While taking a walk two weekends ago, a strange scar on the sidewalk caught my eye at Colfax and 24th in Minneapolis.
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. It was as if a steamshovel had been carelessly dragged along the sidewalk. But the scar was strangely branched. It was hard to imagine what kind of tool could have carved it, even intentionally.
On closer examination, I found the edges of the scar almost completely encrusted with black glass, some of which was easy to pick loose. (The photos below were taken the following weekend, when the scar was filled with organic rubbish.)
It took me ten minutes of standing around staring at the sidewalk — sometimes peering at it very closely on my hands and knees (much to the puzzlement of passersby) — to convince myself that this was created by lightning. It's fulgurite. Whatever made it not only dug a small trench in a municipal sidewalk, it also burned the sand in the sidewalk's concrete into glass.
The scar is immediately below an ordinary city powerline pole, and I can't completely discount the possibility that the scar was created by a downed powerline. I did poorly in the electricity sections of my college physics classes, but my sense is that there's a number of problems with a powerline origin for the scar — not the least of which is that powerlines just don't have the juice to do the job. More likely, the pole attracted the lightning.
Fulgurite is usually found on sand beaches, and online photos of it make it look a little like coral. I think the loose quality of sand eases lightning's path and allows for the dramatically-shaped objects usually associated with fulgurite. Sidewalk fulgurite is not unheard of, as this PDF reprint of a 1947 article in Rocks and Minerals attests.