Module 3, Investigation 2: Sea Floor Spreading
Introduction
Welcome!
In Investigation 1 you learned about Wegener’s theory of continental drift and why the scientific
community never accepted it. Yet, a growing body of evidence suggested that the Earth's
geography had been different in the past.
In this investigation, you will consider evidence from the sea floor that was not available in
Wegener’s time, but has since been critical to helping scientists understand how the geographic
positions of continents could change over long periods of time.
Folder 1: Sea Floor Features
Sea Floor Features
When you look at the satellite imagery in Google Earth, you can see a lot of detail in
the topography of the sea floor. Look at the Atlantic Ocean between South America and Africa.
Can you see the smooth plains extending away from the continents and what looks like a
jagged scar running north-south in the middle?
Turn on the Trans-Atlantic Transect folder, which shows a line across the ocean. While keeping
an eye on the elevation at the bottom of your screen, move your pan tool slowly across the line
from the coast of South America (Station 1) to the coast of Africa (Station 5).
Record the elevation of each station to the nearest 100 m in your field notebook, draw the
profile, and record your interpretations.
Next, use your path tool to trace the scar-like feature as far as you can across the globe. If it
branches, add another path to follow that branch. You can make your paths using any color,
except red.
Look carefully for other similar features elsewhere on the globe and add paths for each one.
Hint: Some of these features are very faint!
Save your paths in your My Sea Floor Features folder and record the observations and
interpretations prompted in your field notebook.
Folder 2: Undersea Volcanoes
Thar She Blows!
In the last placemark, you mapped what is called the mid-ocean ridge system. These are the
most extensive mountain ranges on Earth. Most of these ranges lie in the deep ocean, but in a
few spots, such as Iceland, they reach the surface. You visited Iceland earlier in this class.
What do you remember about the geology of Iceland? You can refer to your field notebook to
refresh your memory.
Next, watch the video below showing volcanic eruptions in the deep sea, and record your
observations and interpretations in your field notebook.
Folder 3: The Hypothesis
The Sea Floor Spreading Hypothesis
Scientists reason that as new rock forms along the mid-ocean ridge that you explored in the last
placemark, the sea floor will spread and the continents will be pushed apart. If this were really
happening, where would the sea floor be youngest? Where would it be oldest?
How can the age of the sea floor be determined so that we can use this evidence to test out our
ideas regarding the age of the sea floor? The answer has to do with the Earth’s magnetic field.
If you have ever used a compass, you know that the Earth has a magnetic field with a
specific polarity. That is why the compass arrow always points toward the magnetic North Pole
rather than toward the South Pole.
Scientists are still not sure what actually creates the Earth’s magnetic field. It has something to
do with the influences of the Earth’s solid metallic core and rotation around its axis on the flow of
electrically conducting fluid within the outer core. This is called the “dynamo theory”.
The polarity of the Earth’s magnetic field (which tells us which way is North) is recorded in
rocks, because when molten rock cools, the metallic minerals in the magma act like
tiny compass needles and align toward the magneticNorth Pole. However, in the 1920’s
scientists discovered that in some rocks, the polarity is reversed. The minerals in the rocks
point south! What?
Scientists later discovered that the reason for this is that the polarity of the
Earth’s magnetic field suddenly reverses on occasion. Scientists are able to determine when in
Earth’s history these flips have occurred, but still are not sure why they happen.
During the 1950s and 1960s research ships used magnetometers to measure changes in
the polarity of the rocks making up the sea floor. Once the data were plotted, scientists could
clearly see what appeared to be regular and continuous stripes of rock with
alternating polarity across the ocean basins. The stripes seemed to be mirrored on each side of
the mid-ocean ridges.
By knowing the approximate age of each magnetic reversal, scientists could then determine the
ages of various sections of the sea floor, and could calculate the rates of sea floor spreading.
In Folder 1, copy your My Sea Floor Features folder and paste it into Folder 3 (right click on
each folder to access the copy and paste menu). Turn on the Sea Floor Age folder and observe
how sea floor age relates to the mid-ocean ridges.
Can you see how the rock making up the sea floor is progressively older as you move away the
ridges? But be forewarned! Scientists are continuing to map the magnetic polarity of the sea
floor, and are now discovering that the picture is much more complicated than this, so stay
tuned for further discoveries!
For now, use the data presented to make the observations and interpretations prompted in your
field notebook.
Summary
Let's Reflect
You’ve now learned how scientists can determine the age of the sea floor and that the age
pattern shows that the sea floor is expanding. Sea floor spreading can explain why the
continents appear to have "drifted" apart over time. But what causes the sea floor spreading?