SEA FLOOR SPREADING – KIT CONTENTS
E Q - 06
• INTERNAL STRUCTURE OF EARTH (use flat color copy, or check out the Globe, EQ-16)
• PLATE PUZZLE MAP (laminated paper copy)
• LARGE LAMINATED MAPS OF ATLANTIC & PACIFIC SEA FLOORS
• SEA FLOOR BASALT ROCK SAMPLE
• SEA FLOOR SPREADING BOX, MAGNETS AND COMPASS
SEA FLOOR SPREADING
EQ - 06
page 1
SEA FLOOR SPREADING
E Q - 06
JULY 1997
OBJECTIVES:
To understand sea floor spreading
To understand the magnetic poles of the earth, and how the reversals of the earth's polarity was
used to derive the theories of plate movements
SCIENCE THEMES:
Patterns of Change, Systems and Interactions
PROCESS SKILLS:
Ordering, Relating
GRADE LEVELS: 4 - 8
FOCUS WORDS:
Crustal Plates
Mantle
Plate Tectonics
Rift Zone
MATERIALS:
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INTERNAL STRUCTURE OF EARTH (use flat color copy, or check out the Globe, EQ-16)
PLATE PUZZLE MAP (laminated paper copy)
LARGE LAMINATED MAPS OF ATLANTIC & PACIFIC SEA FLOORS
SEA FLOOR BASALT ROCK SAMPLE
SEA FLOOR SPREADING BOX AND MAGNETS AND COMPASS
BACKGROUND INFORMATION:
Studies of the ocean floor since 1950 have revealed some of the most important geologic
findings yet uncovered by earth scientists.
The oldest sea floor rocks are around 200 million years old. This is remarkably young compared
to the age of the oldest rock yet found on the continents - around 3.8 billion years old, and the
age of the earth - around 4.5 billion years old. The continents are made of lighter granitic rock
that cannot sink back into the mantle as do the heavier ocean floor basalts. This is why "nonrecycling" continental rock is much older that the "recycling" ocean floors.
Since the earth was formed, the magnetic field surrounding the planet has actually "flipped" a
number of times - the Magnetic North Pole switching with the Magnetic South Pole. In the past
3 million years, this has occurred at least seven times. The most recent reversal occurred about
700.000 years ago. These reversals are recorded in iron particles of newly forming rocks in rift
zones - a seafloor, magnetic "tape recorder."
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In studies of the sea floor, research ships with magnetic sensing devices recorded matching
bands of reversed magnetic fields on either side of ocean ridges or rift zones. This led geologists
to believe that the sea floor was spreading equally in opposite directions from these zones, and
that the ocean floor was a "time tape" with rocks being progressively older the farther away you
move from the spreading zone. Further study of fossils and sediments gathered from the sea
floor has confirmed this theory.
The average rate of spreading in a rift zone is around two inches per year. Since continental
crust rests on the same plates that are moving away from the spreading zone, the continents are
moving at the same average rate. Over geologic time, sea floor spreading has reshaped the face
of the earth, with moving plates, volcanoes and earthquakes the inevitable result.
ACTIVITY: (15 - 20 minute activity)
SUGGESTED INTRODUCTION:
Earthquakes are caused by moving crustal plates. The plates are propelled by heat currents in
the mantle. As a result of this action, plates spread apart along ocean floor "seams." The age of
sea floor rocks and reversal of magnetic fields recorded in them gives strong proof of plate
tectonics theory. The birthplace of new earth crust lies in zones of SEA FLOOR SPREADING.
PREPARE:
PARTICIPANTS:
Place the HEMISPHERE MODEL and the SPREADING BOX on a table for easy
viewing. Have on display the PLATE PUZZLE MAP paper copy. Have SEA
FLOOR BASALT ROCK SAMPLE ready for examination, QUAKE GAME &
CONTINENTAL CUTOUTS and SEA FLOOR ACTIVIT" & COMPASS ready for
use.
* Younger children – Although the concepts are complex, the "QUAKE GAME"
and the SEA FLOOR SPREADING BOX can be used to give a "feel" for the plate
movements. Further understanding can be developed later.
* Older children – Focus on the dynamics of sea floor spreading as the source of
both plate movements and earthquakes.
– Emphasize the ways that the sea floor spreading phenomenon has been proven
and its importance in plate tectonics theory.
DO:
EXPLAIN:
Using the EARTH HEMISPHERE MODEL, review the CORE, MANTLE and
CRUST model of the earth
We are going to examine the area where new crust is being formed and where
plates are spreading apart.
DO:
Use both the CROSS-SECTION and the SURFACE-SECTION to show where plate
spreading is occurring.
