Download sea-floor spreading

Name: _____________________________________________
UNIT 2: Minerals and Rocks
_______________________________
Date: ______________
Period: _____________ Group
LAB 3-3: SEA-FLOOR SPREADING
INTRODUCTION: According to the theory of plate tectonics, the Earth's crust is broken into
many slowly moving plates. Sea floor spreading occurs at the mid-ocean ridge where two
plates are moving away from each other. Here, magma rises up from below as the sea floor
spreads out to either side. This spreading occurs at about the same rate as your fingernails
grow.
The pattern of sea floor spreading can be observed by studying the magnetic field of the
rock on the sea floor. At the mid-ocean ridge, magma rises up from the mantle below and
cools. As it continues to cool, iron in the rock aligns itself with the magnetic field of the
Earth, much like the needle in a compass. When the rock solidifies, this magnetic
"signature" is locked in place.
When Alfred Wegener proposed his theory of Continental Drift, he did not have any
evidence to support a mechanism for this crustal motion. In the 1960's, ocean research
vessels investigated the magnetic field in the rocks on the floor of the Atlantic Ocean. These
ships trailed magnetometers - instruments designed to detect magnetic fields. These
investigations turned up some surprising results, adding to a growing body of evidence that
Sea Floor Spreading is the mechanism of continental drift.
OBJECTIVE: In this investigation, you will use a model of a sea floor spreading zone. The midocean ridge is running north to south down the center of the model. You will use a Magnetic
Field Sensor to map the magnetic field of your model and use it to explain how this is
evidence of sea floor spreading. Using an ocean bottom profile constructed from ocean
depth data, and maps of the ocean floor and of Earth's tectonic plates, you will identify
features of the ocean bottom in regions of diverging plate boundaries.
VOCABULARY:
sea-floor spreading:
mid-ocean ridge:
rift valley:
PROCEDURE A: Investigating the magnetic field of the ocean floor
MATERIALS:
Vernier LabQuest
Magnetic Field sensor
model sea floor spreading zone
ruler
1. Connect the Magnetic Field Sensor to Channel 1 of the LabQuest. The switch on the sensor
should be set on the LOW X 10 amplification setting.
2. Tap File - Open - Sea Floor Spreading
3. Zero the Magnetic Field Sensor. This step removes the effects of the Earth's magnetic field
and any local magnetism from the results. Zero it BEFORE obtaining your model sea
floor spreading zone.
a. Remove anything magnetic from the area where data collection will take place.
b. Hold the sensor parallel to the table over the area where you will be collecting data, with
the white dot near the tip of the sensor facing upwards (as it will be during data
collection).
c. When the magnetic field readings on the screen have stabilized, click
4. Obtain the model sea floor spreading zone and prepare for data collection.
a. Place the model sea floor spreading zone on your table with the side marked North at the
top. The mid-ocean ridge is running north-south down the center of the model.
b. Position the ruler so that it is lined up with the East and West marks on the sides of the
pan. Align the 0 cm mark with the left edge of the pan as shown in the diagram below.
5. Tap
to begin data collection.
6. Position the tip of the Magnetic Field Sensor at the 0 cm mark. Important: For each reading,
make sure the probe is at the same height as the ruler and the white dot is facing up.
7. When the reading has stabilized,
click
8. Type "0" in the edit box (for 0 cm) and press the Enter Key. The first data pair has now been
saved.
9. Move the Magnetic Field Sensor to the 1 cm mark. When the reading has stabilized, click
Type "1" in the edit box and press the Enter key.
10. Repeat the Step 9 procedure at 1 cm intervals until you reach the other side of the pan.
11. When data collection is complete, click
to end data collection.
12. Save your file.
13. Connect to a computer to print a copy of your graph.
PROCEDURE B: Processing the magnetic field data
1. Study your graph. The mid-ocean ridge runs north to south down the center of your model.
Use a ruler or straight-edge to draw a line on your graph representing the mid-ocean ridge.
Label the line "Mid-Ocean Ridge".
