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Journey to the Center of
the EarthÓ Dept. of Earth and
Atmospheric Sciences http://web.ics.purdue.edu/~braile Sheryl J. Braile, Teacher Happy (
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“But in the cause of science men are expected to suffer.” (p. 28, A Journey to the Center of the Earth, Jules Verne, 1864)
Objectives: This virtual journey to the center of the Earth introduces the traveler to the structure, material properties and conditions within the Earth’s interior. The size and scale of the Earth and of the Earth’s internal structure are also emphasized because the journey utilizes a scale model of the depths within the Earth. Opportunities for creative writing and connections to literature are also provided through Jules Verne’s 1864 science fiction novel, A Journey to the Center of the Earth, and the 20th Century Fox 1959 movie adaptation (titled Journey to the Center of the Earth) starring James Mason, Pat Boone, Arlene Dahl, and Diane Baker.
Background:
In the 1800’s there was considerable scientific and popular
interest in what was in the interior of the Earth. The details of the internal structure (crust,
mantle, outer core, and inner core; and their composition and thicknesses;
Figure 1) had not yet been discovered.
And, although volcanic eruptions demonstrated that at least part of the
interior of the Earth was hot enough to melt rocks, temperatures within the
Earth and the existence of radioactivity were unknown. Jules Verne’s book, A Journey to the
Center of the Earth (1864, 272 pages; originally published in France as Voyage
au Centre de la Terre), capitalized on this interest in the Earth and in
adventure with an exciting science fiction story that is still popular
today. Verne introduces us to a
dedicated, and somewhat eccentric professor, and his nephew through whom the
story is told (see selected quotations below), who eventually travel into the
Earth’s deep interior by entering into an opening in the crater of a volcano in
Figure 1. Earth’s interior structure. The Earth’s crust is made up primarily of
silicic (high percentage of Silicon and Oxygen) crystalline (distinct crystals
of individual minerals are visible) rocks.
The mantle makes up about 82% of the Earth by volume and consists of
Iron- and magnesium-rich silicate rocks.
The core is mostly iron, with a small percentage of nickel. The outer core is molten and the inner core
is solid.
“…and my uncle a professor of philosophy, chemistry, geology, mineralogy, and many other ologies.” (p.1, Jules Verne, 1864)
“I loved mineralogy, I loved geology. To me there was nothing like pebbles—and if my uncle had been in a little less of a fury, we should have been the happiest of families.” (p.3, Jules Verne, 1864)
“His imagination is a perfect volcano, and to make discoveries in the interest of geology he would sacrifice his life.” (p. 14, Jules Verne, 1864)
Verne’s novel is science fiction. We know today that such a journey would be impossible. The temperature and pressure conditions within the Earth are so extreme that humans could not survive below a few kilometers depth within the 6371 km radius Earth. Furthermore, we know of no significant openings that would provide access to the deep interior of the planet, and caves or cavities at great depth are nearly impossible based on our knowledge of temperature and pressure within the Earth and the properties of Earth materials. However, Verne’s story is an interesting one and it is the inspiration (along with the desire to provide materials for learning about the Earth’s interior) for this Earth science educational activity.
By the late 1800’s, observations of temperature in mines and drill holes had demonstrated that temperature within the Earth increased with depth, and thus it is possible that the Earth’s interior is very hot. Seismographic recordings in the early 1900’s were used to identify the Earth’s thin (about 5 – 75 km thick) crust (in 1909) and the existence of the core (in 1906). In 1936, Danish seismologist Inge Lehman presented evidence for the existence of a solid inner core. Since then, seismology and other geological and geophysical studies have provided considerably more detailed information about the structure, composition and conditions of the interior of the Earth. These features will be highlighted during our virtual “Journey to the Center of the Earth”.
As it is commonly done, we have represented (Figure 1 and Table 1) the Earth as a layered sphere of 6371 km radius. The Earth is actually not quite spherical. Because of the rotation on its axis, the Earth is approximately an ellipsoid with the equatorial radius being about 21 km larger than the polar radius. Also, in detail, the Earth is not exactly spherically symmetric. Lateral as well as vertical variations in composition and rock properties have been recognized from seismological and other geophysical observations. Finally, because of plate tectonics, there are significant differences in shallow Earth structure in continental versus oceanic areas, near plate boundaries, and at different locations on the surface. For these reasons, the depths to the boundaries that we will encounter in our journey would be slightly different if we chose a different location for the start of our journey. The depths, properties and other descriptions listed in the scale model for our journey are reasonable average values for a continental region.
