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I hiked the Lanikai Pillbox trail (Oahu, HI). I noticed eroded concentric rings in the igneous rock with a hard center. I'm unable to find the geologic cause of these rings. Are they Liesegang rings? There were several instances (~20) I observed on the hike.

Concentric rings in igneous rock in Oahu, HI hike.

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  • $\begingroup$ Maybe try to crash it with a hammer and see what is inside the hard center? By the way, it looks like the model of a cell : ) $\endgroup$ – User123 Apr 13 at 20:06
  • $\begingroup$ It's unlikely to Liesegang rings, they are colored rings which usually occur in sedimentary rocks. They can occur in permeable igneous rocks that have been chemically weathered, where they appear as rings of iron oxide. I don't see any significant color differences in your picture. $\endgroup$ – Fred Apr 14 at 8:59
  • $\begingroup$ Query: Is the circular rock, at the center of the ringed structure, flat or domed? $\endgroup$ – Fred Apr 14 at 9:02
  • $\begingroup$ @Fred circular rock is slightly domed. $\endgroup$ – user3424230 Apr 14 at 11:47
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Not knowing the setting and judging from the image alone in this trampled environment, I think we're looking at a form of chemical weathering called "spheroidal weathering".

The gist is this (from the linked wiki page): Penetrating water alters the bed-rock along cracks or joints, causing volume changes between altered and unaltered parts. These differences result in the formation of spheroidal layers that are removed by erosion (or trampling), leaving a central relatively unaltered boulder/pebble.

Liesegang rings are somewhat enigmatic, not sure if the term is applicable here.

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  • $\begingroup$ Nice work, @Earthworm. Thank you. Your answer helped me find primary resources confirm that this case is most likely "spheroidal weathering." Source 1: "Trace element mobility during spheroidal weathering of basalts and andesites in Hawaii and Guatemala; Patino et al.; doi.org/10.1016/j.chemgeo.2003.01.002; link $\endgroup$ – user3424230 Apr 14 at 14:09
  • $\begingroup$ Yep, I think that's it. But "Tertiary" in a paper from 2003 ? tststs ... :-) $\endgroup$ – user22279 Apr 14 at 15:44

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