Plate Tectonic - Earthquakes (6)
Lab 

   
OBJECTIVES:
  • Experimenting with energy waves.
  • Observing how energy waves go through different substances.
VOCABULARY:
  • frequency
  • seismic waves
  • seismograms
  • waves
MATERIALS:

Students learn how energy transmits through different materials.

BACKGROUND:

As introduced in the Pre Lab, energy waves pass through different substances at different speeds and frequencies. Seismic waves demonstrate this in several ways. First, both P-waves and S-waves go faster through high density rocks like granite than they do through low density materials like soil or sand. Second, when these waves pass through unconsolidated (loose) soil or sediment, the waves slow down and are amplified; wave height increases. Finally, S-waves cannot travel through liquids. The molecules in a liquid "slide past each other" too easily to transmit the shearing motion of S-waves. In this lab the students will experiment with the transmission of energy through different substances. They will not work with seismic waves directly’ the experiments they conduct are direct analogs for their behavior.

The behavior of seismic waves as they travel through the Earth has been used to determine the Earth's interior structure. Through careful study of seismograms, scientists have found that seismic waves change as they travel through the Earth. From this work, they have determined that the Earth has a three-part internal structure.

From its center to the surface, the Earth consists of core, mantle, and crust. The core is composed primarily of two metals, iron and nickel. It has two parts, the outer core where the metal is liquid (not like milk, more like thick honey) and the inner core, which is solid. The mantle surrounds the core, and is composed of very dense silicate minerals. Most of the mantle is a mush of crystals and a little magma (molten rock), perhaps similar to a snow cone in texture (but MUCH hotter!). The upper part of the mantle is solid. It is coupled with the crust the outermost layer of the Earth, to make the plates.   

PROCEDURE:
  1. Set up the lab materials for each student or student group. Explain to the class that energy waves travel through different substances in different ways. A good analogy which might be familiar to the students is the way that sound travels in air versus underwater. Sound seems distorted underwater because it travels faster through this denser medium.
     
    Explain to the students that they will observe the transfer of energy through different substances. The reactions they will record are caused by the differences between the substances and the amount of energy they will apply to the experiment. Stress that the experiments should be done SLOWLY and CAREFULLY.
      
  2. Have the students complete the lab.
      
  3. Have the students try to determine the conclusion independently, then have a group discussion to ensure they understand the correct answer. Stress that their experiment is analogous to the behavior of seismic waves inside the Earth.
      
    ANSWERS TO LAB QUESTIONS:
    Exercise I. When you hit the penny, energy is transferred from you (the energy source) to the penny. As the penny hits another penny it again transfers energy, but there is not a total transfer of energy. The penny will slow down. It is important for students to somehow show that when the first penny hits the second the energy, i.e., speed changes. They can answer the question several ways, as long as they describe a change in the energy pattern. The same is true for the nickel, except the change should look different from with the penny to another penny. This is analogous to seismic waves traveling between two types of rock, where a change in (wave) energy also takes place. Here are suggested answers to each question:
  1. straight, energy transfer slowed down a little
  2. straight, nickel much slower
  3. the direction of pennies goes at an angle
  4. direction of nickel goes at an angle, slower than in #3
  5. energy is not as great as when nickel was used.
    The direction of waves can change if the transfer of energy forces the energy to go into several directions.

In Exercise II and III, the students should recognize that the same energy goes through different substances differently. They can see the difference in Exercise II (one fluid will have bigger waves than the other). In Exercise III students can hear the difference. For the conclusion students should realize that different substances allow energy waves to travel differently. They should also understand how geologists use seismic waves to study the interior of the Earth, expressing the information presented in the Background in their own words.

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