Name ________________________
Investigations Into Earth Science
Seafloor Magnetism
Overview
Two areas of the ocean that were first explored were the Mid-Atlantic Ridge in the North
Atlantic Ocean near Iceland. Another area was the East Pacific Rise west of Chile. For this
activity, you will simulate the discoveries by Vine, Matthews, and Morley by exploring the
magnetism of the seafloor and relating your findings to the theory of plate tectonics.
Data Preparation
You will be simulating the collection of data from two locations.
 Location 1 – The Reykjanes Ridge off of Iceland. (Part of the Mid-Atlantic Ridge.)
 Location 2 – The East Pacific Rise west of Chile. (Another mid-ocean ridge system.)
Cross sections are provided for both locations along with graphs that you will use to plot the data
you collect. Take a moment to observe both cross sections. The heavy line represents the depth
to the sea floor for a path across the mid-ocean ridge. The area below the line represents the
rocks that make up the sea floor, and the space above the line represents the ocean water. Note
that the vertical and horizontal scales are different. Before collecting data, answer the following
questions regarding the cross sections.
1) You are probably more familiar with feet and miles than meters and kilometers. Table 1 lists
the horizontal and vertical distances shown on the cross sections. Given that
1.0
kilometer = 0.62 miles, convert each distance into the other three units. (That is, convert
kilometers into miles, meters, and feet.)
Table 1
Kilometers
Miles
Meters
Feet
Reykjanes Ridge
Horizontal Distance
300 km
Vertical Distance
2250 m
East Pacific Rise
Horizontal Distance
900 km
Vertical Distance
4250 m
Show your work for the three values for the top row.
Investigations Into Earth Science
Spring 2013
EARTHSCI 2500
2) Note that the vertical and scale is provided for each cross section. Why do you think that they
are different?
3) Note that the vertical exaggeration is approximately the horizontal scale divided by the
vertical scale. In general, non-mathematical terms, how would you define vertical
exaggeration? What do you think the profile would look like if the vertical and horizontal
scales were the same (i.e. no vertical exaggeration)?
Now answer some questions for each location.
Location 1 – Reykjanes Ridge (North Atlantic Ocean near Iceland)
4) Describe the general shape of the sea floor along this profile?
5) The deepest location on the profile is 2100 m below sea level and the highest peak is 770 m
below sea level. If you could walk between these two points, how much elevation would you
gain? What would be the approximate slope of your hike in meters per kilometer?
Investigations Into Earth Science
Spring 2013
EARTHSCI 2500
6) Using the vertical and horizontal scales, complete Table 2 by calculating the real-world or
profile distances for the given measurements. (You do not need to calculate any values for
the areas shaded gray.)
Vertical Scale
Horizontal Scale
In The Real World
Table 2
On Profile
1.0
cm
6
mm
mm
m
400
(elevation of Latham hall
above sea level)
km
m
140
cm
(Distance from UNI to Iowa
Legislature in Des Moines)
km
7) Show your work for the first or second row.
Location 2 – East Pacific Rise
8) Describe in general terms the profile for the East Pacific Rise.
9) Compare and contrast this profile to the Reykjanes Ridge profile.
10) How do the horizontal and vertical scales at this location compare to Location #1? At which
location is the topography “stretched” the most? How do you know that?
Investigations Into Earth Science
Spring 2013
EARTHSCI 2500
Data Collection
North = 0° or 360°
Working in pairs, use a compass to collect
NW = 315°
NE = 45°
magnetic data along both the Mid-Atlantic
Ridge and the East Pacific Rise using the
technique demonstrated by your instructor.
East = 90°
West = 270°
For each compass reading, use the meter
sticks to measure the distance in centimeters
from the beginning of the profile. Later, you
SE = 135°
SW = 225°
will convert these distances into kilometers
so you can plot them on the given sea floor
South = 180°
profile. Magnetism will be recorded as the
Figure 1 – Azimuth degrees for different directions
azimuth direction in degrees (0° - 360°)
indicated by the north arrow on the compass.
(See Figure 2 for the azimuth degrees associated with each cardinal Table 3
direction.) Record the distance and magnetism data in your
East Pacific Rise
Distance
notebook, and complete the following follow-up activities and
Magnetism
cm
km
questions. Table 3 shows the first five entries for an example data
0
210
table.
11) In an actual survey, data would be continuously recorded. In our
simulation we will collect data at discrete intervals. Describe the
sampling plan that the class decided to use to collect the data.
10
20
23
30
40
42
50
60
70
80
180
110
90
62
14
0
332
270
242
200
12) Before you begin collecting data, what patterns or trends do you think you will find? Why do
you think this will happen?
