News and views from the geologic realm
Looks like vesicular basalt. I'll hazard a guess a bit further... pillow basalt forms its initial rind very quickly. After the first inch or so of that shell has solidified, and the interior continues to cool and shrink, the interior starts to pull away from the outer shell. This lowering pressure often allows for another, heavily vesiculated shell to form am inch or two inside the outer shell. The vesicular shell can be a "plane" of weakness, so the outer rind can fracture off along that plane, leaving the vesicles exposed.I've seen the phenomenon describe above many times, but I'm not seeing curvature in the photo, and I'm not sure I've seen it on the scale you describe, so I'm doubtful that's correct. Nevertheless, it's my best guess. Here's another possibility: http://outsidetheinterzone.blogspot.com/2013/04/geo-365-april-19-day-109-vesicles-close.html
Or is it borings from some marine critter?
I'm with meta-geologist. I remember seeing this around the beaches in SoCal
it looks similar to a formation here in Alabama (at some outcrops at least), so my guess is crinoidal limestone, dolomite, or chert with the crinoid stems weathering out much faster than the matrix, leaving all the holes.
Wow. A good one. Could it be scoria, or worm holes in mud, or lapilli tuff?
Iron concretions in a sandstone matrix like those found commonly in the deserts of the Colorado plateau.
Another vote for borings of some kind....hardground type?
Tafoni! Which is not really an answer as there are multiple causes. These look like the work of boring (as in drilling) clams -- were you at the seashore? I did a post about them a few years ago: http://plantsandrocks.blogspot.com/2012/07/tafoni-3.html
Garry's previous posts have been from Medicine Lake Highlands, a volcano. And at first glance, this seems to match -- it looks like vesicular basalt.But the holes seem too regular.So my second (and final) thought is that this rock is (or was) in the intertidal zone, and the holes have been left by some boring animal, perhaps piddock clams.
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