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Author Topic: Why are some rocks soft?  (Read 5324 times)

Offline OokieWonderslug

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Why are some rocks soft?
« on: 05/04/2013 16:18:05 »
I have read that the Carolina Slate Belt was once covered in Himalaya size mountains. If that is so, why are the rocks so soft? I saw a geology show where they were at the remains of the Grenville mountains and they were talking about how obvious it was that that area had been deeply buried by the weight of huge mountains. But then I read also that the peidmont of NC had equally as high mountains that eroded away. But if that is so, then why are the rocks here so soft? If you dig them up and set them out they melt away in as little as 2 years. They smash into powder when hit with a hammer. How did rocks that soft support 20,000ft mountains?
« Last Edit: 01/05/2013 08:23:47 by chris »


 

Offline JimBob

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Re: why soft rocks?
« Reply #1 on: 07/04/2013 22:36:34 »
erosion

unweathered Grenville front rocks areas hard as any get
 

Offline OokieWonderslug

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Re: why soft rocks?
« Reply #2 on: 29/04/2013 15:11:25 »
But why did they not go through more intense metamorphism? That is a lot of weight to have on such soft rock. You'd think the pressure would have done more to the rock than lightly bond it together.

And what boggles my mind is the lack of eroded igneous rock in the area. Basalt is orders of a magnitude harder than this tuff around here, and the area is made of ash fall deposits yet the basalt pipes that formed this ash have eroded exactly as fast or faster than the ash itself. I do not think I will ever understand how that happened.
 

Offline JimBob

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Re: why soft rocks?
« Reply #3 on: 29/04/2013 15:47:22 »
But why did they not go through more intense metamorphism? That is a lot of weight to have on such soft rock. You'd think the pressure would have done more to the rock than lightly bond it together.

Slate is  metamorphic rock.  It was shale, became slate and then was weathered to what you see today
 

Offline Mazurka

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Re: why soft rocks?
« Reply #4 on: 30/04/2013 12:59:48 »
Far be it for me to disagree with Jim Bob, [:X] I feel compeled to put my pedants hat on and say that it is weathering not erosion that made the rocks so soft.

I would also add that the mass of rock is not well correlated with its strength or resistance to weathering. For example two different sandstones may appear to be identical (colour, grain size etc) but one will weather more quickly as it is less well cemented than another.   
 
 

Offline OokieWonderslug

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Re: Why are some rocks soft?
« Reply #5 on: 01/05/2013 21:05:35 »
I am not comprehending how rock can be buried 5 miles down and then compressed so hard the layers are turned 90 degrees from horizontal and the rocks be barely metamorphosed. That just does not make a whole lot of sense.

I know it is called the slate belt, but the slate I am used to from WV is hard stuff, sharp and can cut you. The slate here is so soft you can break it up with your hands. More like shale than slate. It dissolves in the weather so fast it is almost a chunk of mud.

And that supported Himalaya size mountains?!? I can sorta see it further west. In the Charlotte area the rocks are mostly harder, the metamorphism much more pronounced, and it feels right I guess. I can see it logically. But here I am having a hard time and you folks are being very patient with me. I am not a stupid man, I learn things quite fast. But I do like for the world to make sense. There is no mystery where I grew up. The geology of WV is more than obvious. It is plain. But here I am having a rough time seeing the sequence work right. I do not know why. All I know is that I will keep asking questions until I understand it. 
 

Offline Bass

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Re: Why are some rocks soft?
« Reply #6 on: 02/05/2013 22:18:45 »
Like a lot of things geologic, the development of the Carolina Slate Belt was not simple.  As you probably know, the Appalachians were formed between 400 and 250 million years ago in a complex series of collisions between Laurentia (North America plate) and island arcs, microcontinents and Africa.  The Piedmont was formed during the Taconic Orogeny (mountain building) towards the end of Ordivician period when an island arc (think Japan) was shoved into the east coast, followed by millions of years of erosion before the Carolina Slate Belt formed when another island arc or microcontinent slammed into Laurentia near the end of the Devonian or early Mississippian periods.  More millions of years of erosion.  During the Allegheny orogeny (late Pennsylvanian-Permian), the ocean finally closed and Africa arrived in a continent-to-continent collision- creating the supercontinent Pangea.  (a quick gloss-over of part of the geologic history of North Carolina).
Fast forward millions of years to the breakup of Pangea and the creation of the Atlantic ocean.  The suture line is probably not far east of the Carolina Slate Belt, and Africa, now with the addition of the island arc/microcontinent that formed the CSB, heads southeast while North America moves northwest.
You're probably wondering what all this has to with your original question (soft rocks)? Everything.
The Carolina Slate Belt (CSB) is composed of the muds, sands and volcanic debris that was washed into the basin between the island arc and North America.  They were lithified and mashed a bit during the collision, but only enough for low-grade metamorphism.  The volcanic rocks are from silicic volcanics, so they are mostly ash and volcanic fragments- with very little in the way of lava flows and practically no basalt- that were deposited in the basin.  Then millions of years (close to 200 million) of weathering and erosion brings us to the present day North Carolina.
Hope this little story helps.  If you want more, google Taconic orogeny and Acadian orogeny in southeast U.S.
 

Offline OokieWonderslug

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Re: Why are some rocks soft?
« Reply #7 on: 06/05/2013 00:33:14 »
That actually makes a lot of sense. Basalt is sort of easy to find though. There is a series of valleys between Unionville and Badin and nearly every single one of them has basalt blocks. There is basalt in the creek behind my house. Thanks!
 

Offline Bass

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Re: Why are some rocks soft?
« Reply #8 on: 06/05/2013 22:28:12 »
The basalt is much younger.  After the collision, Pangea broke apart.  Rift valleys, several of which stayed connected to North America (as failed rifts), were filled with basalt (mantle material).  The basalt is Triassic age and never underwent metamorphism (which is why it is still basalt, and not greenschist).
 

Offline JimBob

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Re: Why are some rocks soft?
« Reply #9 on: 07/05/2013 02:22:29 »
My Bad..  Not enough pedantry but Bass kept me rigorously honest.  Thanks 
Ready to thaw this spring Big Guy?
 

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Re: Why are some rocks soft?
« Reply #9 on: 07/05/2013 02:22:29 »

 

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