Plate Tectonic Intro
Have you ever dropped an egg and the shell broke into an irregular pattern of broken pieces?
 Earths solid outer shell or lithosphere is much like a cracked eggshell
 Scientists are studying the Earth s crust including continents and ocean floors. Scientists
use:
 Seismographs
 Sound Waves
 Submersible vehicles (robots are even more helpful)
 Scientists took the information they have found and they noticed an interesting pattern
 Earthquakes and volcanoes tend to occur in the same areas around the world as well
as deep valleys
 Trenches: Deep valleys under the oceans
 Ridges: Long underwater mountain ranges.
 The mountain range in the middle of the Atlantic Oceans was very important in the
development of Earth’s surface- Mid Atlantic Ridge
 The rock at its top was younger than the rock on the surrounding ocean floor and the
edges of the continents
 Sea Floor Spreading: magma rises up and flows out at the ridge. The further the rock is
away from the ridge the older it is. The earth is not expanding as other areas of the sea
floor is moving deep into the trenches.
 Strong evidence for their theories
1. Most earth quakes and volcanoes are concentrated in specific areas
2. Large areas on Earth where few or no earthquakes and volcanoes occur
3. Ocean floor is spreading away from the mid- ocean ridges
The ocean floor is moving down into deep trenches on or near the edges of continents
Theory
- When all the evidence was pieced together, scientists suggested the Theory of Plate
Tectonics
- Lithosphere broken up into large pieces called plates
- Carry both continental and oceanic crust
- Slowly moving on a semi-solid layer of crust (partly melted layer of the mantle below the
lithosphere: asthenosphere)
- Boundary: Where the edge of the plates meet
Snack Tectonics
Fruit roll-up – oceanic crust
Graham cracker – continental crust
Frosting – asthenosphere
1. Divergent Boundaries
 Students will need two pieces of “oceanic crust.” They should place the two pieces of
crust on the frosting with the edges touching. Explain that these two plates are being pulled
apart. Have the students pull the pieces apart.
 Students should draw the results of the plate motion on their loose-leaf sheets.

Student Question 1. What happens to the molten rock as the two pieces of crust move away
from each other?
 The frosting, or molten rock, should rise up in between the plates as the move apart.
 Explain that this is how new oceanic crust is formed.
 Explain that two continental plates can also move away from each other, but it is less
common.
Student Question 2. If new crust is being created at these divergent boundaries, why isn’t the
Earth expanding?
 This should lead students think about subduction zones and the recycling of crust.
Transform Boundaries
Students will need the two pieces of continental crust. Place them side by side on the
frosting. Push one plate up and the other down
 Students should draw the results of this activity in plan (bird’s-eye) view.
Student Question 3. What happened to the edges of the graham crackers as they moved?
 The graham crackers crumbled slightly due to the friction of the two plates. Rocks at these
boundaries can be crushed and broken. Because of the large amount of friction and pressure
involved, energy can build up over time and be released in the form of earthquakes.
Convergent Plate Boundary (Oceanic – Continental)
Students will need a piece of oceanic crust and a piece of continental crust (a fruit roll-up
and a graham cracker). Line up the oceanic and continental crust on the frosting, making sure
the oceanic crust is pressed firmly into the frosting and the continental is only lightly sitting on
it. Gently push the pieces together. (The oceanic crust should slide underneath the
continental).
Subduction Zone
 Students should draw the result of this activity in profile.
Student Question 4. What happened to the oceanic crust?
 Because the oceanic crust is more dense than the continental crust it subducted, or sank,
beneath the continental crust.
Student Question 5. Did the process happen smoothly?
 There was friction and some resistance on the part of the oceanic crust. Although they are
often too deep to be felt by the people living on the Earth’s surface above them, earthquakes
are common at modern subduction zones. However, the crust usually doesn’t wrinkle or ripple
like the fruit roll-up did.
Convergent Boundary (Continental – Continental)
The students need two pieces of continental crust. They should soften one edge of each
graham cracker by quickly dipping it in the water. Place the continental crust pieces on the
frosting with the softened edges touching. Push the two pieces of crust together. (Mountain
ranges should build along the margins. Some thrust faulting may also occur.)
 Students should draw the results of this activity in profile
Student Question 6. What happened along the margins of the continental crust?
 Explain that as the continents collide, heat is generated. This heat softens the rock, which
we mimicked by softening the graham crackers in water.
Direction of
Movement
Motion
Type
Examples
Effect
Two plates slide
against each
other in opposite
directions
Two plates come
together and
both rising up
Transform
Boundary
San Andreas
Fault
Earthquakes
Convergent
Boundary:
continental crust
to continental
crust
Converging
Boundary: ocean
crust to
continental crust
Divergent
Boundary
Himalayas
Rock mountains
Mountain ranges
Mariana Trench
Trenches
Earthquakes
Volcanoes
Mountains
Mid Atlantic
Ridge
Volcanoes
Tsunami
Two plates come
together, one
sliding under the
other
Two plates move
apart