Convection
Objective: To investigate and observe how material moves within a convection cell.
Background: The lithosphere is divided into at least 13 plates that move independently of
one another. A single plate can be as much as 8000 kilometers across and 100-150 kilometers
thick. The force needed to move such a large object is tremendous!!
One theory about how the plates move is that the layer beneath them--the
asthenosphere--has liquid properties even though it is solid rock! The plates ride on top of
the asthenosphere and the continents are embedded into the plates. If a portion of the
asthenosphere moves in one direction, the plate on top of that portion moves that way too
and so do the continents. Sometimes plates move towards each other and sometimes they
move away from each other.
What causes the asthenosphere to flow in the first place? In this activity, you
will build a model of a convection cell in water, experiment with its ability to flow and
investigate it as a possible driving force for plate tectonics.
Materials:

Clear plastic tub

One small beaker of red food coloring

One plastic pipette and a paper towel

Four foam cups

A white sheet of paper

Data sheets for each student

Three blue ice cubes (please wait until you need them!)
Procedure
Setup
1. Have one member of your group pick up materials from the side table.
2. Place a sheet of white paper on top of your lab bench to make your observations easier
to see.
3. Fill your plastic tub with about 3-4 inches of cool tap water.
4. Place four foam cups up-side-down on your white paper to act as legs for the tub.
Place your tub of water on top of the cups.
5. Leave the apparatus undisturbed for a minute so that the water is COMPLETELY still
when you begin your experiments.
TRIAL 1: CONTROL EXPERIMENT
*Food coloring with NO heat source*
6.
After the water is still, place a quarter-size
drop of food coloring at the bottom of the
center of the tub. To do this, collect the food
coloring in your pipette, wipe off the outside of
the pipette, carefully and slowly place the
pipette tip at the bottom of the tub and slowly
release the coloring. Carefully remove the
pipette, being sure not to disturb the water.
7. Observe the water and coloring for 2-4 minutes,
viewing from the top and sides. Record your
observations on the Data Sheet. Draw what you
see happening to the distribution of the food
coloring. Use arrows to show the direction of
movement.
8. Refill your water for the next trial.
TRIAL 2: *Food coloring with a heat source*
9. After the water is still, select one person from
your group to retrieve hot water in a Styrofoam
cup from your teacher *please be safe!*
10. Being careful not to disturb the water, slide the
cup of hot water under the center of the tub.
11. As in Trial 1, place a quarter size drop of food
coloring at the bottom of the center of the tub;
this time placing the coloring directly above the
hot water.
12. Observe the water and coloring for 3-5 minutes,
viewing from the top and sides. Record your
observations on the Data Sheet. Draw what you
see happening to the distribution of the food
coloring. Use arrows to show the direction of
movement.
13. Refill your water for the next trial. Remove the
cup of hot water from under the tub.
TRIAL 3: *Blue ice with no heat*
14. After the water is still, select one person from
your group to retrieve one blue ice cube.
15. Being careful not to disturb the water, gently
place the ice cube in the center of the tub.
16. Observe the water and coloring for 3-5 minutes
as the ice melts, viewing from the top and sides.
Record your observations on the Data Sheet.
Draw what you see happening to the distribution
of the food coloring. Use arrows to show the
direction of movement.
17. Refill your water for the next trial.
TRIAL 4: *Blue ice, red coloring and heat*
18. After the water is still, select one person from
your group to retrieve another cup of hot water
from the teacher. Select another student to
collect two blue ice cubes.
19. After the water is still, place a quarter-size
drop of food coloring at the bottom of the tub
near one END.
20. Then, being careful not to disturb the water,
gently place and keep the two ice cubes at the
other END of the tub.
21. Observe the water and coloring for at least 5
minutes, viewing from the top and sides. Record
your observations on the Data Sheet. Draw
what you see happening to the distribution of
the food coloring. Use arrows to show the
direction of movement.
22. After completing your observations and
recordings, discard the water. Clean up your
area and place all experimental materials back
where they belong.
23. Answer the questions on the back of the Data
Sheet.
Questions and Conclusions
1.
Review the results of the four trials with your group members. Contrast the different
outcomes to the control experiment (Trial 1).
a. What happens to the red dye when heated in Trials 2 and 4?
b. What happens to the blue dye in the ice in Trials 3 and 4?
2. How is the movement of dyes in Trial 4 different that those in Trials 2 and 3. Based on
what you observe, how would you define a “convection cell”?
3.
Describe the factors that caused the coloring to move upwards and downwards (think
density and temperature).
4. This activity models one of the mechanisms geologists think might drive plate tectonics.
a. In this model, what layer of the Earth does the clear water represent?
b. What layer of the Earth does the hot water represent?
c. What layer of the Earth does the ice represent?
5. If this model accurately represents the asthenosphere, explain how plates (lithosphere)
are moved about on the Earth’s surface.
6. The currents in the water cause the food coloring to move at a rate of two to three cm
per minute. How do you think this may compare to the actual rate of plate movement?
Why are there differences?