Saturday, July 23, 2011

Fulgerites

Good morning all. The temperature here in Grand Marais yesterday was in the mid 80s. It was hot, but not as hot as in other areas. Thankfully Lake Superior does serve to cool us off a bit.

A few customers have brought beach fulgerites into the museum this summer. These are formations of instant sandstone that are formed when lightning hits driftwood. The energy from the lightning goes through the log into the sand below, partially melting and fusing the quartz crystals. Here are a few shots I took a couple of years ago of just such a log that the waves had turned over.




There are two types of fulgerites. One is a beach fulgerite that forms these sandstone-like structures when lightning does not directly hit the sand, but is buffered by driftwood. The other type of fulgerite forms when lightning directly hits the sand. Here are pictures of direct fulgerites that I took off the internet.



Mother Nature makes natural glass each time a large amount of energy is released at the Earth's surface, provided that the soil composition is suitable. Natural glass fulgerites are made when lightning hits compacted wet sand. Technically these direct natural silica glass fulgerites are called "lechatelierite." Natural glass can also be made when meteorites collide with the Earth's surface. This type of natural glass is called meteoritic glass or tektites. Here is a photo of a tektite that I borrowed off the internet.


Fulgerites form when there is a cloud-to-ground lightning discharge. They are named from the Latin "fulgur" which means lightning. Fulgurites come in a great variety of forms and can be viewed as nature's own works of art. It is worth noting that lechatelierite (natural silica glass) is not present in obsidian, a glass-like material associated with volcanic activity. On the other hand, volcanic activity is known to generate lightning which, if it strikes sandy soil, may produce a fulgurite. Silica glass has been also made as a result of nuclear explosions. In 1945, the first nuclear bomb (equivalent to 18,000 tons of TNT) was detonated in the New Mexico desert. The explosion formed a crater 800 yards in diameter, glazed with a dull gray-green silica glass. This glass was named "trinitite" after Trinity Site where the first nuclear bomb test was conducted.

The earliest discovery of a fulgurite was in 1706 by Pastor David Hermann in Germany. Most people have never seen a fulgurite, and if they have they might not have recognized it for what it was. Direct fulgurites can be divided in two classes: sand fulgurites and rock fulgurites. Sand fulgurites are usually hollow, glass-lined tubes with sand adhering to the outside. Rock fulgurites are formed when lightning strikes the bare surface of rocks. This type of fulgurite appears as thin glassy crust with which may be associated short tubes or perforations lined with glass in the rock. Glass of this type may be relatively low in silica and exhibit a wide variety of colors, depending on the composition of the host rock. Rock fulgurites are found on the peaks of mountains. Beach fulgerites, as I call them, are an indirect solid structure that often look like mushrooms.

The diameter of fulgurites ranges from a quarter of an inch to 3 inches, and the color varies, depending upon the type of sand from which they were formed. Direct sand fulgurites as well as indirect beach fulgurites are usually tan, grayish, or black. The inner surface of the direct sand fulgerites is glassy and exhibits numerous bubbles. The walls are usually paper thin. Direct sand fulgurites are quite fragile and very difficult to excavate in one piece. Because the indirect beach fulgerites are solid structures, they tend to be more durable.

Since well formed direct fulgurites are real glasses, they sometimes are resistant to weathering. For this reason they are used as paleoenvironmental indicators. For example, many fulgurites are found in the Sahara desert, where presently there is little lightning activity, confirming that very different conditions existed in this region in prehistoric times. A fossil fulgurite thought to be 250 million years old has been reported.

On average, about 100 lightning discharges occur every second on the Earth. Only about one-third of them involve ground (others occur in the cloud, between clouds, or between cloud and clear air) and potentially can make fulgurites. The Tampa area in Florida receives more than 30 lightning strikes per square mile per year. This is the highest level of lightning activity in the United States.


Each cloud-to-ground lightning involves an energy of roughly 109-1010 Joules. Most of the lightning energy is spent to produce thunder, hot air, light, and radio waves, so that only a small fraction of the total energy is available at the strike point. However, it is well known that this small fraction of the total lightning energy is sufficient to kill people and animals, start fires, and cause considerable mechanical damage to various structures. Lightning is also a major source of electrical disturbances.

The peak temperature of lightning channel is five times higher than the surface temperature of the Sun. The lightning peak temperature is considerably higher than silica's melting point which is why the energy from a lightning strike can form and/or melt sand.

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