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Part A: Modeling Shadow Zones
The structure of the Earth consists of a thin crust, a thick mantle, and a core. These three layers are
further subdivided into lithosphere, asthenosphere, upper mantle, lower mantle, outer core, and
inner core. Each layer has characteristics that set it apart from the other layers. The layers are
composed of different materials with different densities. Temperature, composition, and state of
matter are all characteristics that contribute to the different seismic properties of each layer and
have allowed scientists to deduce the internal structure of the Earth. Earthquakes originate in the
Earth’s crust and energy from the earthquake in the form of waves radiates in all directions through
the Earth.
Around 1910, scientists discovered that direct P waves are not detected in an area from
approximately 104 - 140 degrees from an earthquake. This area is called a shadow zone. Direct
waves are waves that are not reflected or refracted. P waves that are detected between these two
shadow zones are the result of refractions. See the image below:
Direct P waves
104 degrees
140 degrees
Refracted P waves
In this activity, you will model seismic waves as they travel through Earth’s interior.
Purpose: Create a model to demonstrate shadow zones in order to come to the conclusion that the
Earth has a core made of material with a density greater than the mantle.
Important Vocabulary:
Epicenter: the point on the Earth's surface that is directly above the focus
Focus: the point where an earthquake or underground explosion originates
Vertex: a point where two or more straight lines meet; a corner
Diameter: a straight line passing from side to side through the center of a circle
Geocentric angle: angle with reference to, or pertaining to, the center of the Earth
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Materials:
two large piece of paper
protractor
scissors
Procedure:
1. On a piece of paper, use a protractor to draw a circle 10 cm in diameter, or a radius of 5 cm, to
represent a cross-section of the Earth. Mark the center of the circle to represent the center of the
Earth (C). Make an arbitrary mark on the circle’s edge to represent an earthquake epicenter (E).
0
2. With the center of the circle (C) as a vertex, use a protractor to measure an angle of 110 with
0
0
one leg of the angle intersecting the epicenter (E). (We will use an angle of 110 , instead of 104
0
for simplicity of the model). Mark on the circle the other leg of the 110 angle, beginning of the
shadow zone (S). Draw lines connecting all three points: E, C, and S. Why are we using an angle of
0
110 ?
3. Draw a mirror image using the same location for the center of Earth and Earthquake (E), and
using a 110 degree angle in the lower portion of the circle. See figure on the next page.
Note: S1 and S represent seismic recording stations. Recording stations between 0 and 110 degrees receive direct P
waves. Recording stations along the thick line (the arc) receive P waves later than expected because they are refracted
by the matter in the Earth’s interior.
4. Cut out the area that has bolded lines. This cone represents the ray paths from an earthquake
through the Earth at geocentric angle of 110 degrees. It is where P waves showed some
interference during their journey through the Earth. They have been refracted in some way. What
does this say about the material the P waves traveled through?
5. On a second piece of paper, draw another 5 cm radius circle. This is represents the Earth.
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6. Place the vertex of the wedge shaped cut-out on circle you cut from #5. Align the curved arc of
the wedge with the edge of the circle you cut out in #5. The point on the cone depicts an
earthquake epicenter, it will fall on the edge of the circle from #5.
7. Trace the straight edge lines from the epicenter (E) to S and then again from the epicenter (E) to
S1 cutting through the circle (Earth).
8. Repeat this procedure moving the “epicenter” to different locations along the circle. Each time
the wedge is placed and lines are drawn represents an earthquake.
9. What pattern is emerging? What does it represent?
10. Draw a circle, which just grazes the edges of the straight lines drawn in steps 7 and 8. Measure
the radius of this inner circle ______cm. What does this circle represent?
Analysis:
1. Draw a cross section of the Earth. Draw and label its layers. Show how P waves are refracted as
they pass through the mantle and core. Lightly shade the P shadow zone.
2. Paste your diagram of the Earth into your notebook.
3. Calculate the radius of Earth’s Core: The radius of the Earth = 6371km and radius of the
modeled Earth = 5 cm; estimate the radius of the mantle-core boundary.
6371 km (Earth’s radius)
? km (Radius of core)
=
5 cm (modeled Earth radius)
measured inner radius (outer modeled core)
Conclusion:
1. What does the model suggest about the mantle of the Earth in comparison to the core of the Earth?
Part B: Observing a Magnetic Field
Our planet’s magnetic field is believed to be generated deep down in the Earth’s outer core.
Nobody has ever taken the mythical journey to the center of the Earth, but by studying the way
shockwaves from earthquakes travel through the planet, physicists have been able to work out its
likely structure.
Right at the heart of the Earth is a solid inner core, two thirds of the size of the Moon and composed
primarily of iron. At a hellish 5,700°C, this iron is as hot as the Sun’s surface, but the crushing
pressure caused by gravity prevents it from becoming liquid.
Surrounding this is the outer core, a 2,000 km thick layer of iron, nickel, and small quantities of
other metals. Lower pressure than the inner core means the metal here is fluid. Differences in
temperature, pressure and composition within the outer core cause convection currents in the
molten metal as cool, dense matter sinks whilst warm, less dense matter rises. This flow of liquid
iron generates electric currents, which in turn produce magnetic fields. Charged metals passing
through these fields go on to create electric currents of their own, and so the cycle continues. This
self-sustaining loop is known as the geodynamo.
In this activity, you will observe Earth’s magnetic field as well as create your own magnetic field.
Purpose: Observe and induce a magnetic field
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Procedure:
1. Tie a 4 magnets with about 20 cm of string, if not already done. Hold onto the end of the
string and let the magnets hang downwards. Keep still so that the magnet doesn’t move
while it is hanging. Which way do the magnets face? Observe another team’s magnet.
What do you notice about the direction your magnet hangs vs. the direction of another
teams? What does this indicate? Answer these questions in your notebook.
2. Take an aluminum cupcake liner. Is aluminum magnetic? Test it to find out.
3. Fill a petri dish with water. Put enough water in the dish so that a meniscus forms.
4. Put the cupcake liner on top of the water.
5. You will need to dangle your magnet into the center of the cupcake liner while you spin it.
When you dangle the magnet, the magnet should be at least ½ inch to 1 inch shorter than the
diameter of the can so that it does not bang onto the sides of the can while you are
attempting to spin the magnet. See the diagram on the next page.
Cupcake liner
6. As you dangle the magnet from your hand, when you twist the thread between your fingers,
the magnet should spin. You may need to practice so you can be successful.
7. What is happening? When the spinning magnet is lowered into the can, an electrical current
is induced in the aluminum, which is a conductor. This induced current in the aluminum
models the convection that occurs in the outer core; liquid metals are convecting causing an
electric current. The electrical current in the aluminum then creates a magnetic field in the
can. This causes the aluminum can to spin.
Analysis:
1. Using your own words, describe why the cupcake liner started to spin?
2. Illustrate your lab set up. On your illustration, do your best to draw the induced electric
current and the resulting magnetic field that you created in the cupcake liner.
Conclusion:
1. What does this model suggest about the composition of the outer core?