Magnetism Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Magnetic Reversal

In 1930, Motonori Matsuyama, a Japanese mathematician and physicist, began studying magnetism in rocks. He took a closer look at the reason some rocks pointed in one direction, while others pointed in another. Matsuyama studied magnetic anomalies and proposed the idea that they were the result of magnetic reversal .

When geologists took samples of lava flows in Hawaii and other places, they found that some lava samples contained grains with reversed polarity. This meant that thousands of years ago the northern magnetic pole was located where the southern magnetic pole is today and vice versa.

Polarity reversal is when the North Pole location and the South Pole location switch places.

The dating of lavas is possible through the use of radiometric methods using 40 K/ 40 Ar measurements. By using both radiometric dating and magnetic polarity measurements on ancient extruded lava layers, geologists were able to record the average between magnetic reversals. They found that, on average, the magnetic poles flip approximately every 200,000 years. By geological time the flip was overnight, but they actually happened over a gradual period of between 300 and 1000 years.

A magnetic switch begins with the reversal of an area of magma flow deep in the core of the Earth. As the switch area grows larger and more polar, the countercurrent works its way upward and begins to affect the magnetic currents in the crust and atmosphere. When this happens, areas of the Earth’s outer magnetic field begin to weaken. The countercurrent below cancels out the charges above.

Magnetic field strength or field intensity is the force applied to a magnetic pole at any point.

Weakened patches in a magnetic field are called anomalies . A magnetic anomaly may be high or low, subcircular, ridge-like, valley-like, or oval when studying a magnetic topographical map. The range of values of magnetic intensity over an anomaly or an area is called the magnetic relief .

The South Atlantic Anomaly is one of these weakened patches. In this area, the magnetic field is 30% weaker than other areas around the planet and it is growing. Geologists studying magnetic reversal over the past 10 years have used supercomputer programs, along with thousands of lava samples and the compass readings from British Naval officers’ journal notes from the past 300 years, to study magnetism. The result is an excellent prediction method of magnetic reversal.

These studies have revealed that the Earth is long overdue for a magnetic reversal. The last major reversal happened over 700,000 years ago. Knowing this, geologists now think that the South Atlantic Anomaly is the beginning of a magnetic switch. It will not happen in our lifetime, but probably sometime in the next 1000 years if the model holds true.

Magnetic polarity can be minor or major. The tectonic and environmental effects of a magnetic reversal are not known. Scientists are just starting to study and understand the implications of a planet-wide magnetic reversal.

Times of mostly normal polarity, like what we have today, or times of mostly reversed polarity, are called magnetic epochs or chrons . The Matsuyama Epoch , a major polar reversal around 0.5–2.5 million years ago, is named after Motonori Matsuyama.

As lavas from many magnetic epochs pile on top of each other, they build up layers with opposite magnetic polarities. Figure 6-8 shows how these reversed lava layers might look if you were to take a cross-sectional slice.

Igneous Rock Magnetic Reversal


Fig. 6-8. Different lava layers contain igneous rock magnetized in reversed magnetic fields.

Igneous rock provides geologists with many clues to the wild and crazy actions of ancient and recent magmas as they blasted or slowly forced their way to the Earth’s surface in different magnetic fields. Studying these clues will help us better understand magma’s tricks and the Earth’s future.

Practice problems of this concept can be found at: Igneous Rock Practice Test

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