Investigations Into Earth Science
Spring 2013
EARTHSCI 2500
Data Processing and Display
Now that you have your data from the two ridges, you can use your data to draw some inferences
regarding plate tectonics. But first, describe any trends or tentative conclusions that you saw
while collecting the data. (Earth scientists often note patterns, trends that they see while
collecting their data or hunches on what the data will show. These notes often lead to new
insights.)
Your data are in tabular form, but the human mind is better at finding patterns in visual data such
as graphs, maps, and figures, so you will need to convert your data in a form that is easier for
you to analyze.
Step 1 – Convert your measured distances (in cm) to scaled distances along the sea floor (km) by
using the following conversion factors.
Reykjanes Ridge: 1cm = 1 km
East Pacific Rise: 1 cm = 3 km
Step 2 – For each location, plot your magnetism data on the graph provided. For each graph, the
horizontal axis is distance along the survey and the vertical axis is the direction the
compass needle was pointing. Figure 2 shows an example graph for the sample data in
Table 3.
Step 3 – For each location, use your graph to
decide which locations have a north
(normal) polarity and which areas
have a south (reversed) polarity. Use
color to denote the two polarities on
your graphs.
(North) 0°
360° (North)
45
(East) 90°°
315
°
270° (West)
225
°
135°
(South) 180°
180° (South)
0 km
20 km
40 km
60 km
80 km
Step 4 – Once you have decided which parts
Figure 2 – Example graph showing sample magnetism data.
of your survey had normal and
reversed polarities, transfer than
information to the sea floor profiles by using color to denote areas of the sea floor that
have normal and reversed polarity.
Investigations Into Earth Science
Spring 2013
EARTHSCI 2500
Data Analysis and Conclusions
Now you can use your data to draw some conclusions.
13) In general terms, describe any patterns or trends that you see in your graphs.
14) What inferences can you make regarding the sea floor topography and the sea floor magnetic
polarity? Be sure to include the evidence you have to support each claim.
15) For both locations, calculate the distance from the center of the mid-ocean ridge to the first
reversal start of the first area with reversed polarity. Radiometric dating of lavas on land
show that this reversal happened 780,000 years ago. Over that time period, what was the
spreading rate for each ridge?
Investigations Into Earth Science
Spring 2013
EARTHSCI 2500
Extension Questions
16) Given what you know about the Earth, explain how these data support the claims of plate
tectonics.
17) Many people assume that scientists always conduct controlled experiments where they
investigate the changes in just one variable at a time and keep all other variables constant, yet
we have talked in class about ways in which Earth scientists use other methodologies to
gather evidence. Based on your experience from this activity as well as the reading on Vine,
Matthews, and Morely, discuss how this investigation was scientific. Also discuss ways in
which it deviated from the traditional, linear scientific method.
Investigations Into Earth Science
Spring 2013
EARTHSCI 2500
Reykjanes Ridge Near Iceland
Seafloor Magnetic Polarity
(North) 0°
360° (North)
315°
45°
270° (West)
(East) 90°
135°
225°
180° (South)
(South) 180°
0 km
20 km
40 km
60 km
80 km
100 km
120 km
140 km
160 km
180 km
200 km
220 km
240 km
260 km
280 km
300 km
Horizontal Scale = 1:1,275,000
Seafloor Profile
NW
SE
0m
250 m
500 m
750 m
1000 m
1250 m
1500 m
1750 m
2000 m
2250 m
0 km
20 km
Profile provided by Google Earth
(www.earth.google.com)
40 km
60 km
80 km
100 km
120 km
Horizontal Scale = 1:1,275,000
140 km
160 km
180 km
Vertical Scale = 1:31,500
200 km
220 km
240 km
Vertical Exaggeration = 40
260 km
280 km
300 km
East Pacific Rise Off Chile
Seafloor Magnetic Polarity
(North) 0°
360° (North)
45°
315°
(East) 90°
270° (West)
135°
225°
180° (South)
(South) 180°
0 km
100 km
200 km
300 km
400 km
Horizontal Scale = 1:3,850,000
500 km
Horizontal Scale = 1:31,500
600 km
700 km
800 km
Vertical Exaggeration = 123
Seafloor Magnetic Profile
NW
900 km
SE
2000 m
2250 m
2500 m
2750 m
3000 m
3250 m
3500 m
3750m
4000 m
4250 m
0 km
100 km
Profile provided by Google Earth
(www.earth.google.com)
200 km
300 km
Horizontal Scale = 1:3,850,000
400 km
500 km
Vertical Scale = 1:31,500
600 km
700 km
Vertical Exaggeration = 123
800 km
900 km