[Geo4dood (OP)]

Hello everyone!

In a recent hike, I've came across with this rock structure.


TR.jpg (106.08 kB . 494x338 - viewed 4276 times)

I appreciate you to tell me what's it's name, geologically speaking, and what kind of fractures are those.

Thanks in advance.

[Bill S]

Hi Geo4dood, welcome.

The left side of the arch looks as though it has parallel surfaces, which could be bedding planes.  This would suggest it is a sedimentary rock that has been folded.  Having said that, I've seen weathered granite domes that have shown quite similar features. Some more details would be valuable.  Where is it?

The fractures at right-angles to these "planes" are likely to be stress fractures.

The structure is an anticline.

I'm only an amateur, and it's a long time since I dabbled in any geology, so hopefully the experts will chip in.

[chris]

We need @Bass for this one!

[Bill S]

True, but I'd still like to know where it is.

[chris]

True, but I'd still like to know where it is.

Yes, indeed - @Geo4dood - where was this picture taken?

[Geo4dood (OP)]

I thank you guys for your reply!

The picture was taken in a recent hike I have made in the small azorean island of Corvo, (more or less 39° 41.087'N  31° 6.903'W).

I haven't approached to much, but I wonder that it's made of basaltic rock, like the most part of the island.

I agree on Bill S' remarks, but I ask you if a basaltic structure could it have the "bedding planes" and /or "stress fractures" that you've mentioned.

Thanks again!

[Ophiolite]

Since the structure is a product of weathering rather than tectonic deformation it might be more properly called a geomorpholgical structure, rather than a geological one. In that case "arch" would be the approrpiate term.

I wouldn't agree with the suggestion that it is an anticline. The top left to bottom right lineation in the rocks appears to be continued on both sides of the arch. that being the case there is no folding and so it cannot be an anticline.

"Bedding planes" can exist between individual lava flows, but they would not be as thin as is the case here. Thicker bodies of intruded magma (large sills, lacoliths, lopoliths, etc.) often have layering caused by the sequential crystallisation and settling of different minerals. Injections of fresh magma can cause this layering to be repeated many times. That's a possibility here.

The lack of a close up and the exposure of a fresh surface make it difficult to determine just what the rock type is. If much of the native rock colur is showing through lichen and weathered material then this is not basalt - far too light. The jointing is consistent with any body of cooling magma. i.e stress fractures are consistent with any igneous body, but especially a lava, rather than an intrusive mass.

Another possibility that occurs to me is volcanic ash, or pumice. This would explain the apparent layering, since multiple ash falls would be expected during an eruption. This would also be lighter in colour - another match with what we observe.

Summary: insufficient data to reach a conclusion, but a neat photograph and an intriguing feature.

[Bass]

Great photo. I've been to the Azores, but never to Corvo. My recollection is that Corvo (and the rest of the Azores) are volcanic, and that Corvo is a caldera, with both ash, welded ash and some basalt units. I think Ophiolite is correct in calling this a geomorphic feature- I would term it an "arch". Since the island is composed of recent volcanic rocks that have undergone very little strain, this feature is not an anticline. I seem to remember some discussion of this feature being the remnants of a lava tube, the fractures being "joints" that form during contraction of the lava due to cooling.

[Geo4dood (OP)]

Thank you all for your reply!

I never though that this structure could be the remains of a lava tube. Spectacular!

If that's the case, the type of rock has to be basalt, correct?

[Bass]

Correct- or possibly basaltic andesite?

[Bill S]

The structure is an anticline.

Write 100 times; I must not rush in without due care and attention.