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Author Topic: Erosion or deposition?  (Read 5220 times)

Offline OokieWonderslug

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Erosion or deposition?
« on: 18/07/2012 15:41:04 »
As a former amatuer archeologist, I have dug many artifacts out of the ground. One can go down 5,000, 10,000, 20,000 years. I have never found an artifact on bare soil anywhere. Not even in a cave. They were always buried.

Yet when you look at geology, it is always assumed that great amounts of overburden has been removed. They say the Appalachians are eroding at 1 millimeter a year. Yet I have found spear points thousands of years old on ridges in the Appalachians under more than a foot of soil.

Geologists tell us that here in the peidmont we have lost 5 miles of overburden. 5 miles. The soil here is of volcanic origin. There HAD to be a volcano releasing this ash. Yet apparently there is zero remains of any volcanoes here other than the ash deposits and the pumice grains. Extruded lava is much more hard to erode than ash.  Yet we have no huge piles of igneous rock that would have been a volcano.

This makes zero sense. Now they tell me that an area near here that has rhyolite and volcanic breccia was never eruptive. They say the only reason it has a higher elevation than the surrounding piedmont is due to uplift some 5 million years ago.

Please help me understand how you get rhyolite and breccia from an underground pluton. Please tell me how if things are eroding how come everything that is old is buried and not on top of the ground. To me it just does not add up.

Some will say that it goes in cycles and deposition is followed by erosion. But how long are these cycles? There are artifacts in south America 127,000 years old and they were all under dirt in a cave. The Carolina Bays are supposed to be some 80,000 years old and are still on the surface. Uneroded and uncovered. The Sandhills are supposed to exposed dune fields millions of years old. They are supposed to have been buried and then eroded out of the ground. Yet when sand is buried and compressed by overburden it solidifies and becomes sandstone. In the Sandhills the sand is still soft and not rock at all. If it was covered by 5 miles of rock it would not be so unconsolidated. There are fossil dunes in West Virginia. They are easy to make out. But they are rock, not sand. Sand is a rare thing to find in WV and even then it is a soft sandstone that has to be dug and crushed.

There is a layer of ash in the Marsailles shale in WV that has been mapped. It clearly points to a place in Virginia where there should be an extinct volcano. Yet there is no such landform where it should be. No where near it in any way.

I spent the better part of a day scouring Google to find outcrops of extrusive igneous rocks in the piedmont on NC only to come up with nothing. Not one study, not one report of anything.

If we are experiencing erosion like we are told, how come there is not one towering pile of basalt or rhyolite anywhere in NC? Even if there was extensive folding of the sedimentary layers (which there was) there would be a long ridge of extrusive rock somewhere given the differential erosion of hard magma and soft ash. Which there is not.

Please help me understand why this is like it is.


 

Offline Bass

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Re: Erosion or deposition?
« Reply #1 on: 18/07/2012 18:46:46 »
Now they tell me that an area near here that has rhyolite and volcanic breccia was never eruptive.

Please help me understand how you get rhyolite and breccia from an underground pluton.

OW- where exactly is this rhyolite and breccia.  Not all "rhyolite" is extrusive (deposited on the surface as volcanic flows or ash).  There are subvolcanic (underground) dikes and sills that are classified as rhyolite due to their aphanitic (fine grained) nature.

If memory serves me (and it often doesn't), the Carolina Slate belt is full of volcanic rocks.  These rocks are very old and have been extensively folded and faulted.  The source volcanoes would have been eroded quickly, while the flows and ashfalls deposited underwater would have survived- so I wouldn't expect to see the source volcanoes.  The remnants of those volcanoes would be plutonic rocks possibly some distance from the now metamorphosed remnant volcanic deposits.

But maybe I missed the point of your post?
 

Offline OokieWonderslug

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Re: Erosion or deposition?
« Reply #2 on: 19/07/2012 03:59:08 »
The rhyolite is on Morrow Mtn in the Uwharries in Nc. I do not understand why the harder eruptive rocks would be long gone and the ash that fell still be here. It seems counter intuitive. You're right, it was deposited in a marine environment.

