Using physical observations to guess the interior of a globe

Devin Castendyk
State University of New York, College at Oneonta
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This activity introduces students to the need for geophysics and the application of several geophysical methods commonly employed. Students are given a hollow globe that contains various objects and asked to infer the contents of the globe using geophysical observations.

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Introductory-level Physical Geology course and/or and upper-level Geophysics course. This activity is designed for a geophysics course and also can be used to integrate geophysics into a core course in geology. This activity is also suitable for an introductory geology course.

Skills and concepts that students must have mastered

Students should appreciate the Earth is a very large, heterogeneous body and that only a thin veneer of rock can be directly observed on the land surface or by borehole exploration compared to its diameter. Students should be familiar with the structure of the Earth and should ponder "How did scientists conceive of this structure when it can not be directly observed?" Students should feel comfortable making observations and interpreting their findings.

How the activity is situated in the course

In a introductory-level Physcal Geology class, I would place this activity around the presentation of the structure of the Earth. Alternatively, this could be used to introduce the subject of Geophysics in a later phase of the course. In an upper-level Geophysics course, I think this activity would be most appropriate during the first week of class to give students an overview of the methods (seismic, magnetism, electrical resistivity/conductivity, thermal, etc.) that will be discussed in greater detail during the semester.


Content/concepts goals for this activity

Students will learn that simple observations of the physical properties of a globe can be used to make educated guesses on the internal structure and composition of that globe.

Higher order thinking skills goals for this activity

From simple observations, students will formulate a hypothesis that describes the interior of the globe provided to them. To some extent, this hypothesis can also be tested on the globe provided.

Other skills goals for this activity

It is envisioned that students will work together in teams to figure out the content of the globe provided. At the end of their mini investigation, students will give a brief oral presentation to the class stating: (1) what they think their globe contains; (2) what geophysical methods were employed to develop this hypothesis, what was observed, and how were these observations interpreted; (3) if the same methods where applied to the Earth, what could they help identify?

Description of the activity/assignment

The class is divided into small groups of three or four students. Each group is given a globe approximately 1 foot in diameter and asked to formulate a hypothesis for the structure and/or composition of the interior of the globe without looking inside it. Students are provided with several tools with which to analyze the globe, including acoustic sensors (their ears), magnets, paperclips (susceptible to magnetism), strip thermometers, small light bulbs with 9 volt batteries and wires.

Each globe has been designed by the instructor to highlight one or more aspects of geophysics. In the Acoustic Globe, various objects are added to the globe that will generate vibrations when the globe is shaken. These may include pennies, nuts, bolts, glass marbles, or ball bearings. The vibrations generated are analogous to seismic waves generated by a sledge hammer, a shot gun blast, or an earthquake. The seismic waves are recorded at the land surface by a seismometer, or in this case the students' ear drums, and an interpretation is generated. Advanced globes can contain cardboard dividers with small holes that allow the passage of all or some of the internal objects. Students can then interpret the internal structure of the globe.

In the Magnetic Globe, magnets or strips of iron are taped to the inside surface of the globe. The students will use magnets and paperclips to identify changes in the magnetic field of the globe caused by the heterogeneous composition of the surface of the globe, which is analogous to a magnetic survey. This globe could be combined with the Acoustic Globe if some of the objects inside the globe contain iron and some do not.

To create a Thermal Globe, the instructor can fix a gel cool pack to the inside wall of the globe and place the globe in a freezer until an hour before class. Students will used the strip thermometer to map regions of different temperature along the surface of the globe which may be used to infer convective processes occurring within the globe. This globe could easily be combined with either of the previous globes.

Finally, a Conductive Globe is designed by stringing the interior of the globe with wires of different compositions (copper, soldering wire, aluminum foil). The ends of each wire are connected iron nails which extend through the globe and are exposed on the globe surface. When the student completes the circuit using the wire, the 9 volt battery and the small light bulb, the light bulb will turn on. The intensity of the light will be different for each different type of conducting material. Through experimentation, the students can determine regions of homogeneous composition on the surface of the globe similar to an electrical conductivity or resistivity survey.

At the end of the exercise, each team will give an oral presentation to the class that describes their globe. Students should describe:
  1. the methods used to investigate the globe,
  2. the findings of their investigation; and
  3. their interpretation of the interior of the globe.
Students should also discuss one way the same methodology could be used to explore the interior of the Earth.
This activity has minimal/no quantitative component.

Determining whether students have met the goals

Oral presentations provided by students at the conclusion of the activity will indicate the extent of student inquiry using geophysical methods, and their ability to generate a working hypothesis for the internal structure of the globe based on geophysical observations. The teacher should decide whether or not to reveal the true interior of the globe before the activity begins.

More information about assessment tools and techniques.

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Supporting references/URLs

Once globes are constructed, this activity is fairly self contained. The most useful piece of supporting material will be a cross sectional diagram of the interior of the globe which is available in most Physical Geology textbooks.