MATERIALS:

For each student:

• One large graham cracker broken in half (i.e., two square graham crackers)
• Two 3-inch squares (approx.) of fruit roll up
• Cup of water
• Frosting
• Sheet of wax paper
• Plastic knife or spoon
• Directions overheads

DIRECTIONS:

1. Make the model
   1. Give each student about a square foot of wax paper and a large dollop of frosting. Instruct students to spread frosting into a layer about half a cm thick.
   2. Tell students that the frosting in this model represents the asthenosphere, the viscous layer on which Earth’s plates ride. The plates in this model are represented by fruit roll up (oceanic crust which is thin and dense) and graham crackers (continental crust which is thick but less dense).
2. Divergent plate boundary
   1. Instruct students to place the two squares of fruit roll up (oceanic plates) onto the frosting right next to each other.
   2. Press down slowly on the fruit roll ups (because they are dense and will sink a bit into the asthenosphere) as you slowly push them apart about half a cm.
   3. Notice how the frosting is exposed and pushed up where the plates are separated? This is analogous to how magma comes to the surface where real plates are moving apart at divergent plate boundaries. Most divergent plates boundaries are located within oceanic crust. When plates begin to pull apart at continents, rift valleys are made, like the great rift valley in Africa, which can become the bottom of the sea floor if the plates continue to pull apart.
3. Continental-oceanic collision
   1. Instruct students to remove one of the fruit roll ups from the frosting. (They can eat it if they wish!)
   2. Tell students to place one of the graham cracker halves lightly onto the frosting asthenosphere next to the remaining fruit roll up piece. The graham cracker represents continental crust, which is thicker and less dense than oceanic crust (fruit roll up). It floats high on the asthenosphere so don't push it down.
   3. Gently push the continent (graham cracker) towards the ocean plate (fruit roll up) until the two overlap and the graham cracker is on top. The oceanic plate is subducted below the continental one.
4. Continent-continent collision
   1. Tell students that they will next model what happens when tow continents collide. Have them remove both the cracker and fruit roll up from the frosting asthenosphere. (Students can eat or discard the fruit roll up.)
   2. Place one edge of both crackers into the glass of water for just a few seconds.
   3. Place the crackers onto the frosting with wet edges next to each other.
   4. Slowly push the graham crackers towards each other.
   5. Notice how the wet edges crumple? This is how mountains are made at convergent plate boundaries! When continents move towards each other there is nowhere for the rock to go but up!
5. Transform plate boundaries
   1. Pick the two crackers up off the frosting and turn them around so that two dry edges are next to each other.
   2. Push one cracker past the other to simulate a transform plate boundary like the San Andreas fault!
6. Final step: eat all remaining model materials (except, of course, wax paper and plastic utensils!)

ASSESSMENT:

Have students draw what each situation looks like in cross section (by looking at the edge of their model).

BACKGROUND INFORMATION:

The main force that shapes our planet’s surface over long amounts of time is the movement of Earth's outer layer by the process of plate tectonics.
The rigid outer layer of the Earth, called the lithosphere, is made of plates that fit together like a jigsaw puzzle. These plates are made of rock, but the rock is, in general, lightweight compared with the denser, fluid layer underneath. This allows the plates to "float" on top of the denser material. The fluid dense material is called asthenosphere and in this activity it is represented by the frosting. However, plates are not all the same. Plates made of continental crust are thicker but less dense than plates made of ocean crust, which are denser but thinner. In this activity, ocean plates are represented by fruit roll ups and continental crust is represented by graham crackers.

Movements deep within the Earth, which carry heat from the hot interior to the cooler surface, cause the plates to move very slowly on the surface, about 2 inches per year on average. There are several different hypotheses to explain exactly how these motions allow plates to move.

Interesting things happen at the edges of plates. At divergent plate boundaries, rift valleys and spreading ridges form as plates pull away from each other. At convergent plate boundaries, where plates are coming together, subduction zones form when an oceanic plate and a continental plate collide and mountains build when two continental plates collide. Large faults form when plates slide past each other making the Earth tremble with earthquakes.

RELATED SECTIONS OF THE WINDOWS TO THE UNIVERSE WEBSITE:

• Earth’s Layers
• Plate Tectonics
• General Geology

OTHER RESOURCES:

• This Dynamic Earth, a publication of the USGS
• USGS Earthquakes Hazards Program
• Map of Earth’s Plates