Once one realizes that the interior of the Earth is hot, it is natural to ask, why is it hot? Because the Earth is 4.5 billion years old, it would seem logical that the planet would have cooled by now. The heat within the Earth results primarily from two sources – original heat from the Earth’s formation and radiogenic heat (Poirier, 2000). The largest of these sources, radiogenic heat, is mostly produced by three, naturally occurring, radioactive elements, Uranium, Thorium and Potassium. These elements are present in the mantle at concentrations of about 0.015 ppm (parts per million; meaning that only about 15 of every billion atoms in the mantle are Uranium) for Uranium, 0.080 ppm for Thorium and 0.1% for Potassium (Brown and Mussett, 1981). Spontaneous radioactive decay of these elements releases heat. Although the major radioactive elements are more concentrated (10 to 100 times as abundant) in the Earth’s crust, most of the radiogenic heat production comes from the mantle because of the much greater volume. The original heat from formation of the Earth dates from the accretion of the Earth from planetesimals that bombarded the early planet converting gravitational energy into heat.
Modern scientific information about the interior of the Earth comes from a variety of studies including: seismology in which seismic waves from earthquakes and other sources are used to generate images of the interior structure and determine the physical properties of Earth materials; analysis of the Earth’s gravity field indicates density variations; high-pressure mineral experiments that are used to infer the composition of deep layers; thermal modeling of temperature measurements in drill holes; modeling of the Earth’s magnetic field that is produced by convection currents in the electrically-conductive outer core; and chemical analysis of rock samples (called xenoliths) from deep within the Earth that are brought to the surface in volcanic eruptions. More information about the deep Earth and the methods of study of the Earth’s interior can be found in the references listed below. A good starting point is the book by Bolt (1993) or the American Scientist article by Wysession (1995). More advanced readers may wish to refer to Brown and Mussett (1981), Jeanloz (1993), Ahrens (1995), Wysession (1996), Poirier (2000) and Gurnis (2001). For younger readers, examine the children’s book by Harris (1999). Much of the information about deep Earth properties and conditions given in Table 1 comes from Ahrens (1995). Information about microbes in the Earth’s crust (mentioned in the Narrative, Stop number 3) is from Fredrickson and Onstott (1996).
Procedure and Teaching Strategies: A scale model (either a “classroom” scale or a “playground or hallway” scale; Figures 1 and 2 and Table 1) is used to provide the depths and locations of stops for a virtual journey to the center of the Earth.
Using a meter stick or meter wheel, mark out the locations of the 12 stops in the classroom (1:1,000,000 scale model; 6.37 m long) or playground or hallway (1:100,000 scale model; 63.7 m long). Masking tape placed on the floor or pavement is a convenient method for marking the stops. A felt pen can be used to label the stop number on the strip of masking tape. Folded index cards, labeled with the stop number, can also be used and have the advantage that the numbers can be seen from a distance (looking forward or backward to stops along the journey. Depths and the names of the locations can also be labeled using the masking tape, if desired. Provide each student in the class with a copy of the “Tour Guide” that can be produced as described near the end of this document. Folding the page in “thirds” creates a small brochure that each student can use on the tour and take home to help them remember the information that they learned and their experiences on the Journey to the Center of the Earth.
1. With the class, start at stop number 1 (the Earth’s surface) and read the first part of the “Journey to the Center of the Earth Narrative” (below). Proceed to the other stops and read the appropriate section of the narrative at each stop. Be sure to point out the distance that you’ve traveled in each move (by looking forward and backward along the model and using the scaled and actual distances from Table 1) and the distance that is remaining to travel to the Earth’s center. Answer student (traveler) questions at each stop. The information in Table 1 may be useful for answering questions. Other questions may form the basis of class or individual student research (“let’s find out”) using the references listed below or library or Internet searches.
2. When back in the classroom, use transparencies (or copies) of Table 1 and Figures 1, 2 and 3 to review with the students the main features of the Earth’s interior and the properties and conditions at various depths within the Earth. Note the increases in density, temperature and pressure with depth within the Earth and the abrupt changes in density at the major boundaries between layers. Additional questions can be answered or used to prompt additional study (such as other activities related to the Earth’s interior structure or plate tectonics) or research or to provide an assessment of student learning from the activity.
3. As an extension, or to connect to reading, writing and literature study, have the class read Jules Verne’s A Journey to the Center to the Earth (or selected chapters) or watch the movie (it is about 2 hours long, although one could skip the first approximately 30 minutes; starting as the explorers begin to climb the volcano). Relevant writing assignments for the students could be to write their own brief version of A Journey to the Center of the Earth based on the more accurate information about the nature of the Earth’s interior; write a review of the book or movie, or write about the inaccuracies and misconceptions that are evident in the book and movie. The accuracies and misconceptions also can provide material for an effective class discussion and assessment of student learning after reading Verne’s book or viewing the movie.
4. For younger students, reading Journey to the Center of the Earth (Harris, 1999) or The Magic School Bus Inside the Earth (Cole, 1987) before or after completing the journey is a useful extension and connection to literature.
5.
