Earths paleomagnetic field

Over two thousand years ago, philosophers in ancient Greece discovered that certain rocks acted like natural magnets. These rocks, what Produces Earth's Magnetic Field?

Even today, the exact cause of Earth's magnetic field is still somewhat of a mystery. Most geologists believe that it is due to something called the "dynamo effect." It is known that Earth acts like a giant bar magnet with two ends, or "poles." However, scientists are pretty sure that there is not a solid magnet running through the planet. Instead, as Earth rotates, a magnetic field is generated because the center— or core—of Earth is thought to be divided into two parts. The inner core is believed to be a solid mass made mostly of iron and nickel. The outer core is thought to be liquid iron. As Earth spins, the liquid part of the core moves past the solid inner core and it also cuts through the magnetic field of the Sun. Because iron and nickel are good conductors of electricity, this motion generates an electrical current, which in turn generates the planet's magnetic field.

If this sounds complicated, it is. In fact, there are still many questions to be answered. The only thing that we know for sure is that the magnetic field of the planet has two poles that are near the geographic (true north and south) poles of Earth. According to the data derived from ancient rocks, the magnetic field of Earth has completely shut down in the past and the poles have reversed direction. Scientists still have a great deal to learn about how our magnetic field works, but the fact that we have it has helped geologists piece together the plate tectonic puzzle.

which contain the iron-rich mineral magnetite, form from magma that cools and crystallizes. As it cools, iron crystals in the magma line up with Earth's magnetic field. When the rock hardens, these crystals act like little microscopic compass needles pointing to Earth's north magnetic pole. This phenomenon is known as remnant magnetism, and it has become one of the most important tools used by geologists since the discovery of radioactivity.

In the early 1900s, geologists studying the remnant magnetism of thick sequences of lava flows made an amazing discovery. When they measured the magnetism of the rocks from top to bottom, they found that as they went back in time, the magnetism of the iron crystals reversed direction. It was as though the entire magnetic field of Earth had flipped with the magnetic north pole becoming the magnetic south pole. As they went further back in time, the poles flipped again back to their present-day position.

When this discovery was first announced, most geologists just thought that the findings were due to equipment error. The more rocks they studied, however, the more reversals they found. There was definitely something happening to Earth's magnetic field to make it change over time.

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