2. Draw the bands of magnetic reversal on your graph.
a. Study your graph. A change between normal magnetic field ("normal" meaning the north
magnetic pole is aligned with the north geographic pole) and reverse magnetic field
("reverse" meaning the north magnetic pole is aligned with the south geographic pole.)
occurred each time the line crosses zero. Using a straight-edge or ruler, draw a vertical line
on your graph at each point where the line crosses zero.
b. In locations where the magnetic field values are positive, the magnetic field of the Earth
was normal. In locations where the magnetic field values are negative, the magnetic field of
the Earth was reversed. Lightly shade the bands of ocean floor where the Earth's magnetic
field was reversed with a colored pencil.
c. Across the top of your graph, label each band either "normal" or "reverse".
3. Toward the bottom of your graph, draw two arrows indicating the direction of movement of
the ocean floor. Refer to the "Tectonic Plates" map in the Earth Science Reference Tables.
PROCEDURE C: Features of the Ocean Floor
1. On the Ocean Bottom profile provided, label the following ocean floor features: Mid-Atlantic
Ridge, Rift Valley, continental shelf, deep ocean floor and seamounts.
3. Referring to the "Tectonic Plates" map in the Earth Science Reference Tables, draw arrows
representing the directions in which the sea floor is moving.
PROCEDURE D: Complete the following procedures on the "Age of the Atlantic Sea Floor"
map below. The profile from Procedure C is along line AB.
1. Label the Mid-Atlantic Ridge
2. Color the region of youngest rocks on the map. Use red pencil.
3. Referring to the "Tectonic Plates" map in the Earth Science Reference Tables, and your
ocean bottom profile, draw arrows on either side of the Mid-Atlantic Ridge indicating the
direction in which the sea floor is moving.
PROCEDURE E: Determining the rate of Sea-Floor Spreading
Magnetic properties of the ocean floor have made it possible for scientists to establish
the age of vast areas of ocean bottom. The diagram below represents a section of
ocean floor in the central North Atlantic. The numbers on the lines give the ages in
millions of years for parts of the ocean floor that are located along the lines.
The rate at which sections of the ocean floor are moving away from the Mid-Atlantic
Ridge can be calculated by dividing the distance traveled by the time required to travel
that distance.
Determine the rate at which sections of the ocean floor are moving away from the MidAtlantic Ridge as follows:
a. Measure the distance from the 9 MYA line to the Mid-Atlantic Ridge along line A to
the nearest millimeter and record your answer in column B of the Report Sheet.
ALWAYS MEASURE TO THE MID-ATLANTIC RIDGE!!
b. Convert your measurement to kilometers by multiplying by 65 and record your answer
in column C.
c. Convert kilometers to centimeters by multiplying by 100,000 and record your answer
in column D.
d. Calculate the rate of sea-floor motion using the following equation:
Rate of sea-floor motion
(cm/yr)
=
Actual distance sea floor moved (cm)____
Time to move that distance (millions of years)
Important: Remember to convert time to MYA!!! (You need to divide by 9,000,000,
38,000,000, etc.)
e. Repeat the procedure for the other seven lines.
f. Determine the average rate of motion and enter it in the Data Table.
The rate of sea-floor motion is a measure of how fast the ocean floor is moving away
from the Mid-Atlantic Ridge. The rate of sea-floor spreading is a measure of how fast
sections of ocean floor on opposite sides of the Mid-Atlantic Ridge are moving away
from each other.
g. Determine the average rate of sea-floor spreading by multiplying the average rate of
motion by 2.
Report Sheet
A
B
C
D
E
Millions of
years ago
Distance
(mm)
Distance
(km)
Distance
(cm)
Rate of sea-floor
motion (cm/yr)
9
38
53
63
81
135
155
180
Total
Average rate of
motion
Average rate of seafloor spreading
DISCUSSION QUESTIONS: (Answer the following in complete sentences on a separate
sheet of lined paper. You may type your answers.)
1. What prominent sea floor feature is found in the central Atlantic Ocean.
2. Describe the motion of the sea floor that is responsible for the formation of the Mid-Atlantic
Ridge's rift valley.
3. As distance from the Mid-Atlantic Ridge increases, what change in the age of the sea floor is
observed?
4. How old are the oldest rocks of the Atlantic sea floor?
5. Using your value for the Average rate of sea-floor spreading obtained in Procedure E,
calculate how long it would take the Atlantic Ocean to widen another one kilometer. (SHOW
ALL WORK)
CONCLUSION: Explain how the Earth's magnetic reversals provide evidence of sea-floor
spreading.