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Largest melt from lightning strike.

Largest melt from lightning strike

A few years ago, two boys discovered what they thought were dinosaur bones near Winans Lake in Michigan. Their parents, examining one 5-meter-long white, green and gray object, agreed with the boys' interpretation and called the Museum of Paleontology at the University of Michigan in Ann Arbor. In 1984, paleontologist Daniel C. Fisher went to investigate. But what Fisher and petrologist Eric Essene found instead was the world's largest known fulgurite -- a tube-shaped glob of glass that had formed when lightning struck the ground.

Scientists have known for two centuries that lightning bolts, which can heat the air to 10,000 [deg.] Kelvin -- comparable to the sun's surface temperature -- can vaporize and melt any rocks, sand or soil they hit. But Essene and Fisher's study of the Winans Lake fulgurite, published in the Oct. 10 SCIENCE, is among the first quantitative investigations of the chemical and physical processes behind fulgurite formation.

Their studies reveal the presence of two minerals that had never before been found to occur naturally. But more important, the two researchers determined that the Winans Lake fulgurite is one of the most chemically reduced (deoxidized) natural materials known. Moreover, they believe their findings adds a new wrinkle to studies of another ultrahigh-temperature, ultrafast event: the proposed impact of a meteorite or comet on the earth, which may have been responsible for mass extinctions of earth life 65 million years ago (SN: 2/1/86, p.75).

Essene and Fisher found metallic globules embedded in the fulgurite glass. Electron microprobe analysis showed these to be made up of a variety of iron and silicon metal compounds. While purely metallic materials are common in meteorites, they are rarely created on the earth's surface by the normal geologic processes. Usually, they are oxidized or have combined with other elements to form compounds that must be smelted in order for the pure metal phases to be retrieved. But in the Winans Lake fulgurite, the lightning bolt had somehow removed the oxygen from metal oxides in the ground, reducing the metals to a much greater extent than even those found in most meteorites, according to Essene.

Soviet scientists had previously reported finding natural, highly reduced metallic materials, but many skeptical Western scientists suspect the reduced metals are human-made contaminants, according to Essene. Now, in light of the Winans Lake find, the researchers write, "reports of these minerals should not be rejected a priori as requiring impossible geological conditions."

Essene and Fisher's work also has a bearing on studies of meteorite impacts. Because iridium has been found at the geologic boundary between the Cretaceous and Tertiary periods in concentrations exceeding typical iridium levels in the crust, some scientists have speculated that a meteorite had slammed into the earth, spraying the planet with a fine layer of meteoric iridium and other debris. Those scientists have estimated the size of this meteorite by comparing the amount of excess iridium with what is known to occur in meteorites.

But on the basis of their fulgurite studies, Essene and Fisher suggest that much more of the iridium may have come from the earth than is commonly supposed. The researchers found that the Winans Lake fulgurite has enriched in gold; presumably, the metallic melts that were formed by the lightning scavenged the gold from surrounding soils. If highly reduced metallic melts are also formed during impact events, says Essene, then they might collect and concentrate iridium in the same way. The result would be that iridium levels in the impact melts would be much higher than what is normally found on the earth.

"If so," he says, "then the estimate of the size of the [impacting] body is too high, and therefore, perhaps, people may be looking for craters that are too large." In any event, the researchers write that their observations "broaden considerably the range of models that should be considered in investigating the origin and implications of the observed iridium anomalies."
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Title Annotation:largest known fulgurite found in Michigan
Author:Weisburd, Stefi
Publication:Science News
Date:Oct 11, 1986
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