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From: steve@nuchat.UUCP (Steve Nuchia)
Newsgroups: rec.misc,sci.misc
Subject: Re: Holes in Granite
Message-ID: <406@nuchat.UUCP>
Date: Sat, 17-Oct-87 13:08:14 EDT
Article-I.D.: nuchat.406
Posted: Sat Oct 17 13:08:14 1987
Date-Received: Sun, 18-Oct-87 12:58:31 EDT
References: <3013@whuts.UUCP>
Organization: Public Access - Houston, Tx
Lines: 40
Keywords: granite, lightning, mountain
Summary: probably erosion of (weaker) inhomogenaities
Xref: mnetor rec.misc:523 sci.misc:574

I waited to see if anyone who could spell inhomogenaities would
answer this, but no luck.  :-)

In article <3013@whuts.UUCP>, fv@whuts.UUCP (FRANKLIN) writes:
> Does anyone know the explanation for the following phenomenon?
> While hiking up Cannon Mountain in New Hampshire this past weekend
> I noticed some odd holes in the granite on the top of the mountain.
> These holes were about 3-4 inches in diameter, about the same
> amount deep and were fairly circular in appearance.
> They appeared to be randomly placed (?) around one part of the 
> mountain.  There were no buildings or man made objects nearby
> that could explain their existence.  I thought they may
> have been either core sample or anchor holes for some structure
> but their shallowness disproved this.  One theory I was kicking
> around was that they may have been caused by lightning.

> Has anyone seen these elsewhere or know what may have caused them?

Similar structures can occur in almost any exposed rock.  Small inclusions
of weaker material often assume a spherical shape in the deep rock.
When the rock is exposed and begins to weather, the inclusions become
exopsed.  Once the sphere is open to the weather it erodes very rapidly
and leaves a semispherical pit.

There are some formations where the difference in hardness is sufficient
(gas in volcanic rocks is the extreme example) to leave pits with undercut
edges - more than halh the sphere still submerged.  These surfaces are
rather uncomfortable to walk on, since the edges can be pretty sharp.

An inclusion in granite wouldn't have to be too soft to be eroded
ahead of the matrix - perhaps just a local deficiency of some
constituent substance?

Disclaimer - I'm just guessing in the case of granite.  Igneous
and Sedementary rocks have these things - Metamorphic I don't know.
I should shut up now.
-- 
Steve Nuchia	    | [...] but the machine would probably be allowed no mercy.
uunet!nuchat!steve  | In other words then, if a machine is expected to be
(713) 334 6720	    | infallible, it cannot be intelligent.  - Alan Turing, 1947