Plate Tectonic - Plate Tectonics (2)
Pre Lab 

  • Exploring why the Earth's outermost portion moves.
  • Introducing the concept of convection currents.
  • convection
  • core
  • crust
  • mantle
  • pan (optional)
  • water
  • hot plate
  • ladder

Students experiment with convection.

This picture shows convection of the whole mantle. It makes actually happen on a smaller scale. 


Understanding the movement and behavior of the Earth's outermost layers has been a painstakingly long scientific process. The theory of plate tectonics is our current "best explanation" and working model for answering these questions. Plate tectonic theory has developed slowly and progressively since it was developed in the 1960s. It is a theory that truly has the entire world as its experiment.

According to the theory of plate tectonics, the Earth's crust and upper mantle are broken into moving plates of "lithosphere." The lithospheric plates are solid rock. There are several very large plates, each consisting of both oceanic and continental portions. There are a dozen or more smaller plates. The plates average about 80 kilometers (50 miles) in thickness.

All of the plates are moving. They are slow, moving at speeds of centimeters to tens of centimeters per year. They slide along on top of an underlying mantle layer called the asthenosphere, which contains a little magma (molten rock). Many types of evidence indicate that the plates move.

Many lines of evidence indicate that the plates are moving. What is less clear, however, is why the plates move. There are two main scientific ideas for explaining plate movement: gravity and convection currents. All objects on and in the Earth are pulled towards its center by the force of gravity. This may affect the plates at converging plate boundaries in areas called subduction zones, where one plate sinks into the mantle. Some evidence suggests that gravity pulls the sinking plate down. The rest of the plate is dragged along behind it. This is physically similar to slowly pushing a piece of paper off a table; it eventually bends, and slides off, pulling the rest of the paper behind it. The other reason for plate motion relates to convection currents within the upper part of the mantle. Convection is the heat-driven circulation of a fluid. In the mantle, heat from deeper in the Earth causes the overlying mantle to circulate. The mantle can circulate because it contains a little magma (molten rock); it behaves like a very hot mush. Mantle convection currents move very, very, slowly. It is possible that as the mantle convects, it drags the overlying plates along with it. It is likely that both convection and gravity contribute to the movement of the plates.

This unit introduces the importance of convection currents into the overall concept of plate tectonics. Convection currents are common in everyday life. Water that is put over a hot stove heats by convection. The water closest to the heat source becomes less dense and rises. The water that is cooler and on top will sink to the bottom. This sinking of the denser fluid and rising of the less dense fluid causes movement in a circular motion as shown in the diagram below; this is convection.


  1. Explain the physical basis of convection currents to the students. Explain that it may be a force responsible for moving the plates.

    You may wish to demonstrate convection to the class with a glass pan, water, and a hot plate. Another way to demonstrate this is to get a ladder and have the students take turns "feeling" the temperature of the air near the floor of the room, then near the ceiling. It is warmer near the ceiling because warm, buoyant air rises, just like warm buoyant rock rises inside the Earth.

  2. Draw the diagram on the board. Ask the students in which room the air will circulate by convection currents. Emphasize that in room A, the difference in temperature caused by the candle will help circulate the air by convection. Ask the students in which directions they air will circulate. Warm air will rise, cool down and move to the left. When the air cools it will then descend.

  1. Remind the students that a similar process goes on in the Earth’s mantle. Emphasize that convection currents, in a room and in the Earth’s mantle, are not just arrows, but are large three dimensional movements.

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