Geologists’ View of the Earth
A. Teacher introduction
- Geographic or geologic globe (need to be able to show outlines of continents and surface topography on continents and in the ocean basins), or a world map showing the same things
- Dinner plate (preferable unbreakable)
Procedure and Discussion
Plate tectonics is a powerful theory which attempts to explain the dynamic nature of Earth, including the occurrence of earthquakes, volcanoes, formation of mountain ranges, and many other geological features and processes the results of which we see and live with every day. There are two basic ideas which underlie Plate Tectonics:
The outer layer of the Earth is broken into separate pieces called plates.
These plates are in slow motion on top of the inside of the Earth.
You may already be familiar with the crust of the Earth - it is what you live on! The crust is a very thin layer which has a composition different from the inside of the Earth. You know something about the composition of the crust if you have ever picked up a rock or scooped up a handful of sand at the beach or along a stream. The crust is actually part of a strength layer of the Earth which is called the lithosphere. The lithosphere is a strong layer of rock which behaves brittlely when stressed. Since it behaves brittlely, it can cause earthquakes. The lithosphere is the layer which is broken into pieces, called plates, which are moving over the Earth’s inside. Now, you may wonder why these pieces are called plates.
- Hold up the dinner plate for the students to see.
What are the characteristics of this plate? Students may say all sorts of things, but need to get them focused on shape and relative dimensions. The plate is large in 2 dimensions (the eating surface), and much smaller in the 3rd dimension, the thickness.
- Now relate this to the lithosphere. Show a scale drawing of the lithosphere with respect to the whole Earth (Earth is 6370 km in radius, and the lithosphere is 100 km thick). While this lithosphere is broken into large pieces, they are still only 100 km thick, and so each piece resembles a dinner plate. Using a world map, indicate the outlines of some plates: for example, the Pacific Plate encompasses most of the Pacific Ocean, a very large area, but is still just 100 km thick.
Plate Motion and the Relationship Between Plate Motion and Surface Features
B. Student activity
(groups of 2)
- Two telephone books
- Shoe box
- Two partial rolls of paper towels (must fit side-by-side in shoe box)
- Crayons or markers
This activity will demonstrate the motion between two plates moving on Earth. There are three basic directions of motion possible:
- Give each student one telephone book. Each telephone book represents a lithospheric plate. Place the books on the desk, and working in pairs, tell students to move the books/plates towards each other, away, and side-by-side but moving in opposite directions:
These are the three types of motion:
- Convergent - towards each other;
- Divergent - away from each other;
- Conservative - next to each other
- This activity will demonstrate the relationship between plate motion and surface features:
Still using the telephone books, have each student-pair orient their books so that the page edges are towards the other book. Slowly converge the two books/plates. What happens? The pages crumple up and press upwards; the pages may interleave with each other. Imagine that this was happening with real lithosphere, made out of rock. What would happen? Rock pushing against rock, deforming and breaking, leading to earthquakes! What happens to the Earth’s surface? As the crust thickens and responds buoyantly -- Mountains! (The Principle of Isostasy, but that’s an activity for another day .... ) Show mountain ranges on the globe/map which resulted from plates colliding (the Appalachians, Alps, Urals, Himalayas, etc.).
- Have the pairs of students place the two books/plates side-by-side about one inch apart.
Place several marbles in the space between the books/plates, and have the books touching the marbles. Move the two plates in opposite directions, causing the marbles to roll along. Imagine that this is happening with real lithosphere. The marbles represent pieces of rock which break off and slide, causing earthquakes! Where does this happen on Earth? The San Andreas Fault exhibits this type of motion.
- Students work in groups of 4 (or this could be done as a demo)
Give each group a shoe box and two partial rolls of paper towels which will fit side-by-side in the shoe box. Using crayons or markers, have the students (working in pairs) draw a picture on the first piece of towel from each roll to represent land (grass/trees/animals, whatever). On the second piece of towel, have them draw a picture representing ocean (blue water/fish, whatever). Roll towels back up and place each in the box next to each other, so that the top towels on each overlap the adjacent box sides. The rolls of towel represent two different plates. Have each pair of students pull slowly on their towel so that successive pieces are pulled out of the box.
What happens to the two continents? They move farther apart. The new towels appearing from the box represent new ocean floor that is created at divergent plate boundaries.
Where does the material come from that forms the new ocean floor?
- The ocean floor is the top of the ocean crust, which as we saw before, is part of the lithosphere. What is the crust and the lithosphere made of? Rock.
- How is rock formed? In this case, the rock forms from volcanoes which form where the two plates move apart. The molten rock, called magma, rises from inside the Earth to make new lithosphere on the edges of the two plates.
- On the globe/map, indicate the ocean ridges (the mid-Atlantic Ridge in the Atlantic Ocean, the East Pacific Rise in the Pacific Ocean, the Carlsberg Ridge in the Indian Ocean). These ocean mountain ranges represent the divergent boundaries between plates and have active volcanoes and earthquakes along them.
- Students in original pairs from telephone book activity
One student place phone book on table. Give other student one sheet of blue paper. The phone book represents a plate with continent on it, while the blue paper represents a plate with ocean on it. Have the students again converge the two plates (works best if converge using the bound edge of the book), but this time the thin blue plate slides under the thick book/plate.
While all plates are made up of lithosphere, parts of some plates have continents on them, while other parts of plates have oceans on them. What is the difference? The crust of continents is thick, while the crust of oceans is thin (think about removing the water - the ocean floor is far below the surface of the land).
- What happens to the ocean plate that goes under the continental plate? It goes down into the Earth, where it heats up and starts to melt.
- What happens to the molten rock? It rises through the crust forming volcanoes.
- On the map, where are there volcanoes, other than along ocean ridges? (Mount St. Helens in Washington and the other volcanoes of the Cascade Range; Central America; the Andes of South America, the Aleutian Islands; Japan; the Philippines; etc.) These volcanic mountains are a result of convergence between continental and oceanic plates or between two oceanic plates, where the oceanic plate moves into the Earth and melts (a process called subduction). (If you have movable desks, the students can subduct the blue paper down the crack between two desks.)
Plates on Earth
This activity can be done as a demonstration for younger children, or as hands-on for children with good motor-skills!
- Hard-boiled egg(s) (one per student or per pair, or one for teacher demo)
- Knife, dull blade (one per egg)
- Marker which will draw on egg shell (one per egg)
- Draw a circle on the egg, around the pointy-end of the egg.
- Carefully crack the shell along this line using dull knife. You now have a two-plate world.
- Carefully move the circular plate in any direction, and observe what happens at the different plate boundaries.
- You can make mountain ranges at convergent boundaries, conservative boundaries, and divergent boundaries where exposed egg gets larger and larger (ocean basin).
- For the adventuresome, carefully make more plates and see what topography results from plate motion. Have fun! Notice how all the plate boundaries together make a continuous pattern around the “globe”.
- Now look at the real globe. Trace the plate boundaries around the world - ocean ridges continue into other ocean ridges, or “end” and continue as subduction zones or conservative boundaries, or continental collision zones.