sandstones:
• Ask students to draw or photograph the sandstones. Point out the borders between each large layered sandstone bed.
• At the water’s edge, observe how sand particles act in the water. Draw connections to the soil separation test and the tiny particles remain suspended in water but the larger particles rapidly settle to the bottom.
• Point out the sorting of the sediments in each bed.
• Discuss how much sand would need to be dumped at one time to create each bed.
• Ask students what might cause this much sediment to be dumped at one time: under-water landslides.
• Point out how large landslides are known to occur at the edge of a continent, particularly near subduction zones.
6. Lead students on a counter-clockwise walk around Rodeo Lagoon.
7. Stop at the chert outcrop and investigate the chert:
• Ask students to draw or photograph the chert. Pass out hand lenses and encourage students to look for fossils.
• Review the law of original horizontality. Ask students how these layers must have been laid down originally. How did the bodies of these fossils originally form layers? (By millions of years of accumulation on the ocean floor.)
• Ask students for initial hypotheses about how these layers could have become so folded.
• Pass out cookies and model the subduction of an oceanic plate with cookies. Carefully remove the top cookie, leaving the filling on top of the lower cookie. Hold the top cookies still while gradually moving the lower cookie so that it subducts unter the top cookie, scraping off the filling as it goes.
• Discuss the model, pointing out what the cookies and filling each represent. Observe the similarities between the now wrinkled filling and the folded layers of chert.
• Collectively piece together the story of how this chert was originally laid down and how it became wrinkled.
8. Return to the parking lot and go back to school (or play on the beach a little first).
9. At school, review the stories of the 3 rocks that were investigated. Discuss how each type of rock must have formed, specifically focusing on where it must have formed.
10. Spend at least half an hour piecing together the story of how all 3 types of rock, sandstone, chert and pillow basalt, came to all be found in one place. Once the story is filled out, diagram it on the board.
11. Use the relief map of California to show the 3 major geologic zones in California – the Sierra Nevadas, the Central Valley, and the Coast Range mountains.
12. Tell the story of how each of the 3 zones formed. Create a timeline to show students what was happening at different periods.

Getting Ready

1. The Point Bonita Lighthouse is only open on weekends and Mondays from 12:30 to 3:30. To have a docent or ranger open the lighthouse tunnel for you on other days and times, contact the National Park Service at (415) 561-4754.
2. Arrange transportation to the Marin Headlands and around the park.
3. Gather materials:

• small ice chest, ice, water, cup and Magic Shell for the pillow basalt demonstration
• hand lenses and cookies for chert observations
• map of California for putting it all together

Teacher Background
The Marin Headlands offers an exquisite opportunity for students to consolidate everything they have learned about geology and plate tectonics. Students (with a lot of guidance and help) find and piece together evidence concerning the geologic history of California. Students observe 3 different rock types and learn about how each one formed. Then, they take these observations back to the classroom to consider several possible explanations for how these rock types all ended up in the same place.

Pillow Basalt

Marin Headlands: photograph of Marin Headlands from the Golden Gate Bridge by Christopher BelandSummary
The Marin Headlands contain the geologic record of a great deal of plate tectonic action that can be used to piece together the history of the formation of California. Briefly, around 180 million year ago, the North American plate collided with a now subducted plate called the Farallon plate. As the Farallon plate dove under the North American plate, bits and pieces of the Farallon plate were scraped off. These bits and pieces can be found in the Marin Headlands in several distinctive rock formations: pillow basalts (at the Point Bonita Lighthouse), chert (near Rodeo Lagoon), and sandstone (at Rodeo Beach). By closely observing these rocks and figuring out how they formed, an understanding of how California itself was formed may be inferred.

Sources
This activity was inspired by the WGBH production of “Structures”, produced and narrated by Bebe Nixon. In this video designed to introduce teachers to inquiry-based teaching methods, students build towers and bridges out of drinking straws and see what is the maximum amount of weight each tower or bridge can hold. I adapted this lesson as a complement to the plate tectonics unit.

Shake TableTo create your own very simple earthquake table that is more like a trampoline than a standard, motor controlled earthquake table:

1. Cut a piece of board or plywood into a 12” square. If you wish, create a raised edge for your platform by nailing lengths of 1/2” square dowel on top of each of the sides.
2. Mount wood screws on the under side of the plywood at each corner and at the center of each side. Don’t screw the screws in all the way, make sure at least 1/4” sticks up so you can loop a rubber band around it.
3. Construct a frame out of 2” x 4”s that fits around the wood square with around 1/2” clearance between the outer edge of the square and the inside edge of the frame. Make sure the 2” x 4”s are oriented so that the frame is 4” high.
4. Mount wood screws on the top edge of the frame at each corner and at the center of each side. Again, don’t screw in the screws all the way.
5. Loop a rubber band around each pair of screws so that the plywood square is suspended like a trampoline within the frame.

Other earthquake table designs powered by an electric drill are described by John Lahr.

A great, very accessible resource on structural engineering principles with projects that can be adapted for the classroom is the book The Art of Construction: Projects and Principles for Beginning Engineers and Architects by Mario Salvadori, Chicago Review Press (1981).