How does magnetic reversal prove seafloor spreading?

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Seafloor spreading is a process that occurs at mid-ocean ridges. These ridges mark the boundaries between two plates that are diverging, or moving away from one another. As the plates move apart, the crust is stressed and weakened. This allows magma from under the surface to break up through the crust, coming out and creating new sea floor. The plates move apart, carrying the “new” sea floor with it. The idea that this happens was proposed in the 1960s.

At first, seafloor spreading was not proven. However, magnetic reversals have now proven it. Magnetic reversals occur every so often. When they happen, the Earth’s magnetic field reverses its polarity. In other words, north becomes south and south becomes north. Magnetic reversal proves seafloor spreading because we can see the polarity of the Earth’s magnetic field in rocks. As magma cools, particles in it get “frozen” in the direction of the magnetic field. By looking at these particles, scientists can see the polarity of the magnetic field at the time the rocks were created.

If seafloor spreading actually happens, we would expect rocks farthest away from the mid-ocean ridge to be the oldest and we would expect the rocks to get progressively younger as we approach the ridge. Magnetic reversals help to prove this because scientists know when the reversals happened. As you can see in the link below, scientists have found bands of rock with different polarity on either side of mid-ocean ridges. This proves that these bands of rock were laid down at different times. The magnetic evidence shows that the oldest rocks are far away from the ridge and the newest rocks are closest. This is powerful evidence that seafloor spreading does in fact occur.

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