What causes fluids to circulate?

In the Film

Convection currents are identified in Earth's mantle. Heated mantle material is shown rising from deep inside the mantle, while cooler mantle material sinks, creating a convection current. It is thought that this type of current is responsible for the movements of the plates of Earth's crust.

In the ocean, warm water is normally found near the surface while the deeper water is usually cold. Deep, cold-water currents play an important role in creating the ocean oasis of the film.

Concept

Convection currents play a role in the circulation of fluids.

Objective

To observe convection as a result of differential heating

Content

Science, language arts

Background

Convection currents are the result of differential heating. Lighter (less dense), warm material rises while heavier (more dense) cool material sinks. It is this movement that creates circulation patterns known as convection currents in the atmosphere, in water, and in the mantle of Earth.

In the atmosphere, as air warms it rises, allowing cooler air to flow in underneath. Along with the turning of the Earth, this movement of air creates winds. Winds, in turn, create surface waves on the ocean.

Convection also plays a role in the movement of deep ocean waters and contributes to oceanic currents.

Inside Earth, the convection of mantle material is thought to cause the movement of the overriding crustal plates, resulting in events such as earthquakes and volcanic eruptions.

Part A   Density and Convection
Per student or pair of students: three 8 oz. clear plastic cups, 2 medicine droppers, blue and red food color, small containers for ice water and hot water, ice, hot water, room-temperature water

Part B   Convection Current Class Demonstration
Small aquarium, water immersion heater, blue ice cubes, red food color, medicine dropper, small paper cup, tape

Part A   Density and Convection (small groups)

• Discuss or review the concept of density relative to temperature.
• Fill each plastic cup with 6 ounces (200 ml) of water. Allow the water to stand about 5 minutes.
• Fill a small container with very cold water. Add a drop of blue food color.
• Fill another small container with hot water. Add a drop of red food color.
• Use a medicine dropper to release a drop of hot, red water at the bottom of one cup. Observe and record the results. Repeat, releasing the water on the surface. Record the results.
• In the second cup, repeat the process using the cold, blue water. Observe and record the results. What can you determine about warm water? Cold water? Which is more dense?
• In the third cup, simultaneously release hot, red water on the bottom and cold, blue water at the surface. Observe and record the results.

Part B   Convection Current Class Demonstration (small groups)

• Set up the equipment according to the diagram below.
• Poke a few small holes in the paper cup.
• Put the blue ice cubes in the paper cup.
• Tape the cup to the side of the aquarium.
• Turn the heater on.
• Release a few drops of red color at the bottom of the aquarium near the heat.
• Observe by looking through the side of the aquarium. Which is heavier, cold or warm water?
• Record your observations.

Local Connection
Observe local wind patterns. Where is the air warm? Where is the air cold? Which direction does the wind usually blow? Do you live near the ocean or other large body of water? If so, find out about local currents. Do you ever swim or wade in a stream or river? Is the water colder at the surface or near the bottom?

Key Words
convection current

Continue to Activity 5: Onshore and Offshore Winds