Related Earth structure activities include Earth’s
Interior Structure (Braile, 2000) and Three-D Earth Structure Model (Braile and
Braile, 2000). A useful and attractive
color poster (Earth Anatomy poster) illustrating Earth’s interior structure is
available from the
6. Additional extensions are also possible. An interesting assignment is to have each student or pair of students select one stop (depth) along the journey. Have the student or student team learn about the materials and conditions at that depth (some additional reading from the references provided below or from online sources would be necessary) and then draw an illustration that can be used to help describe each stop on the journey. Rock samples, if available (even photographs of rock or mineral samples from a book or from the Internet*), could also be placed at each stop to help illustrate the materials that make up the Earth’s interior. A piece of iron or steel can be used for the Earth’s core remembering that it will be liquid iron in the outer core. The student experts from one class, stationed at each stop, could also be the tour guides that would provide information, show their illustration and rock sample, and read the appropriate section of the journey narrative for another class or group of students. The experience of students learning in-depth information about one area of the tour and serving as “experts” can be an excellent “students teaching students” approach to learning. To emphasize the long journey or tour experience in the “Journey to the Center of the Earth” activity, a glass of water, a piece of candy or other refreshments could be served at one of the stops, probably the core/mantle boundary (stop 10) which is a little less than half way along the journey in terms of depth.
7. Connections of this activity to the National Science Education Standards (National Research Council, 1996) are listed in Table 3 below.
* Photographs of appropriate rocks and minerals can be found at several online sources, including: http://www.soes.soton.ac.uk/resources/collection/minerals/ (these photos can be enlarged by clicking on the photo until the photograph is almost full screen size); examples of sedimentary rocks are appropriate for the surface stop, number 1, click on “Sedimentary Rocks” at top of web page; for example, see sample #8, a sandstone; Granite samples from the “Igneous Rocks” link can be used for stops 2, 3, 4, and 5, alternatively, Gneiss samples could be used to represent crustal rocks, particularly for stops 4 and 5 that are deeper in the upper continental crust; Gabbro or Basalt samples, also from the “Igneous Rocks” link can be used to represent lower crustal rocks; a photograph of Olivine, an iron-magnesium silicate that is a common mineral in the Earth’s mantle – stops 6 – 10 – can be found in the “Minerals” section of the above web site or at: http://www.musee.ensmp.fr/gm/836.html; for the Earth’s core, a photo of an iron-nickel meteorite (http://www-curator.jsc.nasa.gov/outreach1/expmetmys/slideset/IronMet.JPG) is a good representation of the material that forms the core. A selection of photos that are useful for representing typical rocks from the Earth’s interior is provided in Table 2 below.
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Table
1. Journey
to the Center of the Earth |
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Stop Num. |
Depth (km) |
Scaled Depth (m) 1:1 million |
Scaled Depth (m) 1:100,000 |
Name or Location |
Rock/ Material |
Density (g/cm3) |
Pres-sure (MPa) |
Temp. (Deg C) |
|
1 |
0 |
0 |
0 |
Earth's Surface |
Atmosphere Sediments |
0.001 1.5 |
0.1 |
10 |
|
2 |
1 |
0.001
(1 mm) |
0.01
(1 cm) |
Top of "Basement" |
Sed. Rocks Granitic Rk. |
2.0 2.6 |
20 |
16 |
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3 |
3.6 |
0.0036 (3.6
mm) |
0.036
(3.6 cm) |
Deepest Mine |
Granitic Rock |
2.7 |
100 |
50 |
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4 |
10 |
0.01
(1 cm) |
0.1
(10 cm) |
Upper Crust |
Granitic Rock |
2.7 |
300 |
180 |
|
5 |
12 |
0.012
(1.2 cm) |
0.12
(12 cm) |
Deepest Drill Hole |
Granitic Rock |
2.7 |
360 |
200 |
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6 |
35 |
0.035
(3.5 cm) |
0.35
(35 cm) |
Base of Crust ("Moho") |
Mafic Rock Olivine-rich Rk. |
3.0 3.3 |
1100 |
600 |
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7 |
100 |
0.1
(10 cm) |
1 |
Base of Lithosphere |
Olivine-rich Rock |
3.4 |
3200 |
1200 |
|
8 |
150 |
0.15
(15 cm) |
1.5 |
Astheno-sphere |
Olivine-rich Rock |
3.35 |
4800 |
1300 |
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9 |
670 |
0.67
(67 cm) |
6.7 |
Upper Mantle Transition |
Fe-Mg Silicate |
4.1 |
23800 |
1700 |
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10 |
2885 |
2.885 |
28.85 |
Core/Mantle Boundary |
Fe-Mg Silicate Liquid Iron |
5.6 9.9 |
135800 |
3450 |
|
11 |
5155 |
5.155 |
51.55 |
Inner Core/Outer Core Bound. |
Liquid Iron Solid Iron |
12.2 12.8 |
329000 |
4700 |
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12 |
6371 |
6.37 |
63.7 |
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Solid Iron |
13.1 |
364000 |
4800 |
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Table
1. (cont.) Journey to the Center of the Earth |
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Stop Num. |