Interactions between the crust and mantle cause earthquakes and varying landforms. Students will see how friction and convection currents move plates on a liquid surface.
Instructional Method: Demonstration
Goal: Present the concept of crustal plate movement due to convection currents.
Objectives: Students will be able to:
- Diagram how friction and convection currents move plates on a liquid surface.
- Describe in their own words how continents move.
Setup: 15 minutes
Activity time: 15 minutes
- Glass cake pan or aquarium
- Foam cut into continental shapes (map of continents PDF)
- Heat source
- Food coloring or colored syrup
We are most familiar with the crust, which is the layer of the earth on which we live. Surprisingly, it is the thinnest of Earth's layers. The crust ranges in thickness from 5 to 100 km, yet it represents less than 0.1% of Earth's total volume! The crust is ridged and brittle and when it cracks or moves, earthquakes occur. Earth's crust is divided into two types, continental and oceanic crust.
Continental crust is rather thick, about 35 to 100 km. It is the thickest beneath mountain ranges and plateaus. Oceanic crust underlies the world's oceans. Oceanic crust is thinner, about 5-10 km thick, and is more dense.
These two crust types are fragmented into a dozen or more large and small pieces known as plates. A single plate can be made of both continental and oceanic crust. These plates move relative to one another as they ride atop hotter, more mobile magma on the mantle. Plates are constantly in motion, although they move very slowly, only centimeters per year.
The mantle is approximately 2,900 km thick. It is semisolid and rocky, containing iron- and magnesium-rich rock, which makes it more dense than the crust. The mantle is where convection currents occur, and convection currents are responsible for plate motions. Although the mantle is solid it moves plastically, in the same way a pink pearl eraser bends.
Geologists believe that thermal convection cells in the mantle are the driving force behind plate motions.
Convection cells are places where extremely hot magma rises to the upper portion of the mantle, known as the asthenosphere.
Convection cells behave like a lava lamp. Cool liquid sinks to the bottom of the lamp, is heated by the light and rises to the top. The hot bubbles rise because hot liquid is less dense than cool liquid. As the bubbles reach the top, they cool and once again sinking to the bottom. The same heating-upwelling-cooling-sinking process happens in the mantle.
The rising and sinking material moves in a circular motion. This motion is what moves plates on the surface. Plates "stick" to the mantle due to friction, which causes them to move as the mantle moves. It is not known why convection cells form but it is known that they exist.
All of the knowledge scientists have of the mantle is the result of seismic and volcanic studies because we can't see the mantle anywhere. The following activity demonstrates both how convection works and how it drives plate motion.