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Name _______________________________ Teacher _______________ Period _____ Date
Background
The main force that shapes our planet’s surface over long periods 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 oceanic crust, which are denser but thinner. In this activity, oceanic plates are represented by
chocolate squares and the 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 2-3 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 (continental crust and continental crust) and sea-floor
spreading ridges (oceanic crust and oceanic crust) 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.
At transform boundaries large faults form when plates slide past each other making the Earth tremble
with earthquakes.
Objective
Students will learn how Earth’s tectonic plates (lithosphere) ride atop the slow flowing asthenosphere
layer. Students will understand how plates interact at their boundaries.
Materials
1 large graham cracker broken in half (two square graham crackers), 1 square of chocolate, cup of
water, frosting, paper plate, plastic knife
Procedure
Divergent Plate Boundary
1. The teacher will give you a blob of frosting. This will represent the asthenosphere, the viscous
layer on which Earth’s plates ride.
2. Place two graham cracker squares on top of the frosting, so the edges are touching each other.
3. Press down slowly on the crackers as you slowly push them apart about 1-2 cm.
4. Notice how the frosting is exposed and pushed up where the plates are separated. This is
similar to how magma comes to the surface where real plates are moving apart at divergent
plate boundaries. Most divergent plate boundaries are located within oceanic crust and produce
sea-floor spreading like the Mid-Atlantic Ridge. When plates begin to pull apart at continents,
rift valleys are made like the Great Rift Valley in Africa.
5. Draw a picture in the labeled box of what you just did.
Convergent Plate Boundary
Continental Plate-Oceanic Plate Collision
1. Place one of the graham cracker halves lightly onto the frosting asthenosphere next to the
chocolate square. The graham cracker represents continental crust which is thicker and less
dense than oceanic crust (chocolate). Press the oceanic crust down on the asthenosphere so it
sits just below the continental crust (graham cracker.) The continental crust (graham cracker)
floats high on the asthenosphere, so don’t push it down.
2. Gently push the continental crust (graham cracker) towards the ocean crust (chocolate square)
until the continental plate (graham cracker) slides over the oceanic crust (chocolate.) Because
the oceanic plate is below the continental plate, we say it is subducted. We call this
subduction.
3. Draw a picture in the labeled box in the labeled box of what you just did.
Transform Plate Boundaries
1. Remove the chocolate square from the frosting. Pick up the two continental plates (graham
crackers) and set them on top of the aesthenosphere so that the edges are touching each other.
2. Slide the edge of one continental crust (graham cracker) horizontally along the edge of the
other to simulate a transform boundary like the San Andreas Fault in California. Notice the
friction as they pass each other.
3. Draw a picture in the labeled box in the labeled box of what you just did.
Convergent Plate Boundary
Continental Plate-Continental Plate Collision
1. Remove both the graham crackers from the aesthenosphere.
2. Place one edge of both crackers into the glass of water about 1-1 1/2 cm for just a couple of
seconds.
3. Place the graham crackers onto the frosting with the wet edges next to each other.
4. Slowly push the continental crust towards each other.
5. Notice how the wet edges crumble. This is how mountains are made at convergent plate
boundaries. When continental plates move towards each other, there is nowhere for the rock to
go but up!
6. Draw a picture in the labeled box in the labeled box of what you just did.
DIVERGENT Plate Boundaries
(Causes rifts and valleys and sea-floor
spreading, ridges)
TRANSFORM Plate Boundries
(causes earthquakes)
CONVERGENT Plate Boundaries
Continental-Oceanic Crust Collision
(Subduction which causes volcanoes and trenches)
CONVERGENT Plate Boundaries
Continental-Continental Crust Collision
(causes mountains)