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Name: Holton
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: N/A 


Question:
How large a lightning bolt does it take to fuse the minerals etc. in the ground to form large rocks. How much energy or how big of a strike is needed, etc? Or will any strike do it, with the variables being the amount of mineral content in the soil and the size of the lightning bolt determining the size of the rocks? A friend has these on his family property in SE Alabama. He never knew>what kind of rocks they were. So he sent sample to a friend at NASA and was told it had been formed by a strike that had fused the minerals within the soil (iron, etc.). Now he wants to know how big a strike it would take to form them.
Thanks.



Replies:
Dear Holton-

Lightning strikes can cause dramatic changes to the materials they strike, whether it be the ground, a tree or other object in the air, such as a radio or TV tower. Most communications towers are struck frequently, and are equipped with lightning protection devices, to prevent damage to the tower and transmitters and receivers.

Other objects, which may not be good conductors of electricity, can suffer major damage or changes when struck. The ground falls in this category. It would be difficult to accurately estimate the amount of energy required to fuse the minerals in the ground into a glass-like rock. As you mentioned, there are several variables that would affect this formation.

Studies have estimated the amount of energy released or dissipated from lightning strikes, and an "average" strike can be described. Lightning formation is a very complex sequence of events, and not all of them are completely understood by scientists. But observations have shown that lightning initially begins as a "stepped leader" of negative charge that works its way from the cloud towards the ground. A positive charge becomes concentrated in the ground below the thunderstorm. When the leading edge of the stepped leader gets close to the ground, the electrical field becomes strong enough to induce an upward streamer, or ground stroke, called the "return stroke," and this stroke carries the principal electrical current of the flash.

Some characteristics of the return stroke:

      Velocity - about 60,000 miles per second, or about 1/3 the speed of light.

      Current - 20,000 amperes

      Temperature - 45,000 degrees F.

      Duration - 10 milliseconds.

The high temperatures cause some gaseous compounds in the atmosphere to become disassociated, and the rapid cooling, or "freezing" of these substances after the stroke, such that nitrates are formed and washed into the soil by the rain. This is a natural "fertilizer" produced by thunderstorms.

Because of the short duration of the stroke, a typical lightning flash carries to the earth about 1 ampere of current. With an average flash rate of about 3 flashes per minute in a typical thunderstorm, the average electrical power dissipated is about 50 million watts.

The above information was obtained from the "Handbook of Applied Meteorology," edited by David D. Houghton. Most descriptions of weather phenomena in that book are fairly technical in nature...and an easier to understand description can be found in "The Weather Book" by Jack Williams and published by the USA Today newspaper.

I hope this information, though not really answering your question, was helpful in understanding some of the characeristics of lightning.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO



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