**U.S. DEPARTMENT OF THE INTERIOR\
U.S. GEOLOGICAL SURVEY**

**How to Build a Model Illustrating Sea-Floor Spreading and Subduction**

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by

John C. Lahr

Open-File Report 99-132, Paper Edition

This report is preliminary and has not been reviewed for conformity with
U.S. Geological Survey editorial standards. Any use of trade, product,
or firm names is for descriptive purposes only and does not imply
endorsement by the U.S. Government.

Denver Federal Center\
Box 25946, Mail Stop 966\
Denver, CO 80225\
Phone: (303) 273-8596\
Email: lahr@usgs.gov

 **Introduction**

This report describes how to build a model of the outer 300 km (180
miles) of the Earth that can be used to develop a better understanding
of the principal features of plate tectonics, including sea-floor
spreading, the pattern of magnetic stripes frozen into the sea floor,
transform faulting, thrust faulting, subduction, and volcanism.

In addition to a paper copy of this report, the materials required are a
cardboard shoebox, glue, scissors, straight edge, and safety razor
blade.

**Structure of the Earth**

The Earth consists of an iron-rich core with a radius of 3,500 km (2,100
miles), surrounded by a 2,800-km- (1,680-mile-) thick mantle of mostly
silicon, magnesium, and oxygen, and finally an 80-km- (50-mile-) thick
lithosphere. While 96% of the volume of the core is liquid, there is a
solid inner core with a radius of 1,200 km (720 miles). Electric
currents within the metallic-liquid outer core create the Earth\'s
magnetic field. This magnetic field is oriented approximately parallel
to the rotation pole of the Earth.

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.

**Plate Tectonics**

The mantle, which is much more solid than the outer core, is slowly
convecting due to the increase of temperature with depth within the
Earth. This motion can be compared to the convective motion of water in
a pan that is being heated on a stove; however, the movement of the
mantle is much, much slower. The lithosphere, the outer hard shell of
the Earth, is broken into a dozen or so major pieces, called plates, and
these plates are moving with respect to one another. At one time North
America, South America, Europe, and Africa were joined together in one
giant continent that has since broken apart to form the Atlantic Ocean
Basin.  The following figure (Stacy, 1977) shows how these plates once
fit together.

![](media/image3.jpeg){width="4.415972222222222in" height="6.075in"}

The process of sea-floor spreading created the lithosphere under the
Atlantic Ocean. As North America and South America moved away from
Europe and Africa, the resulting crack was filled by mantle material,
which cooled and formed new lithosphere. The process continues today.
Molten mantle materials continually rise to fill the cracks formed as
the plates move slowly apart from each other. This process creates an
underwater mountain chain, known as a mid-ocean ridge, along the zone of
newly forming lithosphere.

**Magnetic Stripes**

Molten rock erupts along a mid-ocean ridge, then cools and freezes to
become solid rock. The direction of the magnetic field of the Earth at
the time the rock cools is \"frozen\" in place. This happens because
magnetic minerals in the molten rock are free to rotate so that they are
aligned with the Earth\'s magnetic field. After the molten rock cools to
a solid, these minerals can no longer rotate freely. At irregular
intervals, averaging about 200-thousand years, the Earth\'s magnetic
field reverses. The end of a compass needle that today points to the
north will instead point to the south after the next reversal. The
oceanic plates act as a giant tape recorder, preserving in their
magnetic minerals the orientation of the magnetic field present at the
time of their creation. Geologists call the current orientation
\"normal\" and the opposite orientation \"reversed.\"

In the figure below, two plates are moving apart. A mid-ocean ridge
marks the location where molten rocks are moving up, cooling, and
forming new ocean floor. The zones of normal magnetization are indicated
by ////// shading of the oceanic crust.

![](media/image4.wmf){width="5.855555555555555in"
height="3.515972222222222in"}

The figure below shows the observed magnetic pattern along the
mid-Atlantic Ridge south of Iceland. This figure is from the excellent
publication \"A Teacher\'s Guide to the Geology of Hawaii Volcanoes
National Park\" (Mattox, 1992), which is available on the World Wide Web
at
http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part9.html.
The Guide starts at
http://volcano.und.nodak.edu/vwdocs/vwlessons/atg.html .

![](media/image5.png){width="3.85625in" height="3.223611111111111in"}

Based on the pattern and spacing of the oceanic magnetic stripes and the
inferred motion of the plates, the age of the ocean floor can be
determined. In the figure below by Müller and others (1997) (
http://gdcinfo.agg.emr.ca/app/jgr_paper.html ), the age of the ocean
floor is depicted by colors
(http://gdcinfo.agg.emr.ca/app/images/agemap.GIF ).

