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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:

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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?

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  • $\begingroup$ Is that sloped at angle? Is it a street curb/gutter? What is the dividing line that cuts through the pattern? How old is the stonework. Location, climate? $\endgroup$
    – Keith McClary
    Nov 21, 2019 at 21:04
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    $\begingroup$ sandatlas.org/dendritic-growth-in-crystals $\endgroup$
    – Keith McClary
    Nov 21, 2019 at 21:10
  • $\begingroup$ Dendrites , gradual crystal growth as the molten rock freezes. very similar to patterns in as cast metals. $\endgroup$
    – blacksmith37
    Nov 22, 2019 at 2:56
  • $\begingroup$ This looks dendritic pyrolusite, manganese oxide. $\endgroup$
    – user18261
    Nov 22, 2019 at 5:26
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    $\begingroup$ see answer several manganese oxides form that dendritic structures, but no pyrolusite $\endgroup$
    – user18261
    Nov 23, 2019 at 13:17

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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.

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They are quite pretty, but they are not fossils. They remind me of the patterns that frost sometimes makes on window panes. A similar process is at work here, though with mineral bearing water infiltration rather than frost being the cause. The water infiltration deposits crystals in the rock.

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