Seafloor Magnetism - In the previous lesson, you learned that magnetometers used on land were important in recognizing apparent polar wander. Magnetometers were also important in understanding the magnetic polarity of rocks in the deep sea. During WWII, magnetometers that were attached to ships to search for submarines discovered a lot about the magnetic properties of the seafloor.
In fact, using magnetometers, scientists discovered an astonishing feature of Earth’s magnetic field. Sometimes, no one really knows why, the magnetic poles switch positions. North becomes south and south becomes north! When the north and south poles are aligned as they are now, geologists say the polarity is normal. When they are in the opposite position, they say that the polarity is reversed.
Scientists were surprised to discover that the normal and reversed magnetic polarity of seafloor basalts creates a pattern of magnetic stripes! There is one long stripe with normal polarity, next to one long stripe with reversed polarity and so on across the ocean bottom. Another amazing feature is that the stripes are form mirror images on either side of the mid-ocean ridges.
The ridge crest is of normal polarity and there are two stripes of reversed polarity of roughly equal width on each side of the ridge. Further distant are roughly equal stripes of normal polarity, beyond that, roughly equal stripes of reversed polarity, and so on. The magnetic polarity maps also show that the magnetic stripes end abruptly at the edges of continents, which are sometimes lined by a deep sea trench.
The scientists used geologic dating techniques to find the ages of the rocks that were found with the different magnetic polarities. It turns out that the rocks of normal polarity are located along the axis of the mid-ocean ridges and these are the youngest rocks on the seafloor. The ages of the rocks increases equally and symmetrically on both sides of the ridge.
Scientists also discovered that there are virtually no sediments on the seafloor at the axis, but the sediment layer increases in thickness in both directions away from the ridge axis. This was additional evidence that the youngest rocks are on the ridge axis and that the rocks are older with distance away from the ridge.
The scientists were surprised to find that oldest seafloor is less than 180 million years old while the oldest continental crust is around 4 billion years old. They realized that some process was causing seafloor to be created and destroyed in a relatively short time.
The scientists also discovered that the seafloor was thinner at the ridge axis and grew thicker as the crust became older. This is because over time, additional magma cools to form rock. The added sediments also increase the thickness of the older crust.
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