![]() ![]() The USGS film Secrets in Stone tells the story of how records of magnetic reversals helped lead to the evolution of the plate tectonics theory. It’s usually done if you’re welding thinner plates. Therefore, large amounts of heat appear at the anode, typically of all the heat. Reversals are not instantaneous they happen over a period of hundreds to thousands of years, though recent research indicates that at least one reversal could have taken place over a period of one year. It’s a condition where the anode is made positive while the work is made negative. The last reversal was about 780,000 years ago. They can happen as often as every 10 thousand years or so and as infrequently as every 50 million years or more. Electronic equipment plugged in to an outlet with reversed polarity will still function, but that doesnt mean its safe. The effect of different chemicals in the covering may change this condition. The bottom line is that reversed polarity at outlets is a shock hazard. Electrode-negative (straight polarity) results in faster melt-off of the electrode and, therefore, faster deposition rate. These reversals are random with no apparent periodicity to their occurrence. With few exceptions, electrode-positive (reversed polarity) results in deeper penetration. Incredible as it may seem, the magnetic field occasionally flips over! The geomagnetic poles are currently roughly coincident with the geographic poles, but occasionally the magnetic poles wander far away from the geographic poles and undergo an "excursion" from their preferred state. Earth's dynamo has no preference for a particular polarity, so, after an excursional period, the magnetic field, upon returning to its usual state of rough alignment with the Earth’s rotational axis, could just as easily have one polarity as another. When lavas or sediments solidify, they often preserve a signature of the ambient magnetic field at the time of deposition. ![]() We can see evidence of magnetic polarity reversals by examining the geologic record. ![]()
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