The time-scale of the
magnetic field reversals is shown at the top. Regions with orange or
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
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
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!
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
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.