I saw this rock while out hiking and I have no idea why it has holes in it. It’s about 7 inches long and the rough, jagged, little holes are a bit smaller than pencil-sized.
Could be a tafoni-type thing. Short explanation (from memory): rock absorbs water, which dissolves key elements (calcium? lime? can’t remember) The outside of the rock dries first, so the outside gets crusty with the strong elements. Inside of the rock is depleted. Now if you have a small breach in the crust, the inside of the rock erodes easily, leaving holes.
There is a Tafoni trail in some park (El Corta Madera???) near Redwood City, CA which has an enormous tafoni. Very interesting. Lovely hike.
Thank you, Karen! I figured it must be something like that, like some other sort of mineral was in there and then slowly eroded or something.
Nature is fascinating!
thats (almost) pure basalt, its a common thing to find holes that are merely exposed gas bubbles in ordinary basalt.
the white stuff isn’t attacked, if that white stuff suggested lime based inclusions, why isn’t the white streak turned into a long streaky erosion pit ??
you won’t find gas bubbles in obsidian - it was granite forming magma that was fast cooled , but it was already degased before being fast cooled.
you won’t find gas bubbles in basalt columns, they were slow cooled.
magma with lots of gas in it that is cooled quickly is pumice or latterite.
basalt with larger crystals cooled more slowly and has less bubbles.
Isilder, are the gas bubbles what they call a vug (a most excellent Scrabble word)?
Very interesting, thank you! This lead me to do a bunch of reading about the formation of Vancouver Island. That rock is probably from 55 million years ago. Apparently most of Van. Isl. is Basalt.
I’m not an expert geologist, but that rock doesn’t look metamorphic, its simple igneous basalt. So it wouldn’t have vug.
vug can mean two things
-
the hole or empty space created by changes in the rock… - but these should be in breccia in basalt… or some sort of metamorphic rock.
-
mineralisation basically growing in the rock and then that mineral being eaten away.
So we now have four suggested types of holes
i. tafoni… No Tafoni is like pigeon holes created by surface erosion, its like a human made brick with the face eroded, it forms a shell. The hole should be a more spherical or oblong shape as its erosion, or representing some matrix of the rock - eg cracks and then subsequent mineralation , that turns a block of rock into bricks of rock, all sitting together almost exactly where created, but then mineralisation and tafoni formation makes it like a brick wall with mortar at the seams and/or bricks eroded and/or bricks missing
2. original minerals eroding away. … But basalt doesn’t have that.
3.vug. No vug creation must damage the rock… or would unless it was under the great pressure and metamorphised at the same time… The vug would be found in metamorphic rock, rock with the crystals visibly stretched, rock turned to glass looking forms, minerals collecting out of the liquid rock (like in very slow cooled magma)… crystals growing in the liquid rock over a very long time…
4. basalt forming magma with little gas cooled to form rock- Some bubbles formed and were stretched slighty by flow of semi-hard rock as it cooled, and then got preserved.
Its ordinary basalt, its just showing some of the bubbles that formed when gases came out.
gas in the magma causes volcanic eruptions that shoot dust straight up… the magna is turned into a fine dust , and then cools so its no longer magma… and just goes with the wind.
If that’s basalt (and I agree with Isilder, it looks like it) then bubbles is most likely (or to use the technical term, vesicles).
it’s definitely not tafoni, that texture generally looks like honeycomb or similar - denser network of often interconnected cavities, also more variation in hole size, etc. Also, a wonderful test for tryptophobia(trigger warning)
Thanks. And I hope you pronounce magma just like this.
Vancouver Island … yeah … the rock looks like what we have down here in regular Oregon … I’m told basalt and the holes are gas bubbles … BIANAGIASOTW …
I *always *do 
The joke, of course, is that technically once it reaches the surface, it ceases to be magma and becomes lava.
Don’t tell Dr. Evil that.