Tuesday, September 29, 2015

A Little Rock Mystery for the Day

A little rock mystery for the day. What are we looking at, and why is it strange? The picture is about 16 inches across.

10 comments:

Lockwood said...

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

meta-geologist said...

Or is it borings from some marine critter?

Anonymous said...

I'm with meta-geologist. I remember seeing this around the beaches in SoCal

Marciepooh said...

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.

RobsGoldVine said...

Wow. A good one. Could it be scoria, or worm holes in mud, or lapilli tuff?

Charles Caldwell said...

Iron concretions in a sandstone matrix like those found commonly in the deserts of the Colorado plateau.

dinogami said...

Another vote for borings of some kind....hardground type?

Hollis said...
This comment has been removed by the author.
Hollis said...

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

Randy A. said...

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.