center762000centerbottomAngel Johnson | Chemistry | May 4, 2018950000Angel Johnson | Chemistry | May 4, 2018center750007132320WHEN LIGHTNING STRIKESHow does Lightning make Glass?950000WHEN LIGHTNING STRIKESHow does Lightning make Glass?
43745157810500When lightning strikes a sandy beach, an amazing glass sculpture is created. This phenomenon was depicted in Reese Witherspoon’s 2002 romcom ‘Sweet Home Alabama’Fig. 1. Whilst this film may have shed some light on one of Mother Nature’s amazing miracles, it lacked scientific accuracy about the spectacular sculptures.
4218480856198Figure 1: Glass sculpture from ‘Sweet Home Alabama’
400000Figure 1: Glass sculpture from ‘Sweet Home Alabama’
There are two possible ways natural silica glass, also called ‘lechatelierite’, can be made on Earth: meteorites and lightning. Glass that is made when a meteorite collides with the Earth’s surface is called meteoritic glass or tektite. Glass (or glassy object, to be exact) that is made from lightning hitting an appropriate target material like sand, soil, rock or clay is called fulgurite. Fulgurite derives from the Latin word ‘fulgur’, meaning lightning. Fulgurite comes in a variety of forms and are considered nature’s work of art.
Characterisation of Lightning
Roughly, about 100 lightning discharges occur on the Earth every second. There are several types of lightning such as ones that occur within the cloud, between clouds, between cloud and clear air or cloud and the ground. Cloud-to-ground strikes make one third of the total lightning discharges, and each strike contains 109-1010 Joules of energy. Most of this energy goes into producing the thunder, light, hot air and radio waves, therefore only a small amount of energy is left for the strike point. However, that small fraction of energy is still sufficient enough to start fires, kill animals and people, cause mechanical damage and electrical disturbances. While lightning’s uncontrollability may cause it to be disruptive and troublesome, it also allows it to create fascinating glass structures.
Fulgurites were reportedly first discovered in 1706, in Germany by Pastor David Hermann. Many did not know about fulgurites, so they would not have been able to identify what it was at the time. Fulgurites can be found all around the world but they are relatively rare. They are a fusion of silica or rock, formed into natural tubes or crusts of glass from a lightning strike. Fulgurites are classified as mineraloids as they are amorphous (meaning they are not crystalline and lack a clear structure). Scientists have identified two forms of fulgurites; sand fulgurites and rock fulgurites.
-3422001774511Figure 2: The glassy interior of sand fulgurites
400000Figure 2: The glassy interior of sand fulgurites
-3615432197500Sand fulgurites are commonly found in beach or desert regions, that contain clean, dry sand. The film ‘Sweet Home Alabama’ presented these fulgurites as perfect root-like glass sculptures Fig. 1, but their actual shape is far from that. Sand fulgurites look more like rocky tubes with branches than glassy tree roots. They are coated in granules of sand, giving them a rough crusty appearance on the outside. They range in colours from greyish white, yellowish brown, tan, brown, or dark grey to black, depending on the type of sand. Sand fulgurites have a colourless glassy interior, as a result of the abrupt heating and cooling of the sand in a short period of time Fig 2. The strength of the lightning strike and thickness of the sand bed can affect the length and size of the fulgurite. There have been cases where fulgurites are almost 20 meters long, but most are less than 30 cm long, and their average diameter is 1-2 inches.
31920801278191Figure 3: Uncleaned rock Fulgurite found in Sedona, Arizona
400000Figure 3: Uncleaned rock Fulgurite found in Sedona, Arizona
30863281471900 When lightning strikes the bare surface of rock, coatings or crusts of glass are formed; these are called rock fulgurites. The shape these fulgurites leave behind resemble the Lichtenburg figure or veins. Rock fulgurites are also short tubes or holes lined with glass inside the rock Fig. 3. They are primarily found at the top of or near mountain summits since mountain peaks are like natural lightning rods, as they are a place of high elevation. Rock fulgurites have low levels of silica, so the glass can appear in a variety of colours, depending on the composition of the rock.
How It Is Made
The arrangement of silicon and oxygen atoms in the compound silicon dioxide(silica), is symmetrical and well ordered. Therefore, it needs a high melting point temperature of 1700 degrees Celsius to break apart the atoms held together by strong covalent bonds. The temperature of a lightning strike can reach up to 27,700 degrees Celsius, which is five times higher than the surface temperature of the Sun (5505 degrees Celsius). The temperature of lightning is considerably higher than the melting point of silica meaning it can melt sand with ease. The heat of the lightning lasts for less than second, so, the liquid silicon dioxide cools down as fast as it was heated up. This means the atoms do not have enough time to return to a stable position before they harden into a glass structure. This can be thought of as a game of Freeze Dance. Whilst the music is playing (heating); the atoms dance around in a liquid, and once the music stops (cooling); the atoms freeze on the spot and reconnect their bonds from where they are Fig. 4.
2374879131138Figure 4: Silicon Dioxide’s atomic structure when it is sand (left) and atomic structure after it has been heated and cooled to make glass.
4000020000Figure 4: Silicon Dioxide’s atomic structure when it is sand (left) and atomic structure after it has been heated and cooled to make glass.
Natural lightning can be unpredictable and it is unknown where a discharge could occur. But this variability can be removed when lightning is artificially triggered, by the rocket and wire technique. Triggered-lightning experiments allow the making of fulgurites to be more controlled and can create some interesting fulgurites. This is what Allan McCollum did, in the summer of 1997, as part of an underground power cable project. He worked in cooperation with the University of Florida’s International Centre for Lightning Research and Testing at Camp Blanding national guard base near Starke, Florida. In the experiment, small rockets were launched into overhead storm clouds, about two to three thousand feet above the ground. The rockets were attached to ultra-thin copper wire, to keep them grounded, and also to direct the lightning to a specific area. The fulgurites that were made of staurolite (75%), ilmenite (15%), and rutile (10%) can be seen in Fig. 5. Many fulgurites of various shapes and sizes were produced in the experiment.
It is because of the intense heat and electricity of the lightning that creates the fulgurites. This heat and electricity can be replicated by other man-made items, like power lines. In Minneapolis, Minnesota, in May 2006, a pedestrian was walking along a concrete sidewalk when something caught his eye. According to the discoverer, “The scar was something like 3 meters long and in about 5 segments, each about 2 cm deep and up to about 5 cm wide… On closer examination, I found the edges of the scar almost completely encrusted with black glass, some of which was easy to pick loose.” This rare man-made fulgurite can be observed in Figure X. The characteristics of the scar had much in common with original fulgurites, but the power lines directed above the fulgurite indicated that it was electrically produced.
15894051843405Figure 5: World’s longest (17 feet) excavated fulgurite, made in 1996 at Camp Blanding, Florida.
400000Figure 5: World’s longest (17 feet) excavated fulgurite, made in 1996 at Camp Blanding, Florida.
One such fulgurite found during McCollum’s experiment led to many days of careful digging and resulted in the discovery of a thin vertical branch, over 2 metres (17 feet) long. This piece of fulgurite was recognized as the world’s longest excavated fulgurite, by the Guinness Book of Records. The fulgurite was carefully separated into sections and then covered in a special plastic material to protect it from breaking. Each section was measure with special instruments and labelled for future reassembling.
When people think of lightning, they usually think of it uncontrollability and the damage it causes; starting fires, injuring animals people and causing electrical disturbances. But, many do not know the amazing glass sculptures it can create. Fulgurites may not be as stunning as how Hollywood displayed it, that doesn’t make it any less spectacular.
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