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Yosemite Nature Notes 47(3) (1978)


PETRIFIED LIGHTNING

Betty Jane Pollan

A fossil record of lightning’s striking the earth’s surface is called “fulgurite,” from the Latin word “fulgur,” meaning lightning. This “petrified lightning” leaves permanent evidence of lightning’s having penetrated the surface of the earth at temperatures high enough to crystallize the available minerals.

Typically, the fulgurite is found on beaches of sand-size particles having a chemical composition high in silicon. The specimen forms when the heat produced in the lightning fuses the minerals.

Silicon is one of the four major chemical constituents of Yosemite granites; others are hornblend, mica and feldspar. It is interesting to consider the possibility of fulgurites existing in Yosemite granites; however, it is unlikely that they exist on the peaks that are routinely capped with the base of the lightning-producing nimbostratus. Lower pressure at the high altitudes induces a lightning discharge, but the extremely tall mountains rarely are struck. This apparent contradiction is explained by the fact that peaks protruding into the cloud base will be in direct contact with the negative charge of the thunderhead. The charge then drains directly into the crust via the mountain peak without igniting a spark which results in the extreme heat (1800-2000 C) necessary to crystallize the chemicals into a fulgurite. Recent studies along the Continental Divide show that lightning has a decreased charge with increased elevation and that likely no lightning ever strikes above 1800 feet.

In considering the Yosemite granites, it is reasonable to assume that those with greater silicon content and rising from approximately 5,000 feet to 10,000 feet above sea level, hold the best potential for fulgurite development.

The accompanying diagram depicts the extent of fulgurite dimension and development.

fulgurite
[click to enlarge]

Dr. Carl Sharsmith, who has spent nearly a lifetime in the high Sierra, reports having seen a fulgurite on top of Mt. McClure. To date, the author has not found a fulgurite in Yosemite, but the search continues.

BIBLIOGRAPHY

1. Anderson, Bette Roda. WEATHER IN THE WEST FROM MIDCONTINENT TO THE PACIFIC, American West Publishing Co., Palo Alto, CA., pp. 137-144. 1975.

2. Matthes, Francois E. GEOLOGIC HISTORY OF YOSEMITE VALLEY, U.S.G.S., U.S. Government Printing Office, Washington, D.C., 1930.

3. Viemeister, Peter E. THE LIGHTNING BOOK, The MIT Press, Cambridge, Mass., pp. 108-109. 1972.



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