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Plate Tectonics

How does magnetic reversals provide evidence for sea floor spreading?


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Evidence for Seafloor Spreading: Magnetic Reversals

Some of the most important evidence of seafloor spreading comes from magnetic reversals recorded in the ocean floor. Throughout Earth's history, the north and south magnetic poles have changed places many times. When the poles change places, the polarity of Earth's magnetic poles changes, as shown in Figure 4. When Earth's magnetic poles change places, this change is called a magnetic reversal.

Figure 4 The polarity of Earth's magnetic field changes over time.

Magnetic Reversals and Seafloor Spreading

The molten rock at the mid ocean ridges contains tiny grains of magnetic minerals. These mineral grains contain iron and are like compasses. They align with the magnetic field of the Earth. When the molten rock cools, the record of these tiny compasses remains in the rock. This record is then carried slowly away from the spreading center of the ridge as seafloor spreading occurs.

As you can see in Figure 5, when the Earth's magnetic field reverses, the magnetic mineral grains align in the opposite direction. The new rock records the direction of the Earth's magnetic field. As the sea floor spreads away from a mid ocean ridge, it carries with it a record of magnetic reversals. This record of magnetic reversals was the final proof that seafloor spreading does occur.

Figure 5 Magnetic reversals in oceanic crust are shown as bands of light blue and dark blue oceanic crust. Light blue bands indicate normal polarity, and dark blue bands indicate reverse polarity.