Thursday, September 18, 2014

More about Fulgurites

Over the years I have had a few postings on this blog about fulgurites including: July 23, 2011 (photos and information about beach fulgurites and direct lighting strike fulgurites); June 26, 2012 (close up photos of beach fulgurite); May 14, 2014 (power line fulgurites).  Below I summarize the previous postings and include some new close up photos I took of the incredibly colorful power line fulgurites.

There are two main types of fulgurites caused by lightning, and one other type caused by power lines. One of the main types is a beach fulgurite that forms sandstone-like structures when lightning does not directly hit the sand, but is buffered by driftwood. The energy from the lightning goes through the log into the sand below, partially melting and fusing the quartz crystals.

Below is a log from the Grand Marais beach that was turned over by waves.  You can see the fulgurites that are still clinging to the log.

Here is a close up of a beach fulgurite.

The second main type of fulgerite forms when lightning directly strikes dirt or sand.  These fulgerites are natural hollow glass tubes that form when lightning with a temperature of at least 1,800 °C (3,270 °F) melts silica on a conductive surface and fuses mineral grains together.   This process occurs over a time span of around one second, and leaves evidence of the lightning path and its dispersion over the surface or into the earth.

Fulgurites may be up to several centimeters in diameter and can penetrate deep into the soil, sometimes occurring as far as 15 m (49 ft) below the surface that was struck.  The color varies depending on the composition of the sand in which they formed, ranging from black or tan, to green or a translucent white. The interior normally is very smooth or lined with fine bubbles; the exterior generally is coated with rough sand particles and is porous. One of the longest fulgurites to have been found in modern times was a little over 4.9 m (16 ft) in length, and was found in northern Florida.

The more unusual type of fulgurite can be produced when the cables of a high voltage electrical distribution network break, and the wires fall onto a conductive surface beneath, in the presence of loose sand or soil.  This is what happened south of Marquette, MI this spring when a tree was hit by lightning and fell on two high tension wires.  Not only did the live wires melt four feet of snow and dry out the grass, but they started a fire in the grass along the highway.  The wires arched for three hours before the power company and fire department were able to respond.  Once everything cooled off, the family was surprised to find two streaks across the ground that intertwined.  All along the streaks there were colorful fulgerite tubes showing on the surface.

All along the streaks there are fulgurite-like tubes sticking up out of the soil.  They are made from amorphous (non crystalline) silica quartz.   Apparently, the current-conducting area around the downed power line continued to expand and glow as electrical current flowed into the ground.  Not only were there rows of the fulgurites, but there were also horizontal structures that spidererd perpendicular from the main lines.  Once power was finally removed, the molten materials solidified into a bubbly, glassy silica rock. 

Here is a photo of the dirt.  I would have expected it to be more sandy -- but it was not.

Last evening I decided to get out my USB microscope camera to get a close up and personal look at the power line fulgurites that I call "electrocuted dirt."

Here are two pictures of the less glassy husk.

More close ups....

LUNA 04:

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