What created the holes found in this rock?

Question

It was found on the beach in Santa Barbara, California.

Top side of rock:
http://farm2.static.flickr.com/1086/1042998177_53ce99f4b6_b.jpg

Bottom side of rock:
http://farm2.static.flickr.com/1088/1042998275_4633c18da4_b.jpg

Answer 1

Hard to tell from the pictures.

To me, it looks like sandstone with limestone (calcite) concretions, and the limestone has weathered away to form the voids. Or it could be limy shell (fossil) fragments that have dissolved.

If you could provide the hardness of the yellowish groundmass and the whitish circular minerals, it would help a lot. Nothing technical here, just let us know if you can scratch it with a nail or not (a carpenter's nail, not a fingernail!).

It's certainly not an igneous rock, meteorite, or conglomerate.

Answer 2

Lobster,

What you have there is a limestone cobble that washed up on the beach from farther out into the ocean. The holes that you see are dissolution features, caused by the selective dissolution of the calcium carbonate that makes up the rock. There are a few mechnisms by which features like this can form. As a previous answerer suggested, one is that colonies of microorganisms produce byproducts that rapidly dissolve the rock. Whatever spots they happened to be living on are where holes have formed. Also, in tidal or fluvial environments small eddy currents can begin eating away at small depressions in a rock, and the continual action of these eddies exaggerates the depressions and scoops out holes. There is also a process called "Rohrenkarren" that dissolves nicely symmetrical holes upward into a piece of limestone under the right chemical conditions. These features are less common, and from the looks of the photo is probably not the cause of your holes. But it is definitely either one of the processes I've described, or a process similar in nature that is based on calcite dissolution.

Answer 3

The holes were probably caused by dissolution of carbonate material to create what is termed "vuggy porosity". Often it was calcium carbonate inclusions (sometimes fossils) which dissolved leaving behind a dolomitic matrix.

I hesitate to agree with the conglomerate answers due to the apparent uniformity of texture in the rock that remains. This looks very similar to the Kimmswick Formation and Plattin Formation (Ordovician Period) of the Mid continent Region which I am familiar with. Both of those formations are as I described.

I don't have much information on that area, but I think the Middle Devonian Simonson Dolomite may be present in that area and sounds like it could look similar to your photographs.

Answer 4

Many times I have seen Sea Urchins inside holes as the ones on that rock. I think this rock is carbonaceous and the urchin can (very slowly) make a hole as a defense to the predators and the turbulent wave-battered. I don't know if the urchin made the hole with the spines or the teeth or by an acid.

Answer 5

I agree. It's congolomerate. Some pebbles that were well rounded from being a stream or ocean beach got covered with some fine-grained sand or mud and eventually turned into rock, which id conglomerate. Then when the pebbles fall out they leave a hole.

Answer 6

Yea looks like a conglomerate. Sandstone with large pieces of rocks that got removed by weathering from the ocean.

Answer 7

These holes are sometimes created by air pockets which were traped inside the rock while it was still molten thousands of years ago. Others are caused by microbes which secreate acidic enzymes that biologically weather the holes in the rock.This is common in rock which contain calciumcarbonate

Answer 8

that is not an igneous rock, it is just a common conglomerates. It is sediment

Answer 9

It might be a meteorite or a piece of igneous rock.