Curious natural patterns on the surface of basalt blocks that make up the sidewalk During one of my walks through the city streets, I noticed that some basalt blocks that make up the sidewalk have in their surface some very curious natural patterns:









The photos above was taken at the Porto Alegre city, Brazil. (Coordinates: 30°01′59″S 51°13′48″W)

What are these fractal-like patterns? How and when are they formed? Is it some kind of organic formation? Is it living being or some kind of fossil?
These are most likely manganese dendrites. It is not a fossil, and not organic. These usually form in cracks in rocks, and most likely this slab was broken along an existing crack.

You can read more about it now: https://www.mindat.org/min-26645.html

One comment: this is not basalt. Basalt is black. Most likely some form of limestone.