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PART IV - CONVECTION The process of convection occurs when a fluid is heated from below and cooled from above. You have probably observed convection within a pot of soup on a hot stove. Use the clear plastic containers to set up and observe a convection experiment as follows (see figure on next page): • Fill 2 containers with cold water to a depth of appx. 6 cm. • Invert 8 paper cups and set each container on a set of 4 "legs." • Allow the water to become motionless. • Using the small plastic pipette, place a drop of food coloring on the bottom center of one container. See figure below. • Place two drops in the other container, one on the bottom just off-center, and the other on the top of the water, off-center to the other side. • Fill 2 other cups with very hot water and gently slide them under the containers until they are centered. (8) Observe the containers carefully and make a series of sketches that illustrate what happens to the food coloring. Holding a sheet of white paper behind the container will make it easier to observe the experiment. Describe, then explain your observations. Set-up for convection experiment: pipette H You can view a computer-simulation of convection at: http://www.geo.cornell.edu/hawaii/220/PRI/convection.html PART V – ISOSTASY Isostasy describes how materials float. When we explored the Earth's interior, we discovered that the outermost layer, the lithosphere, lies above a partially molten layer, the asthenosphere. The lithosphere is less dense and thus "floats" on the asthenosphere. The principle of isostasy is extremely important to plate tectonics--this is what allows the plates to move. You may be more 4
familiar with isostasy as it applies to ice cubes floating in water (especially ice bergs floating in the ocean). The principle is the same, only the materials are different. We can define "The Law of Isostasy," which states that: A body floats when it displaces a mass of fluid equal to its own mass. In this experiment you have two Plexiglas aquaria, a number of blocks of wood, and some water. Fill the cube-shaped container with water (as full as it will go) and set it inside the wide flat container (this is the "flood-prevention container"). We'll use the blocks to perform two related experiments, (1) ice bergs floating in the ocean, and (2) the collision of continents and the creation of mountain ranges. For both of these processes the law of isostasy is very important. Note that two sides of the cube-shaped container are taller than the others. The thickness of the wooden blocks is the same as the distance between the upper and lower walls of the container. Other parameters for this experiment: density of water = 1.0 g/cm 3 density of ice = 0.917 g/cm 3 density of continental crust = 2.8 g/cm 3 density of asthenosphere = 3.17 g/cm 3 Top of box Water surface H 2 O • Add a wooden block to the apparatus. Note that as you hold the block just at the surface of the water (before letting go) the top of the block is even with the top of the apparatus. When you release the block and allow it to float in the water the top is no longer level with the top of the container. (9) Why is this (10) What fraction of the block lies below the water (11) How many blocks can you add before the top of the stack reaches the top of the apparatus (12) Measure the size of the wooden block and calculate its density using the Law of Isostasy. ISOSTASY ON A LARGER SCALE When continents collide the crust is shortened and thickened. For example, the average thickness of continental crust is 35 km, but beneath the Himalaya and Andes mountains the thickness is 70 km. This extra-thick crust is what creates mountains. Once high mountains form 5
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Lab Handout

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