Download Seafloor Spreading

Seafloor Spreading
Activity
Divergent Plate Boundaries
Background
About 50 years ago, geologists and oceanographers discovered
that there are both age and magnetic patterns in the seafloor.
This provided more evidence that plates both exist and move.
It resulted in the development of the theory of plate tectonics,
which describes the motions of plates and the interactions
between them that occur at plate boundaries.
The new discoveries established that new seafloor rock has
continually been forming over millions of years at mid-ocean
ridges, which wind throughout all of Earth’s oceans. Molten
rock, called magma, rises toward the seafloor from inside
Earth. It cools as it rises, and some of it solidifies before it
reaches the surface. Some molten rock spills out into the
surface, solidifying and forming volcanoes at the mid-ocean
ridges. This new rock is pulled apart at mid-ocean ridges,
forming two rock masses that slowly move away from each
other in opposite directions. In other words, the seafloor
“spreads” very slowly away from the ridges. Geologists refer
to this process as seafloor spreading.
In other areas of seafloor, there are long, narrow trenches
where the ocean is extremely deep—the deepest is 10,911 m.
In contrast to the youngest or newest seafloor rock found at
mid-ocean ridges, the oldest rock is found at or close to trenches.
The oldest seafloor rock is “only” about 180 million years old.
Many continental rocks are much older than this; the oldest
continental rock is over 4 billion years old. These age differences
or patterns are an important part of the plate tectonics story.
Vocabulary
Plate tectonics: Geologists’
understanding of the way
rigid plates of the outer solid
Earth move and interact.
Seafloor spreading:
Molten rock rises from below
mid-ocean ridges, cools, and
solidifies (crystallizes) into
new seafloor rock. This new
rock splits into two portions
that move as the plates
move away from each other.
Trenches: Deep linear
areas of the ocean where a
plate made of relatively old
seafloor rock sinks into the
asthenosphere.
Topic: seafloor spreading
Go to: www.scilinks.org
Code: PSCG055
Objective
Construct a paper model to illustrate why seafloor is newest
or youngest at mid-ocean ridges, and is relatively old at and
near trenches.
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Activity 5
Fast Fact
Four billion years is also
4,000,000,000 years,
4,000 million years, or
4.0 x 109 years.
Materials
Each group will need
• one copy of the seafloor
spreading model
• scissors
• tape
• orange-, yellow-, green-,
and blue-colored pencils
or crayons
Time
50 minutes or less
As rocks at the mid-ocean ridges crystallize, some minerals containing iron
line up with Earth’s magnetic field. They point to the magnetic poles, just as
compass needles do. These minerals capture the orientation of Earth’s magnetic
field at the time of their formation. However, Earth’s magnetic field changes
over time. The strength varies, the poles wander, and they switch north for
south and back again—their polarity reverses like flipping a bar magnet 180°.
The record of polarity reversals is frozen in oceanic rocks from the mid-ocean
ridges to the trenches as the seafloor spreads.
In this Activity, you will construct a paper model to investigate patterns that
exist in the rocks or plates that make up the seafloor at mid-ocean ridges.
Procedure
1. Locate BLM 5.1, the seafloor spreading model pattern. Make three cuts the
length of the page or sheet so that you end up with two strips, each labeled
orange-yellow-green-blue. Then, cut along the dashed lines of the left side of
BLM 5.1 (marked Slit A, Slit B, and Slit C) to make the three slits.
2. Color the areas indicated on the two strips with crayons or colored pencils.
3. Tape together the orange ends of the strips with the colored sides facing
each other (Figure 5.1).
Figure 5.1
How to assemble BLM
5.1 and use as seafloor
spreading model
Slit A
Slit B
Tape
Slit C
4. Thread the taped end (orange) of the two strips through slit B of the sheet
with the three slits. Pull one blue end down through slit A and the other
through slit C (Figure 5.1). Be sure to have the colored sides of the strips
facing up.
5. Pull the strips through slits A and C so that the same color on each strip
emerges at the same time from slit B. (The colors that you see on one strip
should be a mirror image of what you see on the other strip—this is the
pattern you are looking for.)
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Activity 5
Questions and Conclusions
1. How does what is happening at slit B correspond to what happens where
the seafloor is spreading and moving in opposite directions? What feature
occurs at the corresponding location on the seafloor?
2. What features on the seafloor are comparable to slits A and C? What is
happening at slits A and C?
3. If you were to sample and date the rocks along the colored strip starting at
slit B and moving toward slit A, what change, if any, would you see in the
age of the rocks?
4. If you were to sample and date the rocks along the colored strip starting at
slit B and moving toward slit C, what change, if any, would you see in the
age of the rocks?
5. In this model, what do the strips represent? What do the colors represent?
6. New seafloor rock is continually being formed at mid-ocean ridges, and old
seafloor rock is continually being removed from Earth’s surface at ocean
trenches. Rock on continents also is continually formed but is not removed.
How then, would the age of the oldest rocks on the continents compare
with the age of the oldest rocks on the seafloor?
7. What are the strengths and weaknesses of this model as a demonstration of
seafloor spreading?
8. Look at the map showing Earth’s plates and the direction of their motion in
Figure 5.2. Is the Atlantic Ocean growing or shrinking? Explain. The Pacific
Ocean is staying about the same size. Why?
Fast Fact
Iceland straddles a mid-ocean
ridge where the seafloor
spreads slowly—about 2.5 cm
per year in the North Atlantic.
As the seafloor spreads, so
does the island. That means
that an Icelander will see his
or her country expand by
about 2 m during the average
lifespan of 82 years.
What Can I Do?
Scientists measure tectonic
plate motions now with
Global Positioning System
(GPS) units. You could learn
to use a GPS unit and set
up a GPS trail for a friend
to follow. You could learn
how GPS works, design
an experiment to use it to
measure plate motion, and
then research how scientists
make the measurements.
Figure 5.2
World map showing
plates and continents
and direction of plate
movement
Project Earth Science: Geology, Revised 2nd Edition
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BLM 5.1: Model to Illustrate Seafloor Spreading
Activity 5: Seafloor Spreading
Slit A
Date____________________
Orange
Orange
Yellow
Yellow
Green
Green
Blue
Blue
Slit B
Slit C
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