Sand can be made up of a few different elements, mostly silicon and oxygen, and lightning itself is very powerful (the hottest temperature within lightning can reach 30,000 Kelvin, which is equivalent to 29,726.85 C or 53,540.33 Farenheit). Sand on many beaches is made up of either silicon dioxide (SiO₂), which also happens to be the most abundant substance in the earth's crust, or quartz (SiO₄), and they melt between 1600 Centigrades (2912 Farenheit) and 1800 Ccentigrades (3272 Farenheit). When the lightning makes contact with the ground, it fuses the silica-based sand. The heat from the lighting is delivered very quickly, which means the sand is only hot for a very short amount of time before cooling. The rapid cooling leaves the cool shapes known as fulgurite.

In general, chemical reactions will happen more frequently when you give them more energy. This is why we usually need a burst of heat energy (from the spark of a lighter or the strike of a match) to light a fire. Without that energy, there's almost no chance that a piece of paper would start burning, even though all the chemicals are already there (carbon in the paper and oxygen in the air).

When lightning strikes, it transfers an enormous burst of energy to the ground. All of that energy comes from a buildup of static electricity in the clouds. Some of it leaves the point of impact as thunder (sound energy), and the rest of it is turned into heat.

When lightning strikes sand, it can reach very high temperatures, higher than 3,000°F. Because of all that energy, a bunch of things happen. All of the different chemicals that make up the sand melt, and some of them react with oxygen (they are oxidized). When all of the chemicals harden again, they form a glassy mess that we call fulgurite.