Petrified Lightning

Petrified Lightning

During the few seconds it takes to read this paragraph, lightning will strike the earth about 700 times. This rough estimate of lightning discharges at the rate of 100 times per second represents, worldwide, over four billion kilowatts of continuous power! Lightning is one of nature's most powerful forces, causing incalculable damage and, quite frequently, death. Utility companies worry about the intense surges of lightning—induced current that play havoc with power and telephone lines. The airlines worry about planes in the air, which essentially become flying lightning rods; commercial jets are struck by lightning on the average of once a year.

Because the extremities of aircraft, such as wingtips, nose, and tail, are usually the targets for lightning, most of these strikes do very little harm. Of course, exceptions occur, as in 1963 when a bolt struck a fuel tank of a Boeing 707 in a holding pattern near Elkton, Maryland. The explosion that followed brought down the plane, killing 81 people.

Nasa, extremely wary of lightning, continues to speculate over how to avoid strikes to space vehicles without calling off a launch every time a cloud is in sight. Their fears are well founded. In 1969, as Apollo 12 rose through what appeared to be a fair—weather cloud, it triggered two successive lightning strikes. Fortunately they caused only nondisabling damage to the outside of the spacecraft and some computer scrambling, so the astronauts continued their trip to the moon. Trouble occurred again in March 1987, when a lightning flash triggered the launching of an unmanned Atlas—Centaur rocket that broke apart under the sudden stress. A few months later nasa was again victimized when a lightning bolt triggered a premature launch of three small rockets; they plummeted unceremoniously into the Atlantic. Ironically one of these rockets was designed to study lightning.

The chances of getting hit by a “bolt from the blue” are proverbially small, yet deaths from lightning strikes average 85 persons per year. This is higher than the death toll from any other act of nature except flash floods. A direct hit by lightning packs enough electrical punch to fry skin and even explode organs, so the unfortunates in its path have a high rate of fatalities. Of course a lightning strike does not wipe out villages, and each victim is usually a single target.

When Benjamin Franklin flew his kite into a thunderstorm in 1752, he was exceptionally lucky to avoid being killed. He managed to draw a lightning bolt down through the kite line and into a key and perceived an electrical spark as he touched his knuckle to the key. Doubtless thrilled with his discovery, he remained unaware that he should have been doubly delighted at having lived through the experiment. A Swedish physicist trying to repeat Franklin's experiment a year later with a lightning rod instead of a kite was killed instantly.

Even nondestructive lightning often leaves a signature. In July 1960 two field geologists were conducting a geological survey in southeastern Arizona when a typical summer thunderstorm descended on them. The rain was so torrential that they could see only a few feet in front of where they stood. The geologists decided to get out of the desert (which was rapidly becoming a lake); their workday was obviously at an end. As they headed for their jeep, one turned to close the gate so the cattle wouldn't get out, and a very loud explosion completely enveloped him. The ground suddenly came up at him, and his feet felt as though they were carrying hundreds of pounds of weight. He went to his knees. As the stricken man regained his posture and composure, his alarmed partner hurried toward him asking if he was able to breathe. Relieved when the answer was “Yes, why?” the partner explained that a near lightning strike usually renders the victim unable to breathe. He was sure that the bolt had struck the gate—closing geologist—who, typical of people struck by lightning, never saw the bolt.

The day after the deluge the geologists returned to the scene of their encounter with lightning. At the precise spot that the bolt had struck, about two feet from where the man had stood, they found a greenish—brown, glassy, encrusted tube sticking out of the ground. One of the geologists remarked, “Looks like we have a fulgurite!”

When a lightning bolt strikes a loose surface of unconsolidated sand, it tends to fuse the sand (which melts at 1,710 degrees Celsius) into a glassy tube of silica, usually about half an inch thick. The tube may extend several feet beneath the surface. The largest ever found, in Cumberland, England, had a diameter of two and one—half inches and was over 40 feet long. As the tube goes downward it tends to branch, giving rise to numerous tubelets. It may contain fine glassy splinters on the surface and is often delicate to the touch. The splintery surface of the tube may be an image of the electrical lightning bolt that created this mineralogical wonder. That's why fulgurites are often referred to as “petrified lightning.”

From the book: 
Petrified Lightning