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Author Topic: Breccia  (Read 2387 times)

Offline OokieWonderslug

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Breccia
« on: 13/12/2013 16:29:06 »
I recently moved to Albemarle, NC. This is a place that at one time had a volcano. There are outcrops of basalt on the surrounding hills and rhyolite on Morrow Mtn.

At the base of a hill near Morrow mtn there is an outcrop of what appears to be breccia.  It is shattered rock with the gaps filled in with quartz.

Can breccia be formed underground? Are there magmas that are nearly 100% quartz?

The rock all around this area is a meta-mudstone. The basalt and rhyolite pierce the mudstone in many places. I have found odd basalt rocks among the mudstone like it was thrown there from some distance.

I have read that pillow lavas are found around here too, but I have no idea how to tell if that is what it is. So how do I tell?

And why do they keep changing the sign on top of Morrow mtn? A few years ago it said Morrow mtn was a volcano on an island some 500 million years ago. Now it has been removed and it just talks about how the indians used the rock for arrowheads. I can see no other history for this mountain other than being the vent of an ancient volcano. What we see now may have never been on the surface, but it still was a volcano. Otherwise why the pillow lavas and rhyolite?

But back to my question, can breccia be formed underground?


 

Offline Bass

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Re: Breccia
« Reply #1 on: 13/12/2013 22:00:54 »
Breccia (rock composed of angular rock fragments) certainly form underground.  All you need is some sort of mechanism to fracture the rock then cement it back together.  Solution collapse breccia forms in caverns and sinkholes. Fault breccia is formed exclusively underground.  As do hydrothermal and intrusive breccias.
Your outcrop may have formed as an intrusive, fault or hydrothermal breccia.  Hydrothermal (hot waters) circulating through the breccia could easily scavenge silica and deposit quartz between the fragments.  Do you notice any bleaching or other changes adjacent to the quartz veins- or are the fragments fairly uniform?
I'd have to look it up, but I suspect that Mt Morrow is part of the Taconic Orogeny- which was an island arc system at the time the rocks were deposited (like Japan today).
 

Offline OokieWonderslug

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Re: Breccia
« Reply #2 on: 14/12/2013 00:44:57 »
There is basalt in the same area so that would make the hydrothermal theory stand out. The fragments are all about 3 to 4 inches and angular. There is no bleaching, ans the quartz is mostly thin except for some places where the fragments are farther apart.

Yes I have read up on this area and it was part of an island arc like Japan. The meta mud stone around here was formed in a shallow lagoon type area. Interspersed with layers of what appears to be tuff.

The breccia occurs in the lower strata of the hill and there are large basalt blocks on the top of it. I suspect this was a dike or something that did not make it to the surface.  It is like less than 5 miles from Morrow mtn so it has to be part of the same vein that fed Morrow mtn.

Another thing:

I was walking my dog here in Albemarle, about 8 miles from Morrow mtn and looking closely at the exposed layers of mud stone. There are foot thick layers of mudstone separated by tuff layers about 3 inches thick. Among this I discovered a block of basalt. It is a single lone block about 4 cu ft in size. Vaguely tear drop shape. It is part of the natural landscape and was not brought there by man.

Could this be a volcanic bomb? I mean, is it possible this rock was thrown there by the volcano that was Morrow mtn? Everywhere else I have seen basalt it was in a vein or outcrop. never a lone block in the mudstone. Am I on the right track?
 

Offline Bass

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Re: Breccia
« Reply #3 on: 14/12/2013 21:14:33 »
It could be a volcanic bomb, but with basalt I would expect to find more bombs and/or scoria nearby.  If it is in the mudstones, could also be a large fragment that got washed in when the muds were deposited.  Transport would be consistent with rounded shape also.
 

Offline OokieWonderslug

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Re: Breccia updated
« Reply #4 on: 19/02/2014 04:24:38 »
After further scouring of Google Earth I found what seems to be the source of that lone piece of basalt. There is an outcrop about 1.2 miles from the rock and it is on a small hill. That is close enough to have been transported to where I found it. And the outcrop is up hill from it. Taking the extreme erosion purported to have happened in the area that would account for the lone rock.



I still can't understand how you can dig down and find things hundreds of thousands of years old, like the Carolina Bays have 80,000 years worth of sediment on them, but geologists say erosion is happening.

It is like in Appalachia. The mountains are supposed to be eroding at 1 millimeter a year. But you can go on ridges and dig down and find arrowheads a foot deep. Why are they not on the surface? They were lost on the surface and now they are a foot deep. Yet the mountain ia supposed to be eroding away.

You can dig in the Carolinas and find the remains of the Younger-Drayas and Clovis points under that. Why would they be buried if erosion is going on?

It seems disingenuous to say we lost 5 miles of sediment if you can dig down and find things buried that were left on the ground. And it still doesn't make sense if you say one part is eroding while other parts are growing. Nothing has been getting higher while the 5 miles went away. It would have to be all of it eroding in order to get the fairly flat plain we have on the Piedmont.

When I was a kid and I went arrowhead hunting with father we dug thousands of arrowheads out of the ground and not a one was at the surface anywhere other than in one creek. And we figured the Indian died in the creek and his quiver rotted there and that is how we found all the good points next to each other like that. What buried those thousands of arrowheads (many on ridge tops) while the rest of the mountain was getting shorter? And how? It seems to eat at me. How does erosion bury things on a ridge of an eroding mountain?
 

The Naked Scientists Forum

Re: Breccia updated
« Reply #4 on: 19/02/2014 04:24:38 »

 

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