DO:
Using the PLATE PUZZLE MAP, point out the spreading zones in the Pacific and
Atlantic. Pass around the SEA FLOOR BASALT ROCK SAMPLE, to show what
new sea floor crust is like.
SEA FLOOR SPREADING
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EXPLAIN:
The BASALT ROCK SAMPLE is from the spreading zone off the coast of Oregon,
where a small plate, the Juan de Fuca plate, is spreading away from the Pacific
Plate and subducting under the continent.
EXPLAIN:
Give one-half of your group the laminated map of the Atlantic floor and the
other half the Pacific floor laminated map. They should try to find the deepest
part of the ocean floor. You can also ask them to find the longest ridge.
EXPLAIN:
To get a feel for how sea floor spreading works, we will simulate its action in a
short activity called, "SPREADING PLATES."
NOTE:
Follow the instructions on the "QUAKE GAME" insert in the folder.
You'll need at least 6 kids (no upper limit) to do this activity, and an amount of
space appropriate to the group size.
Display the SEA FLOOR SPREADING BOX.
DO:
EXPLAIN:
We will demonstrate this spreading action again with the SEA FLOOR
SPREADING BOX. Point out the red mantle material on the side of the box rising
to form new plate material as the plates move away from each other.
EXPLAIN:
Magma rises in the spreading zone as it does in volcanoes. Though ocean floor
volcanoes do exist, most of the magma cools before it reaches the surface.
DO:
Have two volunteers slowly pull out new "crust material" at the same rate from
both sides of the "rift zone."
EXPLAIN:
The actual process of sea floor spreading is very slow; an average of two inches of
new crust being formed per year. It took over 100 million years for South America
and Africa to get where they are today.
DO:
Remind the audience that moving crustal plates are the main cause of earthquakes
and volcanoes.
NOTE:
If you are going to continue and do the "MAGNETIC SEA FLOOR ACTIVITY
ASK:
How do we know, without seeing the bottom of the ocean, that new crust is
forming? What proof do we have?
RESPONSE:
Although the concepts in the"MAGNETIC SEA FLOOR ACTIVITY" may be
difficult, all but the youngest children should be able to see the connections
between magnetic changes, moving plates, and earthquakes.
MAGNETIC SEA FLOOR ACTIVITY
EXPLAIN:
The earth is surrounded by an invisible "field" that causes all compass needles to
point "north." (Show the COMPASS.) This magnetic field is not as permanent as
we once thought. Although compasses point north now, over millions of years of
SEA FLOOR SPREADING
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EXPLAIN:
EXPLAIN:
earth's history the magnetic field has inexplicably "flipped" a number of times –
the North and South magnetic poles actually exchanging places!
If you traveled back in time with a compass in hand, you would see the compass
needle flip around 700,000 years ago, the last recorded reversal. When you have
traveled back to 3 million years ago, the needle would have reversed about seven
times.
Iron particles in the rocks that form along the sea floor spreading zones pick up
the magnetic field of the earth at the time the rocks form. By using the bar
"compass" MAGNETS and the SEA FLOOR SPREADING BOX, we can show how
the magnetic reversals are recorded in the rocks like a giant tape recorder, and
how this has proved sea floor spreading action.
SEA FLOOR SPREADING
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DO:
EXPLAIN
Return the sea floor material on the SEA FLOOR SPREADING BOX to its original
position. Using the COMPASS (or MAGNETS), have one volunteer measure the
magnetic field in the zone where the new plate material is emerging, as two others
slowly pull out the material.
As the compass needle or bar magnet reverses direction, point out that the new
crust has recorded the existing magnetic field of the earth in the rocks on both
sides of the rift zone.
DO:
When the sea floor material is fully extended on both sides of the rift zone, have
someone with the second magnet join the other in slowly passing the MAGNETS
away from the rift zone over the sea floor.
DO:
Point out that the location on the reversals on both sides of the rift zone are
symmetrical – equally distant from the rift zone.
EXPLAIN:
ASK:
RESPONSE:
ASK:
RESPONSE:
ASK:
RESPONSE:
DO:
Originally these changes in magnetic field direction was noticed by passing ships
when their compass needles would swing wildly at certain locations.
Where do you think we would find the oldest sea floor rocks?
At the two points farthest away from the rift zone.
Where are the youngest sea floor rocks?
In the rift zone area.
Where does the old crust go?
At other places on earth, the older rocks are pulled down into the mantle layer by
subduction and are recycled in the mantle, eventually forming new seafloor
materials elsewhere – a very slowly circulating, seafloor "conveyor belt."
Repeat this activity with new participants performing different parts of the
demonstration.
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SEA FLOOR SPREADING
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