Any geologists on here?

[Spinney]

Not an expert either but...

It looks from the photo as if the injection of the brownish stuff came before the formation of the white streaks, as there is a white streak that goes through part of the brown (on the right of your pebble).

Quartz is often the last bit of liquid left in a magma chamber as it has the lowest melting point (anyone correct me here if I'm getting this wrong!), and gets injected into cracks in the surrounding rocks.

So the granite (if that's what the grey mass of the rock is) was formed, and then later more magma was emplaced nearby. Some fluids were forced into the rock forming the brownish stuff, and later, possibly from the same magma chamber, the last bits of liquid quartz were then forced into cracks in the now-solidified brown stuff and the grey.

That's one idea, anyway!

 

[threebikesmcginty]

Mipancertite, I think, but I'm no expert.

I'm being particularly dense today, took me a couple of reads to see that!

 

[andytheflyer]

Not sure whether or not to own up here, but I am a practising professional geologist - but spot the rock is not my field - to get my excuses in first.

Looks to me like an acid volcanic (lava). Probably from Snowdonia or maybe the Lakes, and transported down to the area during the last glaciation. The white veins are probably quartz (try scratching with a kinfe blade - if no real scratches can be made then it's almost certainly quartz).

The reason the various veins intersect is due to different phases of vein formation - during periods that the rock was being stretched, bent and fractured deep underground (a process continuing today - which is why we get earthquakes even in the UK) - and the cracks are then filled with hot aqueous quartz-bearing solutions, you will be able to see veins crossing each other and work out the sequence. These veins are not necessarily anything to do with magma chambers - there's a lot of hot, mineral-rich water circulating deep in the crust.

Can't see the brown veins in your photo - but they may well be iron rich - and could once have been iron sulphide (pyrites - fool's gold) and has now been weathered to a softer iron compound by the action of the sea.

As others have said though, rock identification can be difficult, and there are so many types of igneous rock that merge into one another just by a small variation in chemistry. Doesn't look like a granite to me though - too fine grained, but acid volcanics are the extrusive (comes to the surface) version of granites - so the chemistry may well be similar.

Does that help?

Andy

 

[Globalti]

Not sure whether or not to own up here, but I am a practising professional geologist - but spot the rock is not my field - to get my excuses in first.

Looks to me like an acid volcanic (lava). Probably from Snowdonia or maybe the Lakes, and transported down to the area during the last glaciation. The white veins are probably quartz (try scratching with a kinfe blade - if no real scratches can be made then it's almost certainly quartz).

The reason the various veins intersect is due to different phases of vein formation - during periods that the rock was being stretched, bent and fractured deep underground (a process continuing today - which is why we get earthquakes even in the UK) - and the cracks are then filled with hot aqueous quartz-bearing solutions, you will be able to see veins crossing each other and work out the sequence. These veins are not necessarily anything to do with magma chambers - there's a lot of hot, mineral-rich water circulating deep in the crust.

Can't see the brown veins in your photo - but they may well be iron rich - and could once have been iron sulphide (pyrites - fool's gold) and has now been weathered to a softer iron compound by the action of the sea.

As others have said though, rock identification can be difficult, and there are so many types of igneous rock that merge into one another just by a small variation in chemistry. Doesn't look like a granite to me though - too fine grained, but acid volcanics are the extrusive (comes to the surface) version of granites - so the chemistry may well be similar.

Does that help?

Andy

Thanks. Er.... the brown veins are the biggest feature of all. They are softer and the brown stuff looks randomly crystalline.

 

[Blue]

Given where the rock was found it is possible that it's a piece of stone from Ailsa Craig - off the coast of Scotland. A lot of similar Ailsa Craig stones are seen on the Irish coast as well - they owe their rounded shape to their sea journey. It's possible that the rock is quartz dolerite or granite, both of which would be of volcanic origin.

 

[andytheflyer]

Hi Globalti,

Your Er... is entirely justified - my eyes must be telling porkies - I originally saw the pic slightly differently - the 'brown' veins looked to me like shadows, suggesting some form of fracturing and displacement - which was a bit odd. Looking again I can see exactly what you meant.

The brown veins are cut by the white quartz veins, and so must be earlier - still think it's likely to be some form of iron mineralisation tho, in two phases as I suggested. (And it's very probably not a granite!). I'd expect the brown stuff to be scratched by a penknife, and hence fairly soft. Pretty rock though. Try varnishing it - it will bring out the features.

Oh, and you always need 3 geologists if you have a geo-question - to get a majority verdict - 2 are never enough.............

 

[Globalti]

Thanks. To tell the truth, if I had my time again I would go back to Uni and read geology; as I have aged I have become increasingly aware of the importance of the rocks under our feet in determining how the Earth looks and how we live and I have found it increasingly fascinating. Oh well, too late now.... except maybe an Open University course when I retire.....

 

[heretic]

Too fine for granite, and I've never seen a granite that colour though they can be anything from black'n white to pink. I'll have a punt at diorite, 2 phase hydrothermal injection and shaped by sea transport.
(Ex mine surveyor, it's 30 years since I studied this stuff).

 

[twentysix by twentyfive]

Thanks. To tell the truth, if I had my time again I would go back to Uni and read geology; as I have aged I have become increasingly aware of the importance of the rocks under our feet in determining how the Earth looks and how we live and I have found it increasingly fascinating. Oh well, too late now.... except maybe an Open University course when I retire.....

There must be some good books you could peruse in the meantime Globalti....

 

[XmisterIS]

fossilized 1,000,000 year old dog poo.

 

[Beardie]

fossilized 1,000,000 year old dog poo.

You're talking a load of coprolite!

 

[montage]

does the crack/brown part of the rock run all the way through the pebble or is it just on that side?


same question to the white veins

 

[Spinney]

Thanks. To tell the truth, if I had my time again I would go back to Uni and read geology; as I have aged I have become increasingly aware of the importance of the rocks under our feet in determining how the Earth looks and how we live and I have found it increasingly fascinating. Oh well, too late now.... except maybe an Open University course when I retire.....

I'm hoping to get my OU BSc in Geosciences by the end of this year - go for it!