Have You Seen a Fulgurite?
DOUBTLESS you have heard of trees being split and people being killed by lightning. But have you heard of fulgurites being formed by lightning?
The term “fulgurite” comes from the Latin fulgur, meaning “lightning,” and has been used since 1821 to describe the unusual mineral formation created when lightning strikes a sandy patch of ground. The intense heat of the electrical discharge melts some of the sand and forms a long, narrow and very fragile tube of glass.
Most fulgurites, or “lightning tubes,” are only a few meters long. However, some have attained a length of as much as 20 meters (66 feet). They usually go straight down and, less frequently, run parallel to the surface of the ground.
The tendency for lightning to follow the path of least resistance accounts for the irregular shape of the tubes and the forks or small offshoots protruding from the main stem. The length of a fulgurite seems to be determined by the depth and resistance of the sand, the position of the local water table and the power of the discharge.
Since temperatures of 30,000 degrees Celsius (54,000 degrees Fahrenheit) can be reached in a lightning flash, quartz sand can be heated to the 1,700 degrees Celsius (3,100 degrees Fahrenheit) needed to melt it, and that for quite a depth into the earth’s surface.
These tubes are generally only about one centimeter (0.4 inch) in diameter, but they may reach a diameter of several centimeters. The exterior is characterized by ridges and flanges with semifused and unfused sand grains adhering to them. A fulgurite is roughly circular in shape.
The hollow core is thought to be formed by the rapidly developed outward pressure from the expansion of gases emitted by the instantaneous heating of the damp sand. In most cases, the silica glass wall, smooth on the inside, is only about 1 millimeter (0.04 inch) in thickness. It is generally light gray in color, although some opaque-white to dark-gray ones have been found.
Fulgurites are mainly found in sandhill country that is prone to electrical storms. On occasion, the exact position of the lightning strike has been observed and a fulgurite has been located when the ground around it is still warm. At other times, fulgurite hunters have been rewarded with finds where the surface of the sandy ground has been eroded away, revealing the upper section of the tube.
As you can imagine, it is extremely difficult to extract such a thin glass tube intact, considering its length and frailty. Usually the tube breaks into many pieces, which have to be painstakingly pieced together. Hence, a relatively small number of fulgurites have been preserved.
Although rare, fulgurites have been found all over the world. So, perhaps, when visiting a nearby museum, you may be able to view a fulgurite and see for yourself what can happen when millions of volts of lightning strike the earth.