If you're saying there would still be plutons, ie the "pipe" that the magma flowed through, that is basically what I am asking. If it was folded 90 degrees on it's side, (or any angle really) there would be a line of exposed rock left when the softer surrounding rock weathered away. From what I can tell the "dips" of the folds do not go too deeply as they are mostly eroded away. I don't think there is enough dirt left to cover something that massive. But I can't get enough data due to it not being available.

There is a pluton in Cabarrus County in Concord NC that is what I would imagine the neck of as volcano would look like. It is a round dome shape and has rock around it of another type like one would figure there would be if it erupted after the "pipe" solidified. It is essentially level with the terrain, and I had read it was a pluton and not eruptive so I never figured it to have been the source of the tuff and pumice around here.

Morrow Mtn is a rhyolite and is higher than the surrounding area by about 1,000ft or so. It has breccia and I think I read of flows also. I was convinced it was the source until they changed the sign at the top of the mountain to say it never reached the surface. Something that at first blush appears absurd to me. That changed sign is what has led me to do so much research into just where the ash came from if that is not it.

The original sign (from memory) "You are standing on what what an active volcano 500,000,000 years ago in an island chain"

The new sign "The rock here was once thought to have been a volcano. further research has shown that this rock never reached the surface and was uplifted 5,000,000 years ago during as a result of yet unexplained tectonic activity. It has since eroded to the surface to produce what we see here".

It is shaped somewhat like you'd expect a volcanic pipe to look like. If it wasn't there, then where? It has to be either here, or in Concord. There aren't many other candidates. The place in Concord is what I would expect if it had been leveled by a glacier, not super slow erosion.

There are other places where igneous rock exists around here, but they really appear to be dikes, plutons and sills. My back yard has a basalt kind of rock with what appears to be "squish" marks like it forced it's way through a crack slowly. It has fine grained crystals, but you can see them quite easily. When polished it has a deep greenish black color. The crystals are between sand grain and table salt size which seems to be a hallmark of cooling slowly underground. And they are in straight narrow valleys that start here and regularly repeat all the way to the Uwharries. Most of those valleys contain the same basalt rock. So they seem to be fissures from 200,000,000 years ago.

There are numerous granite outcrops too. Most are boulder groups on ridges, but there is also the Pageland granite South West of here which I feel is impossible to have ever erupted onto the surface. Otherwise there would not be the one inch size matrix of crystals. There is an eroded granite outcrop on Mountain Island Lake in Mecklenburg County that is now nothing but grus and red clay. There is a spot about 1,000ft away from the island where there is nothing but concentrated grus so I figure there was some sort of transport to move them into one place once the feldspar and mica turned to clay.
So that could not be the source.

There has to be a source and it can't be completely gone if the tuff rock it created is soft enough to scratch with a fingernail while rhyolite is hundreds of times harder and needs steel to scratch it.
 

Offline Bass

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Re: Erosion or deposition?
« Reply #3 on: 19/07/2012 05:36:28 »
I know most of the slate belt in the Uwharries is on the order of 400 to 600 million year old, and is composed of both metamorphosed sedimentary and volcanic rock (flows and ash).  The slate belt is intensely folded (compressive strain), which would partly explain why it is difficult to see crosscutting relationships.  The more the rocks are compressed and folded, the more parallel all the layering/dikes become.  When I get time, I'll draw a diagram illustrating this.

I'll have to do some research on Morrow Mtn.  Any idea of the age of the rhyolite there?

The basalts you refer to were (maybe diabase dikes?) were intruded/extruded when North America broke apart from Africa in the Triassic.  Much more recent and relatively undisturbed.

I'll be in Winston-Salem in a couple of weeks- if I have a chance will try to get by Morrow Mtn- but my schedule is pretty tight.
 

