Lab: Evidence for Sea Floor Spreading
Name: ________________________________________________________
Scientists have been able to establish the ages of vast areas of rock on the ocean
bottom. The pattern in the ages of the rocks across the Mid-Atlantic Ridge is used as
evidence of sea floor spreading. In this activity, you will observe the pattern and
learn to calculate the rate of sea floor spreading.
The diagram represents a section of the ocean floor in the North Atlantic. The
numbers give the ages in millions of years for the rocks on the ocean floor.
Part A: Age Pattern
1) Locate the Mid-Atlantic Ridge and highlight it in red.
2) Lightly shade in the age bands using different colored pencils. In the space
below fill in the color that you chose for each age band.
0- 9 million years:
9- 38 million years
38- 63 million years:
63- 81 million years:
81- 135 million years:
135- 155 million years:
Rate of Movement Calculations:
38 Million year average rate of sea floor spreading:
1) Find the marked points on the top of the diagram.
a) Measure the distance between the 38 million year line and the MidAtlantic Ridge in cm.
Measurement on the map = _________________ cm
b) Next you will find the actual amount of movement in terms of kilometers.
Using the scale that 1 cm = 650 km, convert your measurement to km
Actual measurement in km on the ocean floor= ________________ km
c) Next you will convert the number of km the Atlantic Floor grew by into the
actual number of cm. To do this multiply the value above by 100,000.
Actual measurement in cm on the ocean floor= ______________________ cm
d) Finally, calculate the rate of sea floor spreading over the past 38 million
years by using the equation
Rate of Movement (cm/yr) = distance moved in cm
Time in years
Show your work and answer in the space below. Make sure to include units.
81 Million year average rate of sea floor spreading:
2) Find the marked points on the top of the diagram.
a) Measure the distance between the 81 million year line and the MidAtlantic Ridge in cm.
Measurement on the map = _________________ cm
b) Next you will find the actual amount of movement in terms of kilometers.
Using the scale that 1 cm = 650 km, convert your measurement to km
Actual measurement in km on the ocean floor= ________________ km
c) Next you will convert the number of km the Atlantic Floor grew by into the
actual number of cm. To do this multiply the value above by 100,000.
Actual measurement in cm on the ocean floor= ______________________ cm
d) Finally, calculate the rate of sea floor spreading over the past 81 million
years by using the equation
Rate of Movement (cm/yr) = distance moved in cm
Time in years
Show your work and answer in the space below. Make sure to include units.
Questions:
1) Describe the pattern in the ages of rock on the ocean floor (where are the
youngest and where are the oldest in relationship to the mid-Atlantic Ridge).
2) Has the average rate of sea floor spreading remained constant, and if not so, how
has it changed?
3) Using your age, calculate the amount the ocean floor has spread during your
lifetime. Remember that your calculations for the last 38 million years, applies only
to one half of the ocean floors spreading, so make sure you account for that. Show
your work.
4) Draw a diagram of the convection currents found in the Mantle that account for
the sea floor spreading. Make sure you include the location of the Mid-Atlantic
Ridge in your diagram.
5) Look at page 9 in your reference table, and find where it reads “ Pangaea begins
to break up. Using the maps shown, between which period of time did this occur?
6) The oldest rocks on Earth are located on the continents and are about 3.5 billion
years old (3500 million). Explain why the oldest rocks on the ocean floor are only
180 million years old.
7) What is occurring at the Mid-Atlantic Ridge that might explain the pattern in the
ages that you colored in on your diagram?