Although nobody really knows why, Earth’s magnetic poles sometimes switch positions. The north magnetic pole becomes the south magnetic pole and vice-versa. This causes a reversal of Earth’s polarity. When north and south magnetic poles are located where they are now, Earth’s polarity is referred to as normal polarity. When the poles are in the opposite positions, Earth’s polarity is referred to as reversed polarity. During World War II, magnetometers were attached to battleships to help search for underwater submarines. The magnetometers discovered an astonishing feature: seafloor rocks... Show more Although nobody really knows why, Earth’s magnetic poles sometimes switch positions. The north magnetic pole becomes the south magnetic pole and vice-versa. This causes a reversal of Earth’s polarity. When north and south magnetic poles are located where they are now, Earth’s polarity is referred to as normal polarity. When the poles are in the opposite positions, Earth’s polarity is referred to as reversed polarity. During World War II, magnetometers were attached to battleships to help search for underwater submarines. The magnetometers discovered an astonishing feature: seafloor rocks show a pattern of normal and reversed magnetic polarity. Stripes of normal polarity and reversed polarity alternate across the ocean floor. The stripes form mirror images on either side of mid-ocean ridges. The stripes end abruptly at the edges of continents or at deep-sea trenches. Other characteristics of the seafloor also change in a consistent way as distance from mid-ocean ridges increases. At the axis of a ridge, the rocks are youngest, thinnest, and hottest. They also have no sediments on top of them. With increasing distance from the axis of the ridge, the rocks become older, thicker, and cooler. They also have more sediments above them. Seafloor rocks are oldest near the edges of continents or deep-sea trenches. But even the oldest seafloor rocks are less than 180 million years old. This is much younger than the oldest rocks of continental crust. This means that seafloor is destroyed in a relatively short time after it is created. Show less
Although nobody really knows why, Earth’s magnetic poles sometimes switch positions. The north magnetic pole becomes the south magnetic pole and vice-versa. This causes a reversal of Earth’s polarity. When north and south magnetic poles are located where they are now, Earth’s polarity is referred to as normal polarity. When the poles are in the opposite positions, Earth’s polarity is referred to as reversed polarity. During World War II, magnetometers were attached to battleships to help search for underwater submarines. The magnetometers discovered an astonishing feature: seafloor rocks show a pattern of normal and reversed magnetic polarity. Stripes of normal polarity and reversed polarity alternate across the ocean floor. The stripes form mirror images on either side of mid-ocean ridges. The stripes end abruptly at the edges of continents or at deep-sea trenches. Other characteristics of the seafloor also change in a consistent way as distance from mid-ocean ridges increases. At the axis of a ridge, the rocks are youngest, thinnest, and hottest. They also have no sediments on top of them. With increasing distance from the axis of the ridge, the rocks become older, thicker, and cooler. They also have more sediments above them. Seafloor rocks are oldest near the edges of continents or deep-sea trenches. But even the oldest seafloor rocks are less than 180 million years old. This is much younger than the oldest rocks of continental crust. This means that seafloor is destroyed in a relatively short time after it is created.
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