12. Reading
the Magnetic Patterns
Expedition Menu
The time-scale of the magnetic field reversals is shown at the top. Regions with orange or yellow patterns denote times of "normal polarity" or a magnetic direction with the same direction as today's field. The white regions represent times when the field was in the opposite (or reversed) direction from what it is today.
So now back to those patterns of magnetism in the oceanic crust -- how can they be explained using the concept of a flip-flopping Earth's magnetic field?
Vine and Matthews were the first to publish an explanation for the magnetic patterns, which built upon Harry Hess' early idea of seafloor spreading -- by matching the timing of the magnetic reversals with the patterns of magnetism assuming a model of seafloor spreading -- it works beautifully!
Finally in part c above, the situation is repeated, the crust at the mid-ocean ridge (in part b) splits in two by seafloor spreading and new magma rises to fill the gap, this time during a reversed polarity field -- these rocks split again with new magma rising to fill the gap and finally cools in a normal polarity field.
We can see an illustration of seafloor spreading in part (a) of the diagram on the left -- magma or molten rock (lava) erupts on the seafloor and records the direction of the Earth's field during a normal magnetic period. In part (b) seafloor spreading continues, splitting this normal polarity stripe in two, one of each side of the mid-ocean ridge, and more lava erupts to fill the gap -- this time the lava cools during a reversed polarity field. Still in part b, seafloor spreading continues -- splitting the crust of the mid-ocean ridge and more magma rises from the mantle to fill the gap -- this time it cools during another normal magnetic polarity field.
So the oceanic crust acts like a conveyor belt that records the direction of the Earth's magnetic field -- thereby producing the pattern of marine magnetic stripes of anomalies. ..
You will see an animation in the next expedition that may help you better understand this process.
The main point is that the patterns of magnetism in the oceanic crust are symmetric about the rift valley in the mid-ocean ridge, which marks the divergent plate boundary. In other words, the magnetic patterns in the oceanic crust are a mirror image on either side of the divergent plate boundary. Consequently, the process that produced the oceanic lithosphere on one side of the mid-ocean ridge is the same as on the other side of the mid-ocean ridge.
More importantly, you can estimate the age of the oceanic lithosphere at any location from the divergent boundary based on matching the magnetic patterns to the magnetic polarity time scale. By doing so, you can see that the age of the oceanic lithosphere/crust increases with distance away from the divergent plate boundary, located in the rift valley of the mid-ocean ridge, which can only be accounted for by the process of seafloor spreading.
Created By:
Don Reed
Dept. of Geology
San Jose State University
©Copyright 2008
Last Updated on
Sept. 22, 2008
Let's
next examine the rigid surface of the Earth
that form the plates