![](media/image6.png){width="4.603472222222222in"
height="3.634027777777778in"}

**Convergent Margins**

In some places, oceanic plates collide with continental plates. When
this occurs, the heavier oceanic lithosphere sinks beneath the
continental plate. This process, called subduction, creates a very deep
trough near the line of contact between the oceanic and continental
plates. This trough is called an oceanic trench. As an oceanic plate is
subducted into the Earth it is subjected to increased pressure and
temperature. These conditions cause some lightweight materials to melt
and rise to the surface to form volcanoes. As a result, long chains of
volcanoes, called volcanic arcs, are located above subducted plates,
usually above the location where the plate has reached a depth of about
100 km.

 ![](media/image7.wmf){width="6.4944444444444445in"
height="3.667361111111111in"}

**Earthquakes**

Geologic deformation is usually very slow, measured in centimeters per
year, so the dynamic processes that continually reshape the Earth are,
for the most part, unnoticed. Earthquakes are an occasional reminder
that this deformation is indeed taking place, and infrequent, but
potentially damaging, large earthquakes pose a hazard that is all too
often ignored.

Earthquakes occur wherever two plates slip past one another, such as
along the San Andreas Fault in California. This type of faulting is
called \"strike-slip.\" Earthquakes also occur where one plate slides
under the other, as is happening in southern Alaska. \"Thrust\"
earthquakes have been responsible for the two largest earthquakes ever
recorded, the 1960 M 9.5 Chile earthquake and the 1964 M 9.2 Alaska
earthquake. Pull-apart earthquakes occur where the lithosphere is being
stretched, such as along mid-ocean ridges. These earthquakes generate
\"normal-faulting\" events. When two ridges are connected by a fault,
this is called a transform fault, and the fault is the source of
strike-slip events.

When completed, your model will look something like the figure below. 
Follow the directions on the following pages to complete the model.
Coloring with paint or magic markers can be

![](media/image8.wmf){width="6.145833333333333in"
height="4.052083333333333in"}

added to enhance the model.  Note that the shaded portions of the sea
floor have the same magnetization as today, while the unshaded portions
have reversed magnetization.

**Operation of the Model**

Slide the sea floor into the three slits in the shoebox and pull it down
from underneath so that the numbers \"4\" are just visible. Slide the
sea floor into the trench as well. This is the position the sea floor
would have had 4 million years ago. Now slowly pull down with one hand
on the sea floor beneath the trench, while pulling with the other hand
on opposite end of the sea floor. As the sea floor diverges at the
ridges, new sea floor will continually appear until the dark gray bands
indicating the current epoch of normal magnetic polarity have been
revealed.

Now move the sea floor back to the 4 my before present position, and
this time, while the sea floor is moving, watch the boundaries where
slip is occurring. Where would the three styles of earthquake \-- namely
thrust, normal, and strike-slip \-- occur?

The USGS National Earthquake Information Center has placed seismicity
maps for many regions of the World on their World Wide Web site:
http://gldss7.cr.usgs.gov/neis/general/seismicity/seismicity.html . Look
at some of these maps and see if you can tell where the plate boundaries
are located. Where are the deepest earthquakes located and why are they
there?

From the ages of the sea floor and the scale of the model, which is the
same in the vertical and horizontal directions, one can determine that
the plate in this model is being subducted at a rate of about 50 km/my.
This works out to 5 cm/y, which is within the 1 to 10 cm/y range that
most plates are moving. As an interesting comparison, this is about the
rate that fingernails grow!

**Suggested Additional Reading**

Kious, W. J., and Tilling, R I., 1992, This Dynamic Earth, The Story of
Plate Tectonics, U. S. Geological Survey, Booklet 92-TDE, 77 p.

This publication can be found on the World Wide Web at
http://pubs.usgs.gov/publications/text/dynamic.html .

**Acknowledgements**

The author would like to acknowledge the careful reviews by E. Cranswick
and P. Detra. In particular, the latter\'s suggestion to reduce the size
of the model to that of a shoebox was most helpful.

**References**

Mattox, S. R., 1992, A teacher\'s guide to the geology of Hawaii
Volcanoes National Park, published by the Hawaii Natural History
Association, P.O. Box 74, Hawaii National Park, HI 96718, (808)
967-7604.

Müller, R. D., Roest, W. R., Royer, J.-Y., Gahagan, L. M., and Sclater,
J. G., 1997, Digital isochrons of the World\'s ocean floor, Journal of
Geophysical, V. 102 (B2), p. 3211-3214.

Stacey, F. D., 1977, Physics of the Earth, John Wiley & Sons, New York,
414 pages.

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