Offline OokieWonderslug

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Re: Erosion or deposition?
« Reply #4 on: 19/07/2012 16:21:42 »
If there are flows then can they not be traced back to their source? Flow implies that it was eruptive. That should confirm it was a volcano.
 

Offline Bass

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Re: Erosion or deposition?
« Reply #5 on: 19/07/2012 16:47:22 »
Flows obviously confirm that there was a volcano.  However, it doesn't mean that the flow can be traced back to the source volcano.  Assuming that the volcanoes were part of an island arc (and were not submarine), the source volcano would likely be long since eroded away- while flows and ash beds deposited underwater could survive.
 

Offline Mazurka

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Re: Erosion or deposition?
« Reply #6 on: 19/07/2012 17:18:08 »
Bass' point is illustrated by diamonds.

Diamonds are found in 2 distinct circumstances - firstly Kimberlite & lamproite rocks ("primary" sources) and  in "placer" deposits in alluvial deposits ("secondary" sources). 

It is thought that all terrestial diamonds originated deep in the mantle so there should be an obvious "pipe" from which they originate.  However, where they are found in a placer deposit, logically, it should be farily straightforward to explore back up the river system to the point where the volcanic pipe meets the surface and then start mining from there.  As far as I remember, despite this very obvious thing to do, no one has successfully traced a dismond placer deposit back to its original kimberlite/ lamprolite.   

 
 

Offline OokieWonderslug

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Re: Erosion or deposition?
« Reply #7 on: 19/07/2012 21:06:35 »
So what I am getting is that everything that was dry land on the island that smashed into America forming what is now NC eroded away long ago and all that is left is what was deep under water.

Sort of what it would be like if say Mt Surrat in the Caribbean eroded away and there was nothing left of it except the ash it deposited into the deep ocean.  That since the ocean off the coast there is around 17,000ft  deep there would nothing left of the volcano.

But what about the plumbing that fed the volcano? Surely it went far further down than 17,000ft. Hawaii has Mauna Loa rising 36,000ft from the ocean floor. But the source of it's magma goes down hundreds of miles. 

Even in a subduction zone the magma comes from like 20 miles down. So why would there not be the plumbing of the volcano rising above the average local elevation? Solidified magma is harder than most sedimentary rock, isn't it? Stone Mtn GA goes down 9 miles below ground level. So when erosion claims another mile of dirt won't Stone Mtn be a tall mountain of grantite chunks towering over the area?

I am not getting the erosion rate being close to the same for ash, mud and basalt/granite. We have Pilot Mtn in Mt Airy rising 1500ft above the surrounding plain because it is basically made of sand melted together. That is harder than basalt? 

Please explain how erosion can take basalt, rhyolite, and granite just as fast as it does soft tuff, argillite and mudstone.
 

Offline Bass

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Re: Erosion or deposition?
« Reply #8 on: 20/07/2012 03:41:35 »
OW
You have the basic idea.  The roots of the volcanoes are still around- but those rocks aren't volcanic.  Roots of rhyolite (ash) are granitic rocks, andesite roots are diorite, basalt is gabbro.

Rocks more resistant to physical erosion (like basalt, rhyolite and granite) will not wear down as fast as less resistant rocks.  That being said, they may be more susceptible to chemical erosion (especially if full of feldspar crystals). 

Remember that thousands of feet of material has been removed from the Appalachians by erosion- so what remains tends to be the deeper material- not the original surface rocks like volcanoes.

BTW, I have a garage full of subvolcanic rocks (got close but never made it to the surface) that most geologists would identify as volcanic.
 

Offline Geezer

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Re: Erosion or deposition?
« Reply #9 on: 20/07/2012 06:41:16 »

I have a garage full of subvolcanic rocks (got close but never made it to the surface) that most geologists would identify as volcanic.


Makes a change. Most of us have a garage full of worthless crap.
 

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Re: Erosion or deposition?
« Reply #9 on: 20/07/2012 06:41:16 »

 

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