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Egg Tectonics
by
Sherry Weisgarber
Plate
tectonics, or the continental drift
theory, was first suggested in 1912
by the German scientist Alfred
Wegener. The theory, which states
that the Earth's surface, or crust,
is divided into six to nine major
plates that slowly move and change
in size, was not widely accepted
until the late 1960's. The theory
supposes that all the continents
were once part of a supercontinent
called Pangaea. This theory explains
why continents that are now widely
separated from each other possess
rocks and fossils of the same
extinct plants and animals. Geologic
events and features such as
earthquakes, volcanoes, mountain
ranges, ot springs, and geysers also
can be explained using plate
tectonics.
The
slow (1-4 cm per year) movement of
tectonic plates causes one of three
types of boundaries: divergent
boundaries, where plates separate;
convergent boundaries, where plates
collide; and transform boundaries,
where plates slide past each other.
The following activity simulates
these plate boundaries using a
cracked eggshell.
Materials:
3 (or
more) hard-boiled eggs
3 (or more) water-based markers
Gently
tap the eggs repeatedly on a table
while rotating them to produce
cracks all around the eggs. Trace
along the major cracks with a
water-based marker. Gently squeeze
the eggs until slight movement of
the shell pieces occurs. Look for
places where pieces of the eggshell
separate. This area represents a
divergent boundary. Most divergent
boundaries on the Earth are hidden
beneath the oceans and are
characterized by volcanism,
earthquakes, and massive heat flow
due to molten rock (magma) rising up
from the mantle, which is the thick
layer of rock separating the crust
from the core at the center of the
Earth. The Mid-Atlantic Ridge on the
bottom of the Atlantic Ocean is an
example of a divergent boundary;
here the North American Plate and
the Eurasian Plate are separating,
causing sea-floor spreading and new
oceanic crust to form. Next, look
for places where two pieces of
eggshell are colliding. This area
represents a convergent boundary.
Two events can occur when plates
converge. If denser oceanic crust
collides with lighter continental
crust, the oceanic crust will buckle
under the continental crust down
into the mantle. This process is
called subduction and is
characterized by earthquakes, rock
deformation, and volcanism. The
volcanic Cascade Range of the
Pacific Northwest was formed by
subduction of the Juan de Fuca Plate
under the North American Plate. If
two equally dense continental crusts
collide, both plates will resist
being subducted. In this process,
the continental crust folds and
deforms into a mountain range. The
Himalayas are an example of a
mountain-building episode which
began 25 million years ago and is
still occurring today as India
travels northward, colliding with
Asia. Finally, look for places where
one piece of eggshell slides past
another. This area represents a
transform boundary. The crust is not
destroyed here as it is at a
convergent boundary, nor is crust
created as it is at a divergent
boundary. As the two plates slide
past each other, earthquakes occur.
The San Andreas fault in California
is an example of this type of
boundary.
NOTE: After this experiment, use the
eggs to illustrate the layers of the
Earth. Cut the egg in half. The
shell represents the crust. The
thick egg white represents the
mantle. The egg yolk represents the
core.
Courtesy of Ohio Division of
Geological Survey "HANDS ON EARTH
SCIENCE" Webpage.
http://www.ohiodnr.com/geosurvey/edu/handson.htm
SOURCE: Terrific Science & Math
(Miami University), Fall 1993, and
Earth and Its Resources, Creative
Teaching Press.
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