Lightning Strikes and Minerals
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.
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
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
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
Click here to return to the Environmental and Earth Science Archives
Update: June 2012